Unveiling a Technosol-based remediation approach for enhancing plant growth in an iron-rich acidic mine soil from the Rio Tinto Mars analog site

This paper explores the potential of Technosols made from non-hazardous industrial wastes as a sustainable solution for highly acidic iron-rich soils at the Rio Tinto mining site (Spain), a terrestrial Mars analog. These mine soils exhibit extreme acidity (pHH2O = 2.1-3.0), low nutrient availability (non-acid cation saturation < 20 %), and high levels of Pb (3420 mg kg-1), Cu (504 mg kg-1), Zn (415 mg kg-1), and As (319 mg kg-1), hindering plant growth and ecosystem restoration. To address these challenges, the study systematically analyzed selected waste materials, formulated them into Technosols, and conducted a four-month pot trial to evaluate the growth of Brassica juncea under greenhouse conditions. Technosols were tailored by adding varying weight percentages of waste amendments into the mine Technosol, specifically 10 %, 25 %, and 50 %. The waste amendments comprised a blend of organic waste (water clarification sludge, WCS) and inorganic wastes (white steel slag, WSS; and furnace iron slag, FIS). The formulations included: (T0) exclusively mine Technosol (control); (T1) 60 % WCS + 40 % WSS; (T2) 60 % WCS + 40 % FIS; and (T3) 50 % WCS + 16.66 % WSS + 33.33 % FIS. The analyses covered leachate quality, soil pore water chemistry, and plant response (germination and survival rates, plant height, and leaf number). Results revealed a significant reduction in leachable contaminant concentrations, with Pb (26.16 mg kg-1), Zn (4.94 mg kg-1), and Cu (2.29 mg kg-1) dropping to negligible levels and shifting towards less toxic species. These changes improved soil conditions, promoting seed germination and seedling growth. Among the formulations tested, Technosol T1 showed promise in overcoming mine soil limitations, enhancing plant adaptation, buffering against acidification, and stabilizing contaminants through precipitation and adsorption mechanisms. The paper stresses the importance of tailoring waste amendments to specific soil conditions, and highlights the broader implications of the Technosol approach, such as waste valorization, soil stabilization, and insights for Brassica juncea growth in extreme environments, including Martian soil simulants.

The impact of microbial community structure changes on the migration and release of typical heavy metal (loid)s during the revegetation process of mercury-thallium mining waste slag

The effect of changes in microbial community structure on the migration and release of toxic heavy metal (loid)s is often ignored in ecological restoration. Here, we investigated a multi-metal (mercury and thallium, Tl) mine waste slag. With particular focus on its strong acidity, poor nutrition, and high toxicity pollution characteristics, we added fish manure and carbonate to the slag as environmental-friendly amendments. On this basis, ryegrass, which is suitable for the remediation of metal waste dumps, was then planted for ecological restoration. We finally explored the influence of changes in microbial community structure on the release of Tl and As in the waste slag during vegetation reconstruction. The results show that the combination of fish manure and carbonate temporarily halted the release of Tl, but subsequently promoted the release of Tl and arsenic (As), which was closely related to changes in the microbial community structure in the waste slag after fish manure and carbonate addition. The main reason for these patterns was that in the early stage of the experiment, Bacillaceae inhibited the release of Tl by secreting extracellular polymeric substances; with increasing time, Actinobacteriota became the dominant bacterium, which promoted the migration and release of Tl by mycelial disintegration of minerals. In addition, the exogenously added organic matter acted as an electron transport medium for reducing microorganisms and thus helped to reduce nitrate or As (Ⅴ) in the substrate, which reduced the redox potential of the waste slag and promoted As release. At the same time, the phylum Firmicutes, including specific dissimilatory As-reducing bacteria that are capable of converting As into a more soluble form, further promoted the release of As. Our findings provide a theoretical basis for guiding the ecological restoration of relevant heavy-metal (loid) mine waste dumps.

Soil arthropod community responses to restoration in areas impacted by iron mining tailings deposition after Fundão dam failure

In 2015, the failure of the Fundão dam in Mariana, Brazil released ~43 million m3 of iron mining tailings into the environment. Despite restoration initiatives in the following years, few studies-and most focused on revegetation-have evaluated the effectiveness of the restoration process in areas impacted by the disaster. We aimed to evaluate the responses of the arthropod community in areas impacted by iron mining tailings deposition from the Fundão dam that is in the restoration process. We defined sampling units in the riparian zone of the Gualaxo do Norte River, which is under restoration, and in a native not impacted riparian zone. We collected soil arthropods using pitfall traps and sampled environmental variables in the same sites. We used generalize least squares models (GLS) to test if the restored areas already presented values of arthropod diversity and functional group abundance similar to the reference area and to test which environmental variables are influencing arthropod diversity. We also tested how large the differences of arthropod community composition between the study areas and used the index of indicator species (IndVal) to verify which species could be used as an indicator of reference or restoration areas. The diversity of arthropods and the functional groups of detritivores and omnivores were higher in the native riparian zone. Understory density, soil density, organic matter content, and microbial biomass carbon were the environmental variables that significantly explained the diversity and species composition of arthropods. We show that restoration areas still have different soil arthropod diversity values and community composition when compared to reference areas. Evaluating the response of the arthropod community to the restoration process and long-term monitoring are essential to achieve a satisfactory result in this process and achieve a self-sustaining ecosystem.

Chinese sapindaceous tree species (Sapindus mukorosii) exhibits lead tolerance and long-term phytoremediation potential for moderately contaminated soils

Heavy metal pollution in metropolitan soils poses significant risks to human health and the entire ecosystem. Effective mitigation strategies and technologies are crucial for addressing these environmental issues. Fast-growing trees are an essential part of phytoremediation projects all over the world and provide long-term ecological benefits to mankind. This study assessed the lead tolerance and phytoremediation potential of a fast-growing soapberry tree species (Sapindus mukorossi) in moderately contaminated soil. Two independent experiments were conducted to assess its tolerance at (i) germination level and (ii) prolonged growth stage. In the germination experiments, seeds were exposed to lead (II) nitrate Pb (NO₃)₂ at various concentrations (0, 5, 10, 20, 50, 100, 200, 300, 400 and 500 μM) for 120 days. Results showed significant differences in germination time, germination index, seedling vigor index, energy of germination, final germination, germination inhibition, seedling height and root/shoot weight compared to the control experiments. In the prolonged growth experiments, seedlings were grown for six months in soils amended/spiked with different Pb concentrations (T0 = 0, T1 = 20, T2 = 50, T3 = 100, T4 = 150 and T5 = 200 mg kg-1 soil) and their biomass was determined. The highest biomass achieved in six months (T0: 12.62 g plant-1), followed by (T1: 12.33 g plant-1), (T2: 12.42 g plant-1), (T3: 11.86 g plant-1), (T4: 10.86 g plant-1) and (T5: 10.06 g plant-1) respectively. S. mukorossi showed no visible signs of Pb toxicity over a six-month period. During six months of exposure, the total Pb content in S. mucrossi tissues were classified as roots > leaves > stems. The highest cumulative absorption of Pb occurred between the fourth and fifth months of exposure. Maximum transfer factor (TF) was detected during the fourth month ranging from 0.888 to 1.012 for the different Pb concentrations. Furthermore, the growth behavior, lead accumulation, bioconcentration factors (BCF) and tolerance index (TI) indicated that S. mucrossi may tolerate moderate Pb concentrations for longer periods. These findings suggest that S. mukorossi may be deployed for long-term phytoremediation coupled with urban forest applications in the future.

5-year leaching experiments to evaluate a modified bauxite residue: remediation of sulfidic mine tailings

The ALTEO company produces approximately 300,000 tons per year of bauxite residue after alumina extraction, which is washed and dried in a press filter to produce Bauxaline®. In this study, different ways for recovering and reusing this residue were explored, namely transformation into a vegetated soil, use in acid mine drainage depollution, and application in sulfide-mine tailings remediation. The Bauxaline® was therefore transformed into modified bauxite residue (MBR), resulting in reduced alkalinity, salinity, and sodicity. To counterbalance the net acid generation potential of two sulfidic mine tailings with 1 mol H+ kg-1 (1.5% sulfide) and 3.3 mol H+ kg-1 (5.3% sulfide), respectively, various treatments were applied. These treatments included the addition of 10% MBR or 10% MBR plus limestone, or by limestone only, within 40-l lysimeters. Six lysimeters were monitored over a 5-year period to assess the long-term emissions from treated materials. Vegetation was tested under various conditions, and its impact on emission was evaluated. The emissions of mine tailings treated with MBR and limestone were very low. The mine tailings with limestone showed intermittent peaks of emission, probably due to the coating of calcite grain by ferric oxide, hindering contact with percolating water. Vegetation successfully grew in the treated tailings. This study demonstrated that the alkalinity of limestone can temporarily immobilize elements in sulfidic mine tailings, with a reduction factor of emissions of 300 and 40 for the two mine tailings, respectively. For long-term immobilization, the alkalinity provided by both limestone and MBR and the Al and Fe oxides of MBR are more effective and necessary for long-term immobilization, with a reduction factor of 300 and 900, respectively.

Restoring South African subtropical succulent thicket using Portulacaria afra: exploring the rooting window hypothesis

Drought prone, arid and semi-arid ecosystems are challenging to restore once degraded due to low levels of natural recruitment and survival of reintroduced plants. This is evident in the restoration of degraded succulent thicket habitats in the Albany Subtropical Thicket Biome located in South Africa. The current restoration practice for this ecosystem focuses predominantly on reintroducing Portulacaria afra L. Jacq., which is naturally dominant in terms of cover and biomass, but largely absent in regions degraded by domestic livestock. This has been achieved by planting unrooted cuttings with limited consideration of soil water availability in a drought-prone ecosystem. This study tests the effects of the timing of water availability after planting on the root development of P. afra cuttings. Cuttings were harvested from seven individual plants and grown in a glasshouse setting. Eighty four cuttings were taken from each individual, twelve for each of the seven watering treatments per individual plant. The treatments represented a time-staggered initial watering after planting, including: on the day of planting, 4 days, 7 days, 14 days, 21 days, and 28 days after planting. After 32 days, all treatments were watered on a bi-weekly basis for two weeks; a control treatment with no watering throughout the experiment was included. The proportion of rooted cuttings per treatment and dry root mass were determined at the end of the experimental period (day 42). The early onset of watering was associated with a higher percentage of rooting (X2(5) = 11.352, p = 0.045) and had a weak, but non-significant, impact on the final dry root mass (F5,36 = 2.109, p = 0.0631). Importantly, no clear rooting window within 28 days was detected as the majority of cuttings exhibited root development (greater than 50% of cuttings rooted for each individual parent-plant); this suggests that watering at the time of planting P. afra cuttings in-field for restoration may not be necessary. An unexpected, but important, result was that parent-plant identity had a strong interaction with the accumulation of root mass (F36,460 = 5.026, p < 0.001; LR7 = 122.99, p < 0.001). The control treatment, which had no water throughout the experiment, had no root development. These findings suggest that water availability is required for the onset of rooting in P. afra cutting. However, the duration of the experiment was insufficient to detect the point at which P. afra cuttings could no longer initiate rooting once exposed to soil moisture, and thus no rooting window could be defined. Despite harvesting material from the same source population, parent-plant identity strongly impacted root development. Further work is required to characterise the rooting window, and to explore the effect of parent-plant condition on in-field and experimental restoration results; we urge that experiments using P. afra closely track the parent-source at the individual level as this may be a factor that may have a major impact on results.

How harmful are exotic plantations for soils and its microbiome? A case study in an arid island

The plantation of exotic species has been a common practice in (semi-) arid areas worldwide aiming to restore highly degraded habitats. The effects of these plantations on plant cover or soil erosion have been widely studied, while little attention has been paid to the consequences on soil quality and belowground biological communities. This study evaluates the long-term (>60 years) effects of the exotic species Acacia cyclops and Pinus halepensis revegetation on soil properties, including microbiome, in an arid island. Soils under exotic plantation were compared to both degraded soils with a very low cover of native species and soils with well-preserved native plant communities. Seven scenarios were selected in a small area (~25 ha) with similar soil type but differing in the plant cover. Topsoils (0-15 cm) were analyzed for physical, chemical and biochemical properties, and amplicon sequencing of bacterial and fungal communities. Microbial diversity was similar among soils with exotic plants and native vegetation (Shannon's index = 5.26 and 5.34, respectively), while the most eroded soils exhibited significantly lower diversity levels (Shannon's index = 4.72). Bacterial and fungal communities' composition in degraded soils greatly differed from those in vegetated soils (Canberra index = 0.85 and 0.92, respectively) likely due to high soil sodicity, fine textures and compaction. Microbial communities' composition also differed in soils covered with exotic and native species, to a greater extent for fungi than for bacteria (Canberra index = 0.94 and 0.89, respectively), due to higher levels of nutrients, microbial biomass and activity in soils with native species. Results suggest that reforestation succeeded in avoiding further soil degradation but still leading to relevant changes in soil microbial community that may have negative effects on ecosystem stability. Information gained in this research could be useful for environmental agencies and decision makers about the controversial replacement of exotic plants in insular territories.

Using Unmanned Aerial Vehicles to Evaluate Revegetation Success on Natural Gas Pipelines

The Appalachian region of the United States has experienced significant growth in the production of natural gas. Developing the infrastructure required to transport this resource to market creates significant disturbances across the landscape, as both well pads and transportation pipelines must be created in this mountainous terrain. Midstream infrastructure, which includes pipeline rights-of-way and associated infrastructure, can cause significant environmental degradation, especially in the form of sedimentation. The introduction of this non-point source pollutant can be detrimental to freshwater ecosystems found throughout this region. This ecological risk has necessitated the enactment of regulations related to midstream infrastructure development. Weekly, inspectors travel afoot along new pipeline rights-of-way, monitoring the re-establishment of surface vegetation and identifying failing areas for future management. The topographically challenging terrain of West Virginia makes these inspections difficult and dangerous to the hiking inspectors. We evaluated the accuracy at which unmanned aerial vehicles replicated inspector classifications to evaluate their use as a complementary tool in the pipeline inspection process. Both RGB and multispectral sensor collections were performed, and a support vector machine classification model predicting vegetation cover were made for each dataset. Using inspector defined validation plots, our research found comparable high accuracy between the two collection sensors. This technique displays the capability of augmenting the current inspection process, though it is likely that the model can be improved further. The high accuracy thus obtained suggests valuable implementation of this widely available technology in aiding these challenging inspections.

Response of soil erodibility of permanent gully heads to revegetation along a vegetation zone gradient in the loess-table and gully region of the Chinese Loess Plateau

Revegetation has been proven to significantly affect soil erodibility of gully heads, and climate conditions are expected to affect soil erodibility by determining the vegetation characteristic. However, there are crucial scientific/knowledge gaps regarding the change in the response of soil erodibility of gully heads to revegetation along a vegetation zone gradient. Therefore, we selected the gully heads with different restoration years along a vegetation zone gradient encompassing the steppe zone (SZ), forest-steppe zone (FSZ), and forest zone (FZ) on the Chinese Loess Plateau to clarify the variation in soil erodibility of gully head and its response to soil and vegetation properties from SZ to FZ. Furtherly, we systematically and comprehensively reveal driving factors of changes in soil erodibility in three vegetation zones. Results showed that: (1) Vegetation and soil properties were affected positively by revegetation and differed significantly in three vegetation zones. (2) Soil erodibility of gully heads in SZ was significantly higher than in FSZ and FZ, by 3.3 % and 6.7 % on average, respectively, and it showed a significantly different decrease with restoration years in three vegetation zones. (3) Standardized major axis analysis proved that the sensitivity of response soil erodibility to vegetation characteristics and soil characteristics presented a significant difference as the revegetation proceeded. Vegetation roots were the primary driver in SZ, but soil organic matter content dominated the change in soil erodibility in FSZ and FZ. (4) Structural equation modeling indicated that climate conditions played an indirect role in regulating soil erodibility of gully heads by mediating vegetation characteristics. This study offers essential insights for assessing the ecological functions of revegetation in the gully heads of the Chinese Loess Plateau under different climatic scenarios.

A slurry approach to identify nutrient critical source areas from subtropical catchment erosion

Targeting catchment nutrient critical source areas (CSAs) (areas contributing most of the nutrients in a catchment) is an efficient way to prioritize remediation sites for reducing nutrient runoff to waterways. We tested if the soil slurry approach - with particle sizes and sediment concentrations representative of those in streams during high rainfall events - can be used to identify potential CSAs within individual land use types, examine fire impacts, and identify the contribution of leaf litter in topsoil to nutrient export in subtropical catchments. We first confirmed the slurry approach met the prerequisite to identify CSAs with relatively higher nutrient contribution (not absolute load estimation) by comparing the slurry sampling with stream nutrient monitoring data. We validated that: 1) differences in slurry total nitrogen to phosphorus mass ratios from different land uses were consistent with stream monitoring data; and 2) our estimated nutrient export contribution from agricultural land, via the slurry approach, was comparable to that derived from monitoring data. Additionally, we found nutrient concentrations in slurries differed across soil types and management practice within individual land uses, correlating with nutrient concentrations in fine particles. These results indicate the slurry approach can be used to identify potential small-scale CSAs. Slurry results from burnt soils were also comparable to other studies showing increased levels of dissolved nutrient loss and higher nitrogen than phosphorus loss, than non-burnt soils. The slurry method also showed the contribution of leaf litter to slurry nutrient concentrations from topsoil was greater for dissolved nutrients than particulate nutrients, indicating different forms of nutrients need to be considered for impacts of vegetation. Our study reveals that the slurry method can be used to identify potential small-scale CSAs within the same land use from erosion and can account for impacts of vegetation and bushfires, providing timely information to guide catchment restoration actions.

Revegetation-induced changes in vegetation diversity improve soil properties of gully heads

Revegetation is among the most efficient methods to improve gully headcut erosion. However, the influencing mechanism of revegetation on the soil properties of the gully head (GHSP) is still unclear. Thus, this study hypothesized that the variations in the GHSP were influenced by vegetation diversity during nature revegetation, and the influence pathways were mainly root traits, aboveground dry biomass (ADB), and vegetation coverage (VC). We studied six grassland communities of the gully head with different natural revegetation ages. The findings showed that the GHSP were improved during 22-year revegetation. The interaction effect of vegetation diversity, roots, aboveground dry biomass, and vegetation coverage on the GHSP was 43 %. In addition, vegetation diversity significantly explained >70.3 % of the changes in the root traits, ADB, and VC of the gully head (P < 0.05). Therefore, we combined vegetation diversity, roots, ADB, and VC to establish the path model to explain the GHSP changes, and the goodness of fit of the model was 82.3 %. The results showed that the model explained 96.1 % of the variation in the GHSP, and the vegetation diversity of the gully head affected the GHSP through roots, ADB, and VC. Therefore, during nature revegetation, vegetation diversity dominates the improvement of the GHSP, which has important significance for designing an optimal vegetation restoration strategy to control gully erosion.

Molecular insights into the chemodiversity of dissolved organic matter and its interactions with the microbial community in eco-engineered bauxite residue

Dissolved organic matter (DOM) plays an important role in the biogeochemical function development of bauxite residue. Nevertheless, the DOM composition at the molecular level and its interaction with microbial community during soil formation of bauxite residue driven by eco-engineering strategies are still relatively unknown. In the present study, the DOM composition at the molecular level and its interactions with the microbial community in amended and revegetated bauxite residue were explored. The results showed that the amendment applications and revegetation enhanced the accumulation of unsaturated molecules with high values of double bond equivalent (DBE) and nominal oxidation of carbon (NOSC) and aromatic compounds with high values of modified aromaticity index (AI_mod) as well as the reduction of average weighted molecular mass of DOM molecules. Significant correlations between DOM molecules and the microbial community and Fe/Al oxides were found. DOM molecules were decomposed by the microbial community and sequestered onto Fe/Al oxides, which were the main driving factors that changed DOM chemodiversity in the amended and revegetated bauxite residue. These findings are beneficial for understanding the biogeochemical behaviours of DOM and providing a critical basis for the development of eco-engineering strategies towards soil formation and the sustainable revegetation of bauxite residue.

Revegetation affects the response of land surface phenology to climate in Loess Plateau, China

Land surface phenology (LSP), defined as the plant's growth rhythm retrieved from satellite sensing products, is proven to shift with climate change and affect the carbon cycles of terrestrial ecosystems. Global afforested area is largely increasing and consequently affecting local and global climate. However, how and to what extent revegetation affects LSP remains relatively unexplored. Here we investigated the difference in four LSPs (i.e., greenup, maturity, senescence, and dormancy) and the response of LSP to climate between restored and native vegetation on Loess Plateau, China, where a remarkable process of vegetation restoration happened during 1982-2015. Most study regions showed a longer growing season (LOS) over time, specifically, with a slight delay in greenup but a relatively large delay in senescence. We found that air temperature was the dominant factor affecting greenup and maturity, while precipitation mostly controlled the senescence and dormancy in the study area. Under similar climate conditions, the LSP of restored vegetation (i.e., restored forest and grassland) showed a significant difference (p < 0.05) from native ones during 1999-2015. Compared to the native forest, restored forest from cropland and grassland showed a delayed greenup date by 0.3 and 3.6 days (p < 0.05) and an advanced dormancy date of 6.6 and 9.0 days (p < 0.05), respectively. Furthermore, the restored vegetation became less sensitive to air temperature than native vegetation, while the restored forest was more sensitive to precipitation, and its growth was affected by the water limitation to a larger extent in the study area. Our study highlights the necessity of considering land use management and its effect on the LSP change to better understand the effect of afforestation on global climate and carbon cycles.

Socioeconomic development alters the effects of 'green' and 'grain' on evapotranspiration in China's loess plateau after the grain for green programme

Revegetation has been conducted extensively to restore degraded ecosystems, thereby accelerating water consumption and affecting water availability for other human demands. Examining evapotranspiration (ET) can guide regional management to promote revegetation sustainability and address the contradiction in water demand. We characterised ET variation on China's Loess Plateau from 2003 to 2013, after the 'Grain for Green' revegetation programme implementation. Annual ET significantly increased, with an average trend of 4.87 mm yr^-2; the highest increasing trends were in the southern part of the plateau. Combining zero-order correlation and partial correlation, we found that climate and crop production were the key factors influencing ET, while revegetation also had significant effects. We also explored how multiple influencing factors affected ET through partial least-squares path modelling. Revegetation and socioeconomic development were found to impose indirect effects on ET by promoting rural household income and altering agricultural production. The specified linkages and regulating pathways among revegetation and human needs including socioeconomic development and agricultural production should be considered in solving the conflicts between the ecosystem and human water use in water-limited regions.

Indigenous rhizosphere microbial community characteristics of the phytostabilizer Athyrium wardii (Hook.) grown in a Pb/Zn mine tailing

Plant rhizosphere microbiome usually changes dramatically in adaptation to the mine environment to endure high heavy metal concentration, which in turn improves the process of revegetation and phytostabilization of mine tailing and deserves deep investigation. A field study was conducted to investigate the indigenous microbial community of a mining ecotype (ME) of the phytostabilizer Athyrium wardii (Hook.) grown in a Pb/Zn mine tailing and a corresponding non-mining ecotype (NME) grown in an uncontaminated adjacent site. Our study found a slight difference in microbial α-diversity between the ME and NME, and no significant difference between the rhizosphere and bulk soil. Both bacterial and fungal community compositions differed between the ME and NME, for which the differences were mainly driven by pH and metal contaminants. The ME harbored a unique microbial community in the rhizosphere soils different from the bulk soil and NME counterparts. The dominant phyla in the ME rhizosphere were Proteobacteria, Chloroflexi, Actinobacteria, Nitrospirae, and Ascomycota. Several genera from Proteobacteria, Acidobacteria, and Ascomycota were more abundant in the ME rhizosphere than in the NME rhizosphere. Network analysis revealed that keystone taxa were different in the two sites. Some keystone taxa from Gemmatimonadaceae, and Burkholderiaceae and Ascomycota played a critical role in microbial interactions within the mine tailing network. The unique microbial community with high tolerance in the rhizosphere soils of ME may show great benefit for plant growth and metal tolerance of the ME and thereby contributing to the process of revegetation and phytostabilization of mine tailings.

Revegetation of vineyard terrace embankments: A matter of seed mixture and seeding technique

Maintaining traditional agricultural management to preserve agrobiodiversity remains one of the major challenges for biodiversity conservation in Europe. In Germany, viticulture on steep slopes has shaped cultural landscapes of high conservational value but has declined strongly in recent decades due to insufficient profitability. One promising approach to keep management economically viable is modern vineyard terracing. Here, vineyard rows run parallel to the hillside, thus facilitating management and lowering production costs. At the same time, terrace embankments offer large non-cropped areas between the vines that could make a significant contribution to biodiversity. However, to fully exploit that potential sustainable revegetation of terrace embankments is mandatory but barely studied. For three consecutive years, we evaluated the effects of different seed mixtures (hay threshing, regional and commercial mixture), seeding techniques (manual vs. hydro-seeding), and the effect of nurse plants, hay mulch, and fertilizer on the establishment and maintenance of vegetation on three terraced vineyards in the Upper Middle Rhine Valley, Germany. The regional mixture best met the demands of biodiversity conservation by providing high herb and flower cover. Hay threshing and the regional mixture provided sufficient vegetation cover to reduce the risk of erosion, whereas the commercial mixture performed poorly in terms of vegetation cover and plant diversity. For hydro-seeding, target species richness was higher and bare ground cover was significantly reduced compared to manual seeding. Nurse plants, hay mulch, and fertilizer application only marginally improved plant establishment. Together these results emphasize that hydro-seeding with regional seeds is the best approach for revegetation of vineyard terrace embankments by combining high biodiversity and sufficient vegetation cover to reduce erosion risks at the best cost-benefit ratio for winegrowers. Vineyard terracing contributes to maintaining economically viable viticulture on steep slopes and may concurrently act as a conservation tool for biodiversity in vineyard landscapes.

Pseudomonas simiae augments the tolerance to alkaline bauxite residue in Atriplex canescens by modulating photosynthesis, antioxidant defense enzymes, and compatible osmolytes

In situ revegetation is effective in improving water-stable aggregation, preserving structural stability, and decreasing groundwater pollution to reduce the environmental risks posed by alkaline bauxite residue (ABR). Pseudomonas simiae, a plant growth-promoting rhizobacteria (PGPR), was used to promote Atriplex canescens growth challenged by ABR. The mechanism of P. simiae-induced plant growth promotion and tolerance against ABR stresses has been investigated. P. simiae was shown to alleviate ABR-induced stress in A. canescens by regulating photosynthesis and transpiration, inducing antioxidant defense, causing osmolyte accumulation, and altering plant morphology. Shoot dry weight, root dry weight, and root length of A. canescens were increased by 5.9%, 6.7%, and 11.5%, respectively, after inoculation with P. simiae for 60 days. Thus, it seems that P. simiae systemically regulated physiological processes in A. canescens favoring its growth under ABR treatments.

Cattle manure compost and biochar supplementation improve growth of Onobrychis viciifolia in coal-mined spoils under water stress conditions

Surface mining is a critical anthropogenic activity that significantly alters the ecosystem, while the use of appropriate revegetation techniques can be considered an important and feasible strategy in the way to improve the ecosystem services of degraded land. In the present study, we carried out a pot experiment to investigate the effects of three different variables on morpho-physiological and biochemical parameters of Onobrychis viciifolia to assess the capability of this species to be used for restoration purposes. Specifically, the variables studied were: (a) water (W) regime, working at five values as regards field capacity (FC) (i.e., 80% FC = highest, 72% FC = high, 60% FC = moderate, 48% FC = low, and 40% FC = very-low dose); and (b) rates of cattle manure compost (CMC) and wood biochar (BC) (weight/weight ratio), working at five rates (i.e., 4.0% = highest, 3.2% = high, 2.0% = moderate, 0.8% = low, and 0% = either no-CMC or no-BC dose). In addition, soil physical-chemical properties and enzyme activities were also investigated at the end of the experimental period. It was found that morphological growth attributes such as plant height, maximum root length, and dry biomass significantly increased with W, CMC and BC applications. Compared to control, moderate-to-high W, CMC and BC doses (W₈₀CMC₂BC₂) increased net photosynthesis rate (by 42%), stomatal conductance (by 50%), transpiration rate (by 29%), water use efficiency (by 10%), chlorophyll contents (by 73%), carotenoid content (by 81%), leaf relative water content (by 33%) and leaf membrane stability index (by 30%). Under low-W content, the application of CMC and BC enhanced osmotic adjustments by increasing the content of soluble sugar and the activities of superoxide dismutase, catalase, peroxidase and ascorbate peroxidase, decreasing the oxidative stress, as verified by low levels of hydrogen peroxide, superoxide anion, malondialdehyde and proline contents in leaf tissues. Moreover, application of W, CMC and BC significantly improved soil water holding capacity, available nitrogen, phosphorus and potassium, urease and catalase activities, which facilitate plant growth. These results would aid in designing an appropriate strategy for achieving a successful revegetation of O. viciifolia, providing optimum doses of W (64% field capacity), CMC (2.4%) and BC (1.7%), with the final aim of reaching ecological restoration in arid degraded lands.

Insights into the nurse effect of a native plant Ficus tikoua on Pb‒Zn tailing wastelands in western Hunan, China

Lead‒zinc (Pb‒Zn) processing and extraction activity generates large volumes of highly toxic and bare tailing (BT) wastelands which poses a potentially extreme risk to the surrounding environment. Revegetation in the Pb‒Zn tailing wastelands is usually considered a beneficial approach. Ficus tikoua is a native vine which can successfully colonize on Maoping Pb‒Zn mine tailing wastelands in western Hunan, China. This study involved examination of the nurse effect of F. tikoua on Pb‒Zn tailing wastelands, to provide insights into the potential mechanism of F. tikoua influencing soil quality and vegetation succession. The vegetation characteristics, nutrient properties, and heavy metal contents of three different types of vegetation patches associated with F. tikoua in Pb‒Zn tailing wastelands, representing different stage of succession, were investigated. The height, coverage, and aboveground and underground biomass of these vegetation patches showed an increasing trend from vegetation patch I (VP-I) to patch III (VP-III). The nutrient pool and chemical properties of these tailing wastelands gradually re-established from BT wasteland to VP-III. From VP-I to VP-III, the total heavy metal contents (i.e., Pb, Zn, Cu, and Cd) and DTPA-extractable Pb, Cu, and Cd contents significantly decreased, while the DTPA-extractable Zn content remained unchanged. Our findings suggested that F. tikoua exerts a distinct nurse plant effect by increasing the essential nutrient content of soil, reducing the available heavy metal content, and subsequently increasing the number of plant species and the biomass. Therefore, F. tikoua may be used as a promising nurse plant for triggering revegetation and phytostabilization of Pb‒Zn tailing wastelands at the initial stage of remediation.

Quantifying impacts of climate dynamics and land-use changes on water yield service in the agro-pastoral ecotone of northern China

Large-scale revegetation practices have lasted approximately two decades in the agro-pastoral ecotone of northern China (AENC), and their impacts on hydrological and ecological effects remain poorly understood. Previous studies largely focused on assessing water yield service (WYs) based on several fixed time points, whereas time series information-continuous WYs dynamics were more reliable and valuable in decision-making about water sustainability goals. This study analyzed the interannual WYs trend and relative roles of its drivers in the last 20 years based on a newly proposed approach, and revealed the past, present and future impacts of revegetation on WYs. The final results indicated that the annual WYs averaged approximately 97 mm and exhibited an increasing trend of 1.96 mm year^-1 (p = 0.086) during 2000-2019, in which climate and land-use changes were responsible for 88% and 12% of WYs variations, respectively. From 2000 to 2019, WYs was pronouncedly 1.47 mm year^-1 (p = 0.119) lower in the afforestation area than in the nonafforestation area, but the precipitation in the two regions had a statistically insignificant difference (p = 0.97). Future revegetation scenarios showed great potential for the shrinkage of WYs provision, even approaching a maximum of 50 mm at a local scale. Even so, the afforestation-induced reductions in blue water and benefits in green water both should receive equal attention. Specifically, any attempts to assess WYs or other climate-driven ecosystem services using discontinuous years as the study period must be taken with extreme caution.

Amending mine tailing cover with compost and biochar: effects on vegetation establishment and metal bioaccumulation in the Finnish subarctic

In the northern boreal zone, revegetation and landscaping of closed mine tailings are challenging due to the high concentrations of potentially toxic elements; the use of nutrient-poor, glacigenic cover material (till); cool temperatures; and short growing period. Recycled waste materials such as biochar (BC) and composted sewage sludge (CSS) have been suggested to improve soil forming process and revegetation success as well as decrease metal bioavailability in closed mine tailing areas. We conducted two field experiments in old iron mine tailings at Rautuvaara, northern Finland, where the native mine soil or transported cover till soil had not supported plant growth since the mining ended in 1989. The impacts of CSS and spruce (Picea abies)-derived BC application to till soil on the survival and growth of selected plant species (Pinus sylvestris, Salix myrsinifolia, and grass mixture containing Festuca rubra, Lolium perenne, and Trifolium repens) were investigated during two growing seasons. In addition, the potential of BC to reduce bioaccumulation of metals in plants was studied. We found that (1) organic amendment like CSS markedly enhanced the plant growth and is therefore needed for vegetation establishment in tailing sites that contained only transported till cover, and (2) BC application to till soil-CSS mixture further facilitated the success of grass mixtures resulting in 71-250% higher plant biomass. On the other hand, (3) no effects on P. sylvestris or S. myrsinifolia were recorded during the first growing seasons, and (4) accumulation of metals in cover plants was negligible and BC application to till further decreased the accumulation of Al, Cr, and Fe in the plant tissues.

Additions of optimum water, spent mushroom compost and wood biochar to improve the growth performance of Althaea rosea in drought-prone coal-mined spoils

Ecosystem degradation as a result of coal mining is a common phenomenon in various regions of the world, especially in arid and semi-arid zones. The implementation of appropriate revegetation techniques can be considered crucial to restore these degraded areas. In this regard, the additions of spent mushroom compost (SMC) and wood biochar (WB) to infertile and degraded soils have been reported to enhance soil fertility and plant growth under water (W) deficit conditions. However, the combined application of W, SMC and WB to coal mine degraded soils, to promote Althaea rosea growth and facilitate subsequent restoration, has not been explored yet. Hence, in the current study a pot experiment was carried out by growing A. rosea on coal mine spoils to assess the influence of different doses of W, SMC and WB on its morpho-physiological and biochemical growth responses. The results indicated that several plant growth traits like plant height, root length and dry biomass significantly improved with moderate W-SMC-WB doses. In addition, the simultaneous application of W-SMC-WB caused a significant decrease in hydrogen peroxide (H₂O₂) (by 7-56%), superoxide anion (O₂^●‒) (by 14-51%), malondialdehyde (MDA) (by 23-46%) and proline (Pro) contents (by 23-66%), as well as an increase in relative water content (by 10-27%), membrane stability index (by 2-24%), net photosynthesis rate (by 40-99%), total chlorophylls (by 43-113%) and carotenoids (by 31-115%), as compared to the control treatment. The addition of SMC and WB under low-W regime enhanced leaf water use efficiency, and soluble sugar content, also boosting the activity of superoxide dismutase, catalase, peroxidase and ascorbate peroxidase in leaf tissues, thus reducing the oxidative stress, as proved by low levels of H₂O₂, O₂^●‒, MDA and Pro contents. Finest growth performance under optimum doses of W (60% field capacity), SMC (1.4%) and WB (0.8%) suggest that revegetation of A. rosea with the recommended W-SMC-WB doses would be a suitable and eco-friendly approach for ecological restoration in arid degraded areas.

Seawater neutralization and gypsum amelioration of bauxite refining residue to produce a plant growth medium

Alumina production waste (bauxite refining residue) is highly alkaline, saline, and sodic, and hence cannot support plant growth for revegetation. Gypsum (CaSO₄.2H₂O) amendment of bauxite residue can lower alkalinity and improve the residue Ca status, but given the large gypsum requirement, efficient gypsum use is imperative. We investigated gypsum amelioration of residue sand (RS), examining changes in RS chemistry, and growth of Rhodes grass (Chloris gayana). Furthermore, we examined whether gypsum amelioration of RS should occur before or after seawater neutralization. We found that Ca from gypsum (20 t ha^-1) was retained within the surface 0.2 m of RS, regardless of whether the gypsum was applied before or after seawater neutralization. This Ca was retained both as exchangeable Ca and as a precipitate (either calcite or hydrotalcite), with ca. 50% retained as exchangeable Ca in both approaches. Gypsum at 20 t ha^-1, or even lower, provided sufficient Ca for maximum growth of Rhodes grass, in the surface, but higher rates would be required to allow Ca movement down the Na-dominated profile to ameliorate a larger rooting depth - this being important in environments where there are prolonged periods of water stress. The information presented will guide the efficient use of gypsum to ameliorate bauxite refining wastes.

How does drought affect native grasses' photosynthesis on the revegetation of iron ore tailings?

The revegetation of areas degraded by iron ore mining is a difficult challenge mainly due to water availability and impoverished metal-rich substrates. We sought to understand the photosynthetic responses to drought of native tropical grasses Paspalum densum (Poir.) and Setaria parviflora (Poir.) grown in iron ore tailing. The grass P. densum presented better photosynthetic adjustments when grown in the iron ore tailing and S. paviflora in response to water stress. Both species accumulated iron above the phytotoxic threshold when grown in an iron ore tailing. The net photosynthesis, stomatal conductance, transpiration, and water use efficiency decreased followed by a reduction in leaf relative water content in response to water stress for both species. The photochemical efficiency of photosystem II only decreased at the point of maximum drought. At this point, the water-stressed grass grown in the iron ore tailing presented higher H₂O₂ concentrations, particularly S. parviflora. After rehydration, full recovery of photosynthetic variables was achieved with decreased malondialdehyde concentrations, increased catalase activity, and, consequently, decreased H₂O₂ concentrations in leaves for both species. The fast recovery of the native grasses P. densum and S. parviflora to drought in the iron ore tailing substrate is indicative of their resistance and potential use in the revegetation of impoverished mined areas with high iron content and seasonal water deficit.

Chemically degraded soil rehabilitation process using medicinal and aromatic plants: review

In recent decades, the increasing number of degraded lands worldwide makes their rehabilitation essential and crucial. Various techniques have emerged to fulfill these needs but most of them are expensive and difficult to be applied. Revegetation is a cost effective, environmental friendly, and aesthetically pleasing approach suitable for degraded areas. However, the use of edible crops, especially for areas with heavy metals (HM) contamination, is not ecologically suitable because the HM may enter the food chain. Alternatively, non-edible, fast-growing, deep-rooting, and metal-stabilizing plants with high biomass, which can produce high-value products hold a great potential and have been regarded as potential candidates of edible crops. This current review presents the benefits of using aromatic and medicinal plants (AMPs) and their associated microorganisms for revegetation of degraded sites as they are high-value economic crops. We discussed the effect of various stress on productivity of secondary metabolites in AMPs in addition to the potential health risk with human consumption of these plants and their products. A focus was also given to the effect of HM stress on the essential oil (EO) content of certain AMPs. Reported data showed that AMPs growing on HM-contaminated soils are safe products to use as they are not significantly contaminated themselves by HM.

Phytoremediation ability of naturally growing plant species on the electroplating wastewater-contaminated site

The presence of heavy metal in soil and water resources has serious impact on human health. The study was designed to examine the phytoremediation ability of plant species that are growing naturally on the Zn-contaminated site. For the study, six plant species and their rhizospheric soil as well as non-rhizospheric soil samples were collected from different parts of the industrial sites for chemical and biological characterization. Visual observations and highest importance value index (IVI) through biodiversity study revealed potential plants as effective ecological tools for the restoration of the contaminated site. Among the plants, almost all were the most efficient in accumulating Fe, Mn, Cu and Zn in its shoots and roots, while Cynodon dactylon, Chloris virgata and Desmostachya bipinnata were found to be stabilizing Cr, Pb and Cd (bioconcentration factor in root = 7.95, 6.28 and 1.98 as well as translocation factor = 0.48, 0.46 and 0.78), respectively. Thus, the results of this study showed that the naturally growing plant species have phytoremediation potential to remediate the electroplating wastewater-contaminated site. These plant species are successful phytoremediators with their efficient metal stabilizing and well-evolved tolerance to heavy metal toxicity.

Effects of land-use patterns on soil carbon and nitrogen variations along revegetated hillslopes in the Chinese Loess Plateau

In water-limited areas, revegetation of abandoned croplands may lead to extensive land-use changes and considerable variations on soil carbon (C) and nitrogen (N). However, the impact of land-use patterns (i.e., the spatial combinations of different land-use types) on soil C and N variations following revegetation remains unclear. In this study, we measured soil organic carbon (SOC), total carbon (TC), and total nitrogen (TN) stocks to a depth of 200 cm in grassland (GL), shrubland (SL), young forestland (YF), and mature forestland (MF) under four land-use patterns in a catchment located in the Chinese Loess Plateau. The highest SOC, TC and TN stocks occurred in MF and the lowest was found in GL. Compared to every single land-use type, soil C and N stocks significantly increased under different land-use patterns. The highest SOC stock (6.51 kg m^-2) was found in the GL-YF-SL pattern, and the highest TC stock (47.25 kg m^-2) and TN stock (0.70 kg m^-2) were both observed in the MF-YF pattern. SOC stocks showed significantly positive correlations with TC and TN stocks under different land-use patterns (p < 0.05), except for the GL-MF. The soil C-N interactions were stronger in the MF-SL and GL-YF-SL patterns compared to the GL-MF and MF-SL. Redundancy analysis indicated that the SOC, TC, and TN variations were well explained by aboveground biomass and land-use patterns, with accumulated variance of 41.6% and 54.2% in Axis 1 and Axis 2, respectively. The differences of soil C and N accumulation among land-use patterns were mostly related to different vegetation coverage and the intensity of soil erosion. This study indicates that creating proper spatial distribution of land-use types on hillslopes could benefit soil C and N sequestrations and ecosystem restoration in semi-arid environments.

Soil carbon status after vegetation restoration in South West Iceland

Reclamation of degraded land using revegetation is one way of sequestering carbon into the soil. In this study an assessment was done to estimate the status of soil carbon amounts after revegetation with trees and grass in South West Iceland (Hafnarmelar). Natural woodland and eroded plots were part of the assessed plots as controls making four treatments. Soil samples were analysed for bulk density, carbon content and soil texture. Total % carbon (C) was analysed using vario MAX CN analyser (measured % C) and Loss on Ignition (calculated % C). The results showed that natural woodland had higher (9.32%) C than the tree treatment (4.91%), and both had significantly higher C than the grass (1.12%) and the eroded (0.76%) treatments (p < 0.0001). The amount of C in the grass and the eroded treatments were not statistically different (p > 0.0566). Notably, the grass treatment had carbon below the minimum expected level of 1.5% in Icelandic Andosol under vegetation. The natural woodland and the tree treatment had fine soil texture than the grass and eroded plots. Results suggest that where land has been properly restored or kept in natural condition, soil properties improve significantly especially when trees are part of the restored vegetation. The natural woodland had possibly not lost the old carbon-rich soil, as was the case with the tree, grass and eroded plots hence more time for development of various soil properties. Moreover, more litter deposits in natural woodlands and partly in the tree treatments might have contributed to higher carbon than in the grass and eroded treatments. Furthermore clay content variations (natural woodlands and the tree treatments had finer soil texture) might also be responsible for C limitations in the grass and the eroded treatments. Therefore, more restoration efforts are encouraged. The results also showed that LOI is a good method for C estimation but not very accurate estimator of soil organic carbon unless equations are developed with respect to known carbon content of particular soil type.

Plant available Al and Na in rehabilitated bauxite residue: a field study assessment

Bauxite residue is a high volume by-product generated during the extraction of alumina from bauxite ore (Bayer process). The long-term containment of residue is associated with environmental risks due to potential dusting and surface run-off. While rehabilitation of residue is viewed as a suitable approach for minimizing this risk, there is need for completion criteria. In particular, elevated sodium and aluminium are cited as inhibitory to plant growth and guideline values for satisfactory exchangeable sodium percentage (ESP) vary. Further, there is little information on the efficiency of standard soil assessment techniques in predicting plant available amounts of Al and Na for rehabilitated residue. This work aimed to evaluate the efficiency of soil extractants (NH₄OAC and NH₄Cl for cation; KCl for Al) for determining cation and Al content in field rehabilitated bauxite residue after 1 and 8 years. Depending on the method used, the ESP of residue varied significantly, and all were above the standard guideline values. Successful establishment of Holcus lanatus L. (Yorkshire fog) over 8 years together with absence of nutrient deficiency or elevated uptake of Na and Al indicates that the proposed ESP target of < 9.5 is both stringent and difficult to achieve under field conditions. Findings indicate that at ESP of ca. 20% (determined using NH₄OAc) and 30-40% (using NH₄Cl) may be more realistic target values for establishing vegetation in rehabilitated bauxite residue.

Improvement of growth performance of Amorpha fruticosa under contrasting regime of water and fertilizer in coal-contaminated spoils using response surface methodology

BACKGROUND

Water availability and nutrient-status of soils play crucial roles in seedling establishment and plant survival in coal-spoiled areas worldwide. Restoration of spoils pertains to the application of proper doses of nutrients and water, and selection of particular plant species for efficient revegetation. This study aimed at examining the potential effects of different combinations of soil-water and fertilizers (nitrogen, N and phosphorus, P) on morpho-physiological and biochemical attributes of Amorpha fruticosa grown in coal-mined spoils. Three factors five-level central-composite-design with optimization technique response surface methodology (rsm) was used to optimize water irrigation and fertilizer application strategies.

RESULTS

Our results revealed a strong correlation between experimental data and predicted values developed from the rsm model. The best responses of A. fruticosa in terms of plant height, stem diameter, root length, and dry biomass were observed under a high-water regime. Low-water regime caused a notable reduction in growth-associated parameters, and fertilization with either N or P did not show positive effects on those parameters, indicating that soil-water was the most influential factor for growth performance. Leaf water potential, gas-exchange parameters, and chlorophyll content significantly increased under high levels of soil-water, N and P, suggesting a synergistic effect of these factors for the improvement of photosynthesis-related parameters. At low soil-water contents and N-P fertilizer application levels, enhanced accumulation of malondialdehyde and proline indicated that A. fruticosa suffered from oxidative and osmotic stresses. Amorpha fruticosa also responded to oxidative stress by accelerating the activities of superoxide dismutase, catalase, and peroxidase. The effects of both fertilizers relied on soil-water, and fertilization was most effective under well-watered conditions. The maximum growth of A. fruticosa was observed under the combination of soil-water, N-dose and P-dose at 76% field capacity, 52.0 mg kg^- 1 and 49.0 mg kg^- 1, respectively.

CONCLUSION

Our results demonstrate that rsm effectively designed appropriate doses of water and N-P fertilizer to restore coal-spoiled soils. Furthermore, A. fruticosa responded to low-water and fertilizer-shortage by upregulating defensive mechanism to avoid damage induced by such deficiencies. Finally, our findings provide effective strategies for revegetation of coal-contaminated spoils with A. fruticosa using appropriate doses of water and N-P fertilizers.

Revegetation and reproduction: do restoration plantings in agricultural landscapes support breeding populations of woodland birds?

Restoration plantings are frequently occupied by native wildlife, but little is known about how planting attributes influence breeding by, and persistence of, fauna populations. We monitored breeding success of woodland birds in restoration plantings in a fragmented agricultural landscape in south-eastern Australia. We documented nest fate and daily nest survival (DSR) in plantings and remnant woodland sites. We analysed the influence on breeding success of patch attributes (size, shape, type) compared to other potentially influential predictors such as nest-site and microhabitat variables. We found that, in general, patch attributes did not play a significant role in determining breeding success for woodland birds. However, we examined a subset of species of conservation concern, and found higher DSR for these species in restoration plantings than in similarly sized woodland remnants. We also found negative effects of patch size and linearity on DSR in species of conservation concern. The primary cause of nest failure was predation (91%). We used camera trap imagery to identify the most common nest predators in our study sites: native predatory bird species, and the introduced red fox (Vulpes vulpes). Our findings are further evidence of the value of restoration plantings and small habitat patches for bird populations in fragmented agricultural landscapes. We recommend controlling for foxes to maximise the likelihood that restoration plantings and other woodland patches in Australia support breeding populations of woodland birds. More broadly, our study highlights the importance of taking a detailed, population-oriented approach to understanding factors that influence habitat suitability for fauna of conservation concern.

Revegetation of a barren rare earth mine using native plant species in reciprocal plantation: effect of phytoremediation on soil microbiological communities

Over-exploration of rare earth elements causes soil desertification and environmental degradation. However, the restoration of rare earth mine tailings requires the recovery of both vegetation and soil microbiota. Accordingly, the present study aimed to compare the efficacy of restoring mine tailings using organic compost and native plants (Miscanthus sinensis, Pinus massoniana, Bambusa textilis, or a mixture of all three). After three years, the mixed plantation harbored tenfold greater plant richness than that in the barren land. Among these, M. sinensis played a dominant role across all restored areas. The microbial communities of the soils were assessed using high-throughput 16S rDNA gene sequencing. A total of 34,870 16S rDNA gene sequences were obtained and classified into 15 bacterial phyla and 36 genera. The dominant genus across all the restored soils was Burkholderia, and the bacterial diversity of restored soils was greater than that of soils from either unrestored or natural (unexploited) areas, with the M. sinensis plantation yielding the greatest diversity. The effects of phytoremediation were mainly driven by changes in nutrient and metal contents. These results indicate that M. sinensis significantly improves phytoremediation and that mixed planting is ideal for restoring the soils of abandoned rare earth mines.

Influence of CaO-activated silicon-based slag amendment on the growth and heavy metal uptake of vetiver grass (Vetiveria zizanioides) grown in multi-metal-contaminated soils

Few plant species used for revegetation grow well in multi-metal-contaminated soils. Vetiver grass (Vetiveria zizanioides) is known to be tolerant of heavy metals. Vetiver has been reported to be effective for revegetation and heavy metal phytoextraction by applying targeted amendments due to its large biomass. In this study, a greenhouse vetiver pot experiment and soil incubation were performed to investigate the growth and Cd, Cr, Cu, Pb, and Zn uptake of vetiver grown in multi-metal-contaminated soils treated with a CaO-activated Si-based slag amendment (0, 0.5, 1.0, and 2.0% w/w). The results showed that the effects of slag amendment on plant growth and heavy metal uptake and distribution were dependent on the amendment dosages and metal species. Although vetiver could grow in contaminated soils, its growth was obviously inhibited. The slag amendment enhanced the vetiver growth and the highest biomass (2.62-fold over the control) was determined at a 1.0% amendment rate. The slag amendment improved plant growth by alleviating the toxicity of heavy metals in plants. This result was mainly attributed to the increases in soil pH and citric acid-extractable Si caused by alkaline amendment. The results suggest that vetiver can be applied to remediate multi-metal-contaminated soils in conjunction with the application of CaO-activated Si-based slag amendment.

The influence of planting size and configuration on landscape fire risk

Revegetating cleared land with native trees and shrubs is increasingly used as a means of addressing loss of biodiversity, degraded soil and water resources and sequestration of carbon. However, revegetation also brings a potential to alter fire risk due to changing fuel types across the landscape. Previous research has found that increasing the area of revegetation does not increase the risk of fire at a landscape scale, but it remains unclear whether the design of revegetation can be optimised to minimise risk. We evaluated if size and arrangement of revegetation affects fire size and intensity within an agricultural setting using a simulation modelling approach. Three revegetation planting designs were assessed, including small (3.2 ha) dispersed plantings, small (3.2 ha) plantings clustered into one third of the landscape, and large (29.2 ha) dispersed plantings, all resulting in the same overall percentage of revegetation (approximately 10% of the landscape). We simulated fires using Phoenix Rapidfire under varying planting design, weather, surrounding pasture conditions, and fire suppression. Planting design had little effect on fire sizes across the landscape, with larger plantings resulting in slightly larger fire sizes. Fires were smaller in landscapes with all planting designs compared with current landscape patterns. There was no significant influence of planting design on fire intensity. Weather and suppression had the strongest influence on both fire size and intensity, with larger and more intense fires under extreme weather conditions, with higher adjacent pasture loads and with no simulated suppression. Management of fuel loads in the pasture surrounding revegetation, weather and suppression are far greater risk factors for fire in these landscapes than planting design.

Differences of Acer platanoides L. and Tilia cordata Mill. Response patterns/survival strategies during cultivation in extremely polluted mining sludge - A pot trial

The study aimed to evaluate the physiological mechanisms underlying differences in metals and metalloid uptake and tolerance of two tree species cultivated in mining waste material. Two-year old Acer platanoides L. and Tilia cordata Mill. were cultivated in mining sludge characterized by high pH, salinity and an extremely high concentration of As. Both species were able to develop leaves from leafless seedlings, however, their total biomass was greatly reduced in comparison to control plants, following the severe disturbances in chlorophyll content. Phytoextraction abilities were observed for T. cordata for Ba, Nb, Rb and Se, and phytostabilisation was stated for Pd, Ru, Sc and Sm for both species, Ba and Nd for A. platonoides and Be for T. cordata only. Metal exclusion was observed for the majority of detected elements indicating an intense limitation of metal transport to photosynthetic tissue. A diversified uptake of elements was accompanied by a species-specific pattern of physiological reaction during the cultivation in sludge. Organic ligands (glutatnione and low-molecular-weight organic acids) were suppressed in A. platanoides, and enhanced biosynthesis of phenolic compounds was observed for both species, being more pronounced in T. cordata. Despite its higher accumulation of key metabolites for plant reaction to oxidative stress, such as phenolic acids, flavonoids and organic ligands, T. cordata exhibited relatively lower tolerance to sludge, probably due to the increased uptake and translocation rate of toxic metal/loids to aerial organs and/or restricted accumulation of salicylic acid which is known to play a decisive role in mechanisms of plant tolerance.

A field assessment of bauxite residue rehabilitation strategies

Bauxite residue, the by-product of the alumina industry, is mainly stored in land-based bauxite residue disposal areas (BRDAs). Environmental concern has been raised due to the large volumes in stockpile, the high alkalinity of the material, as well as the presence of elevated concentrations of trace elements. If not adequately managed, BRDAs can act as a source of pollution. In order to minimize the environmental risk, revegetation is implemented to stabilize the residue against water and wind erosion. Currently, two main approaches are used: the use of amendments or the installation of a capping layer. However, few studies evaluating the long-term success and self-sustainability of the rehabilitation programs have been published. A series of field-established rehabilitation strategies reflecting both direct revegetation and revegetation on capping layer were assessed in terms of both soil and plant quality. Soil physico-chemical properties, including pseudo-total and plant-available fractions of nutrients and trace elements, were determined over a summer and winter seasons and aerial portions of vegetation were analysed for nutrients and trace elements. Failure to adequately lower alkalinity remains the major constraint to long-term rehabilitation success of bauxite residue. This is evidenced from poor soil properties in unamended residue and in residue capped with a shallow soil layer, as well from vegetation displaying excessive concentrations of certain elements. Certain elements exceeded typical ranges for non-contaminated soils (i.e. Cr, Fe, Na, Ni and V), with some showing excessive plant-available fractions (i.e. of Al, As, Cr, Hg and V). Vegetation analysis found excessive uptake of some elements (i.e. of Al, Na, Fe, Cr and V). Future attempts for bauxite residue rehabilitation should include both gypsum and organic amendments, while a capping layer may only be effective if either a deep layer (>1 m) is installed or if the underlying residue is sufficiently treated prior to capping.

Reclamation of coal mine spoil and its effect on Technosol quality and carbon sequestration: a case study from India

A field study was carried out to assess the impact of revegetation on Technosol quality in the post-mining sites (Central Coalfield Limited, India). The study evaluated community structure, biodiversity, Technosol quality, and carbon (C) dynamics in the post-mining ecosystem (PME). The multivariate statistical tool was used to identify the key soil properties, and soil quality was evaluated by using Technosol quality index (TQI). One unreclaimed site (0 years) and four chronosequences revegetated coal mine sites (3, 7, 10, and 15 years) were studied and compared with an undisturbed forest as a reference site. Plant biodiversity indices [Shannon index of diversity (2.42) and Pielou's evenness (0.97) and Patric richness (12)] were highest in 15-year-old revegetated sites. Soil physicochemical and biological properties were recovered with the revegetation age. Soil organic C (SOC) stock significantly increased from 0.75 Mg C ha^-1 in 3 years to 7.60 Mg C ha^-1 after 15 years of revegetation in top 15 cm of soils. Ecosystem C pool increased at a rate of 5.38 Mg C ha^-1 year^-1. Soil CO₂ flux was significantly increased from 0.27 μmol CO₂ m^-2 s^-1 in unreclaimed sites to 3.19 μmol CO₂ m^-2 s^-1 in 15-year-old revegetated site. Principal component analysis (PCA) showed that dehydrogenase activity (DHA), available nitrogen (N), and silt content were the key soil parameters that were affected by reclamation. A 15-year-old Technosol had a greater TQI (0.78) compared to the control forest soils (0.64) that indicated the suitability of revegetation to recuperate soil quality in mining-degraded land and to increase C sequestration potential.

Properties of seawater neutralized bauxite residues and changes in chemical, physical and microbial properties induced by additions of gypsum and organic matter

Seawater neutralization is a technique that can be used to treat bauxite residue prior to its storage but, as yet, no attempts have been made to revegetate it. Seawater neutralized bauxite residue was found to have a pH_1:5 of 9.3 and to be highly saline (EC_1.5 16.5 dS m^-1). After leaching pH_1:5 rose to 9.7 and the residue was still highly sodic (ESP = 64-69%). Addition of 1% gypsum, prior to leaching, arrested this increase in pH while with 5% gypsum addition the pH_1:5 was lowered to 8.9. Addition of 5% gypsum also reduced ESP to 38% and increased watercress germination in the residue from 58% in control treatments to 88%. The major ions in leachates were Na⁺ and Cl^- and gypsum application increased the quantities of Na⁺, Ca²⁺ and SO₄^2- leached. Addition of 6% biosolids or 6% poultry manure added exchangeable cations to the mud and lowered ESP by 5-11%.The EC was 2.8-3.7 (mean 3.1) times higher and pH 0.2-0.7 (mean 0.43) units lower in saturation paste compared with 1:5 soil:water extracts. Addition of amendments had only small effects on physical properties. While organic C content was increased more by biosolids than poultry manure addition the reverse was the case for soluble organic C, microbial biomass C and basal respiration. It was concluded that although seawater neutralization initially lowers the pH of bauxite residues it is unlikely to increase the ease with which they can be revegetated.

Increased leaching and addition of amendments improve the properties of seawater-neutralized bauxite residue as a growth medium

Laboratory and greenhouse experiments were carried out to investigate the chemical, physical, and microbial properties of seawater-neutralized bauxite residues and the effects of additional leaching (1 pore volume of deionized water versus an additional 6 pore volumes) and amendment with gypsum (5%) and/or cattle manure (6%) on its properties and on the growth of Rhodes grass (Chloris gayana). Additional leaching resulted in a decrease in EC, exchangeable Na, SAR, and ESP. For unamended control treatments, additional leaching induced a rise in pH_SE from 8.5 to 9.6 and pH_1:5 from 9.1 to 10.1 due to dissolution of residual alkalinity. Addition of gypsum arrested this pH increase resulting in a final pH_SE of 7.5 and pH_1:5 of 8.8. In control treatments, additional leaching resulted in a pronounced decrease in Rhodes grass yields. However, in gypsum and cattle manure-amended treatments, it led to substantial yield increases and decreases in tissue Al and Na concentrations and increased K/Na ratios. Upon drying for the first time, bauxite residue was shown to contract and form a solid massive structure. The aggregates formed from crushing this material were water stable (as measured by wet sieving). Additions of cattle manure or gypsum to residue aggregates did not affect pore size distribution. Addition of cattle manure increased organic C and microbial biomass C content and basal respiration rate while additional leaching increased basal respiration and metabolic quotient. It was concluded that a combination of drying and crushing the residue, amending it with gypsum and organic manure followed by extensive leaching results in the formation of a medium that supports plant growth.

Natural revegetation of a semiarid habitat alters taxonomic and functional diversity of soil microbial communities

Revegetation of degraded lands has a profound impact on the maintenance and stability of ecosystem processes. However, the impacts of this land use change on functional diversity of soil microbial communities are poorly understood. Here, using 16S rRNA gene amplicon and shotgun metagenomic sequencing, we compared the taxonomic and functional communities of soil microbiome, and analyzed the effects of plant diversity and soil chemical properties, in a chronosequence of restored ex-farmland that had been naturally revegetated to grassland over periods of 5, 15 and 30years with adjacent farmland, on the Loess Plateau, China. We found that microbial taxonomic diversity was positively correlated with plant diversity and was higher in the revegetated sites. Functional diversity increased significantly in the oldest grassland. Actinobacteria, commonly considered a copiotrophic phylum, was more abundant in the revegetated sites, while Acidobacteria, an oligotrophic phylum, was more abundant in farmland. Furthermore, the structure of taxonomic and functional communities was significantly different between revegetated sites and farmland, and organic matter was the best environmental predictor in determining these microbial communities. Compared with the farmland, revegetation increased the proportion of genes associated with energy metabolism, carbohydrate metabolism and xenobiotics biodegradation and metabolism. Notably, the higher proportion of carbohydrate degradation gene subfamilies in the revegetated sites indicated higher levels of soil nutrient cycling. These results elucidate the significant shifts in belowground microbial taxonomic and functional diversity following vegetation restoration and have implications for ecological restoration programs in arid and semi-arid ecosystems.

Frankia communities at revegetating sites in Mt. Ontake, Japan

In 1984 at Mt. Ontake in Japan, an earthquake caused a devastating landslide, and as a result, the vegetation on the south slope of the mountain was completely eliminated. In higher elevation (2000 m) areas, revegetation has not yet been completed even 30 years after the landslide. Revegetation progress throughout the area was heterogeneous. In the partially revegetated areas, actinorhizal plant species such as Alnus maximowiczii and Alnus matsumurae have been found. In the present study, we investigated the Frankia communities in the higher-elevation area using sequence analysis of the amplified nifH (dinitrogenase reductase) gene from nodule and soil samples collected in the disturbed region, undisturbed forest, and in the boundary between the disturbed region and the undisturbed forest. Phylogenetic analysis of partial nifH sequences revealed the presence of six clusters, each of which consisted of highly similar (> 99%) sequences. Four clusters showed significant sequence similarity to Frankia (three Alnus- and a Casuarina-infecting strains). Diversity in the Frankia community was relatively low-only one or two clusters were detected in a site. At most of the sampling sites, a dominant cluster in a nodule coincided with that in rhizosphere soil, indicating that community structure in the rhizosphere is a primary factor that determines occupancy in a nodule. No significant difference in community structure was observed between plant species. Diversity in the Frankia community varied depending on revegetation progress. Cluster A, which was the most dominant in the disturbed region, was likely to have invaded from undisturbed forest.

Revegetation of the riparian zone of the Three Gorges Dam Reservoir leads to increased soil bacterial diversity

As one of the most active components in soil, bacteria can affect soil physicochemical properties, its biological characteristics, and even its quality and health. We characterized dynamics of the soil bacterial diversity in planted (with Taxodium distichum) and unplanted soil in the riparian zone of the Three Gorges Dam Reservoir (TGDR), in southwestern China, in order to accurately quantify the changes in long-term soil bacterial community structure after revegetation. Measurements were taken annually in situ in the TGDR over the course of 5 years, from 2012 to 2016. Soil chemical properties and bacterial diversity were analyzed in both the planted and unplanted soil. After revegetation, the soil chemical properties in planted soil were significantly different than in unplanted soil. The effects of treatment, time, and the interaction of both time and treatment had significant impacts on most diversity indices. Specifically, the bacterial community diversity indices in planted soil were significantly higher and more stable than that in unplanted soil. The correlation analyses indicated that the diversity indices correlated with the pH value, organic matter, and soil available nutrients. After revegetation in the riparian zone of the TGDR, the soil quality and health is closely related to the observed bacterial diversity, and a higher bacterial diversity avails the maintenance of soil functionality. Thus, more reforestation should be carried out in the riparian zone of the TGDR, so as to effectively mitigate the negative ecological impacts of the dam. Vegetating the reservoir banks with Taxodium distichum proved successful, but planting mixed stands of native tree species could promote even higher riparian soil biodiversity and improved levels of ecosystem functioning within the TGDR.

Reclamation of a lignite combustion waste disposal site with alders (Alnus sp.): assessment of tree growth and nutrient status within 10 years of the experiment

Combustion wastes are characterised by extremely low N contents. Therefore, introduction of nitrogen-fixing species at the first stage of their biological reclamation is required. This paper presents an assessment of the growth parameters of alders (Alnus sp.) 10 years after their introduction to a disposal site of lignite combustion waste in Central Poland. Black (Alnus glutinosa) and grey alders (Alnus incana) were planted directly in the combustion waste. The soil amendment included three variants: control with pure combustion waste, admixture of lignite culm and addition of acid sand. Both alder species displayed good growth parameters comparable to those of alders in natural habitats. However, black alder had better growth parameters, such as stand density index (SDI), diameter at breast height (DBH) and height (H) than grey alder. The lignite amendment exerted a positive effect on tree growth, reflected in a higher SDI and H, whereas the acid sand amendment did not affect any of the growth parameters of the studied alder species. Despite the good growth parameters, the measured N:P and N:K ratios in the alder leaves largely differed from the optimal values indicating insufficient P and K supply at the combustion waste disposal site. This may pose a threat to further development of the introduced tree plantings. The introduction of alders along with the lignite addition into the planting holes seems to be a successful method of combustion waste revegetation.

Plantation performance of chestnut hybrids and progenitors on reclaimed Appalachian surface mines

Reclamation of surface mined sites to forests is a preferred post-mining land use option, but performance of planted trees on such sites is variable. American chestnut (Castanea dentata (Marsh.) Borkh.) is a threatened forest tree in the eastern USA that may become an important species option for mine reclamation. Chestnut restoration using backcross hybrids that incorporate blight resistance may be targeted to the Appalachian coal mining region, which corresponds closely with the species' native range. Thus, it is important to understand how chestnut hybrids perform relative to progenitors on reclamation sites to develop restoration prescriptions. Seeds of parents and three backcross generations of chestnut (100% American, 100% Chinese, and BC₁F₃, BC₂F₃, and BC₃F₂ hybrids) were planted into mine soils in West Virginia, USA with shelter treatments. Survival for all stock types was 44% after 8 years (American 39%, Chinese 77%, BC₁F₃ 40%, BC₂F₃ 28%, and BC₃F₂ 35%). Height for all stock types was 33 cm after 8 years (American 28 cm, Chinese 67 cm, BC₁F₃ 30 cm, BC₂F₃ 21 cm, and BC₃F₂ 20 cm). At another site a year later, seedlings of the chestnut stock types were planted into brown (pH 4.6) or gray sandstone (pH 6.3) mine soils and seedling survival across all stock types was 58% after 7 years. Chinese had the highest survival at 82%, while the others ranged from 38 to 66%. Height was 63 cm for all stock types after 7 years. More advanced backcross hybrids (BC₂F₃ and BC₃F₂) had the lowest vigor ratings at both sites after 7-8 years. Our results indicate that surface mines in Appalachia may provide a land base for planting blight-resistant chestnuts, although Chinese chestnut outperformed American chestnut and later generation backcross hybrids. As blight-resistant chestnuts establish and spread after planting, chestnut trees may become a component of the forest canopy again and possibly occupy its former niche, but their spread may alter future forest stand dynamics.

Dynamics of natural revegetation of hydrocarbon-contaminated soil and remediation potential of indigenous plant species in the steppe zone of the southern Volga Uplands

The result of monitoring of natural vegetation growing on oil-contaminated (2.0-75.6 g/kg) and uncontaminated (0.04-2.0 g/kg) soils of a petroleum refinery for a period of 13 years is presented. Floristic studies showed that the families Poaceae, Asteraceae, Fabaceae, and eventually Brassicaceae were predominant in the vegetation cover of both types of soils. Over time, the projective vegetation cover of the contaminated sites increased from 46 to 90%; the species diversity increased twofold: in the ecological-cenotic structure of the flora, the number of ruderal plant species decreased; and the number of steppe, i.e., zonal, plant species increased. Using 62 dominant plant species, we conducted a field study of plant characteristics such as resistance to oil pollution, the ability to enrich the rhizosphere soil with microorganisms and bioavailable mineral nitrogen, and reduction of the concentration of petroleum hydrocarbons. The results enable us to characterize the phytoremediation potential (PRP) of the native plants and identify species that, probably, played a key role in the natural restoration of oil-contaminated soils. Statistical analysis showed correlations between the PRP constituents, and the leading role of rhizosphere microorganisms in the rhizodegradation of petroleum hydrocarbons was proven. A conditional value of PRP was proposed which allowed the investigated plants to be ranked in 11 classes. The study of a large sample of plant species showed that some plants held promise for the use in reclamation of soils in arid steppe zone, and that other species can be used for the rehabilitation of saline soils and semideserts.

Effect of biochar on the presence of nutrients and ryegrass growth in the soil from an abandoned indigenous coking site: The potential role of biochar in the revegetation of contaminated site

Little is known regarding how biochars' feedstock and pyrolysis temperature affect soil function and plant growth. To address this gap in knowledge, 12 biochars (walnut shells, corn cobs, corn straws, and rice straws were separately pyrolyzed at 250, 400, and 600°C for 4h) were applied to soil from an indigenous coking site with application rate of 2.5% (w/w) in a pot experiment to determine the impact of biochar types on macro-nutrients (total and available N, P, and K) and ryegrass growth in the soil from an indigenous coking site. Generally, the total N, P, and K in the soil was not significantly different from that of the control group. However, biochars decreased the available N from 21.76mg·kg^-1 for the control to 14.96mg·kg^-1. Corn straw and rice straw biochars increased the available P from 2.14mg·kg^-1 for the control to 28.35mg·kg^-1, specifically at higher pyrolysis temperature, while walnut shell and corn cob biochars had little influence on it regardless of pyrolysis temperature. Biochars increased the available K from 173.58mg·kg^-1 for the control to 355.64mg·kg^-1, varying as their feedstocks of corn cob>rice straw>corn straw>walnut shell and increasing with the increase of pyrolysis temperature. Correlation analysis suggests that it is responsible for the competition of soluble cations from biochars with K for adsorption sites on the soil surface. Biochars increased the ryegrass biomass from 0.07g·pot^-1 for the control to 0.16g·pot^-1, with the generally most effective stimulation by biochars produced at 400°C. Ryegrass biomass had obviously positive correlation with available K, indicating its essential role in the growth of ryegrass in the studied soil.

The footprint of marginal agriculture in the Mediterranean mountain landscape: An analysis of the Central Spanish Pyrenees

Agriculture forms an essential part of the mountains of the Mediterranean. For centuries, large areas were cultivated to feed the local population, with highly marginal slopes being tilled at times of heavy demographic pressure, using the shifting agriculture system. A great deal of agricultural land was abandoned during the 20th century, giving rise to secondary succession processes that tend to eliminate the agricultural footprint. However, revegetation is a highly complex process leading to areas with dense, well-structured plant cover, and other open areas of scrubland. This article studies the role of traditional agriculture in the deterioration of the landscape. By using experimental plots in the Central Pyrenees to reproduce traditional agriculture and abandonment, maps of field types, and current uses and ground cover, it could be confirmed that shifting agriculture has caused very heavy soil loss, which explains the deterioration of the landscape on several slopes. Burning scrub and adding the ash to the soil as a fertilizer did not greatly help to improve soil quality, but caused high rates of erosion and a very slow process of regrowth. The average data obtained from the shifting experimental plots recorded losses of 1356kgha^-1years^-1, 1.6 times more than the plot of fertilized cereal, and 8.2 times more than the dense scrub plot. Following abandonment, losses in the shifting agriculture plot were almost three times higher than the abandoned sloping field plot. Traditional shifting agriculture in the Pyrenees is the main cause of the deterioration of the landscape 50-70years after agriculture ceased.

Response of soil physico-chemical properties to restoration approaches and submergence in the water level fluctuation zone of the Danjiangkou Reservoir, China

With the completion of the Danjiangkou Dam, the impoundment and drainage of dams can significantly alter shorelines, hydrological regime, and sediment and can result in the loss of soil and original riparian vegetation. Revegetation may affect soil properties and have broad important implications both for ecological services and soil recovery. In this work, we investigated the soil properties under different restoration approaches, and before and after submergence in the water level fluctuation zone (WLFZ) of the Danjiangkou Reservoir. Soil physical (bulk density and soil moisture), chemical (pH, soil organic carbon, nitrogen, phosphorus and potassium contents), and heavy metals were determined. This study reported that restoration approaches have impacts on soil moisture, pH, N, soil organic carbon, P, K and heavy metals in the WLFZ of the Danjiangkou Reservoir. Our results indicated that different restoration approaches could increase the soil moisture while decrease soil pH. Higher soil organic carbon in propagule banks transplantation (PBT) and shrubs restoration (SR) indicate that PBT and SR may provide soil organic matter more quickly than trees restoration (TR). SR and TR could significantly improve the soil total P and available P. PBT and SR could improve the soil total K and available K. SR and TR could significantly promote Cu and Zn adsorption, and Pb and Fe release by plant. Submergence could significantly affect the soil pH, NO₃^－-N, NH₄^＋-N, total P and available P. Submergence could promote NO₃^－-N and available P adsorption, and NH₄^＋-N and total P release by soil. The soil quality index (SQI) values implied that TR and PBT greatly improved soil quality. The present study suggests that PBT and TR could be effective for soil restoration in WLFZ of the Danjiangkou Reservoir.

Evaluation of pyritic mine tailings as a plant growth substrate

At the Kidston gold mine, Australia, the direct establishment of vegetation on tailings was considered as an alternative to the use of a waste rock cover. The tailings acid/base account was used to predict plant growth limitation by acidity, and thus methods capable of identifying tailings that would acidify to pH 4.5 or lower were sought. Total S was found to be poorly correlated with acid-generating sulfide, and total C was poorly correlated with acid-neutralizing carbonate, precluding the use of readily determined total S and C as predictors of net acid generation. Therefore, the selected approach used assessment of sulfide content as a predictor of acid generation, and carbonate content as a measure of the acid-neutralizing capacity available at pH 5 and above. Using this approach, the majority of tailings (67%) were found to be non-acid generating. However, areas of potentially acid-generating tailings were randomly distributed across the dam, and could only be located by intensive sampling. The limitations imposed by the large sample numbers, and costly analysis of sulfide and carbonate, make it impractical to identify and ameliorate acid-generating areas prior to vegetation establishment. However, as only a small proportion of the tailings will acidify, a strategy of re-treating acid areas following oxidation is suggested. The findings of the present study will assist in the selection of appropriate methods for the prediction of net acid generation, particularly where more conservative measurements are required to allow vegetation to be established directly in tailings.

Remediation of metalliferous mines, revegetation challenges and emerging prospects in semi-arid and arid conditions

Understanding plant behaviour in polluted soils is critical for the sustainable remediation of metal-polluted sites including abandoned mines. Post-operational and abandoned metal mines particularly in semi-arid and arid zones are one of the major sources of pollution by soil erosion or plant hyperaccumulation bringing ecological impacts. We have selected from the literature 157 species belonging to 50 families to present a global overview of 'plants under action' against heavy metal pollution. Generally, all species of plants that are drought, salt and metal tolerant are candidates of interest to deal with harsh environmental conditions, particularly at semi-arid and arid mine sites. Pioneer metallophytes namely Atriplex nummularia, Atriplex semibaccata, Salsola kali, Phragmites australis and Medicago sativa, representing the taxonomic orders Caryophyllales, Poales and Fabales are evaluated in terms of phytoremediation in this review. Phytoremediation processes, microbial and algal bioremediation, the use and implication of tissue culture and biotechnology are critically examined. Overall, an integration of available remediation plant-based technologies, referred to here as 'integrated remediation technology,' is proposed to be one of the possible ways ahead to effectively address problems of toxic heavy metal pollution. Graphical abstract Integrated remediation technology (IRT) in metal-contaminated semi-arid and arid conditions. The hexagonal red line represents an IRT concept based on remediation decisions by combination of plants and microbial processes.