Biogeochemical cycles of nutrients, rare earth elements (REEs) and Al in soil-plant system in ion-adsorption REE mine tailings remediated with amendment and ramie (Boehmeria nivea L.)

Amelioration of the physicochemical properties enhanced the resilience of bacteria in bauxite residues

Bacteria-driven strategies have gained attention because of their effectiveness, viability, and cost-efficiency in the soil formation process of bauxite residues. However, further investigation is needed to enhance the extreme environment of bauxite residues and facilitate long-term sustainable development of bacteria. Here, soil, phosphogypsum, and leaf litter were selected as amendments, and soil and leaf litter were also used as bacterial inoculants in a 12-month microcosm experiment with bauxite residues. The results showed significant improvements in physicochemical properties, including alkalinity, organic carbon content, nutrient availability, and physical structure, when bauxite residue was mixed with amendments, particularly when different amendments were combined. The diversity, structure, and function of the bacterial community were significantly enhanced with the amelioration of the physicochemical properties. In the treated samples, especially those treated with a combination of different amendments, the relative abundance (RA) of alkali-resistant bacterial taxa decreased, whereas the RA of some common taxa found in normal soil increased, and the structure of the bacterial community gradually changed towards that of normal soil. A strong correlation between physicochemical and biological properties was found. These findings suggest that rational application of soil, phosphogypsum, and leaf litter effectively improves the environmental conditions of bauxite residues and facilitate long-term sustainable bacterial communities.

Pioneer plants enhance soil multifunctionality by reshaping underground multitrophic community during natural succession of an abandoned rare earth mine tailing

Spontaneous natural succession in metal mine tailings is fundamental to the rehabilitation of bare tailing. Here, an abandoned rare earth element (REE) mine tailing with spontaneous colonisation by pioneer plants with different functional traits was selected. Soil nutrient cycling, fertility, organic matter decomposition as well as underground organismal communities and their multitrophic networks were investigated. Compared with the bare tailing, the colonisation with Lycopodium japonicum, Miscanthus sinensis, and Dicranopteris dichotoma increased soil multifunction by 222%, 293%, and 525%, respectively. This was accompanied by significant changes in soil bacterial and protistan community composition and increased soil multitrophic network complexity. Rhizospheres of different plant species showed distinct microbial community composition compared to that of bare tailing. Some WPS-2, Chloroflexi, and Chlorophyta were mainly present in the bare tailing, while some Proteobacteria and Cercozoa were predominantly seen in the rhizosphere. Pearson correlation and Random Forest revealed the biotic factors driving soil multifunction. Structural equation modelling further revealed that pioneer plants improved soil multifunction primarily by decreasing the microbial biodiversity and increasing the multitrophic network complexity. Overall, this highlights the importance of subterrestrial organisms in accelerating soil rehabilitation during natural succession and provides options for the ecological restoration of degraded REE mining areas.

Co-cropping vetiver grass and legume for the phytoremediation of an acid mine drainage (AMD) impacted soil

Acid mine drainage (AMD) is a form of environmental pollution from mining activity that can negatively affect soil environments by acidification, salinisation, and metal(loid) contamination. The use of plants to remediate (phytoremediation) these impacted environments while generating plant-based value is a promising approach to more accessible and cost-benefiting restoration of post-mining, marginal lands. In this study, a 3-month growth-chamber pot experiment was conducted to investigate the influence of co-cropping two plant species, Chrysopogon zizanioides (vetiver grass) and the legume Medicago truncatula (barrel clover) with a wheat straw biochar amendment on the phytostabilisation of metal(loid)s Cr, Zn, and As and the phytoextraction of rare earth element (REE) in an AMD impacted soil from a gold mining region in South Africa. The results showed that co-cropping with vetiver significantly lowered the legume's Cr, Zn, and As root contents by 80%, 32% and 54%, respectively, and improved the plant's overall metal(loid) tolerance by increasing its translocation from root to shoot tissue. The biochar further inhibited root uptake of Cr and Zn, by 71% and 36%, and increased the legume biomass by 40%. Both plant species and cropping treatments exhibited low REE extraction capabilities by shoot tissue, which accounted for less than 0.2% of total soil REE contents. The study shows that co-cropping with vetiver and biochar amendment are effective tools for the phytoremediation of AMD impacted soil mainly by lowering plant uptake and improving plant metal(loid) tolerance. Likely mechanisms at play include the alteration of rhizosphere chemistry and species-specific physiological and molecular responses. These effects offer support for the phytostabilisation of AMD impacted soil with the generation of plant-based value through dual (and safe) cultivation (phytoprotection) rather than through REE recovery from plant biomass (phytoextraction). These techniques could allow for the simultaneous restoration of post-mining, mining-impacted and marginal lands with agricultural production.

Accumulation, translocation, and fractionation of rare earth elements (REEs) in fern species of hyperaccumulators and non-hyperaccumulators growing in urban areas

The issue of ion-adsorption type rare earth deposits (IADs) in urban areas of South China has garnered significant attention due to its environmental implications. Hyperaccumulator-based phytoremediation is a potentially effective solution for reducing the environmental impact of IADs in urban areas, particularly using ferns as they are known to be REE hyperaccumulators. However, the ability of different fern species to accumulate REEs in urban areas remains unknown. In this study, four fern species, including known hyperaccumulators (Dicranopteris linearis and Blechnum orientale) and other ferns (Pteris ensiformis and Cibotium barometz), were studied to investigate their REE accumulation abilities in the Guangzhou urban area. The aboveground parts of Dicranopteris linearis (848.7 μg g-1) and Blechum orientale (1046.8 μg g-1) have been found to accumulate high concentrations of REEs, demonstrating they probably can be applied for phytoremediation in the natural environments. Despite having lower REE concentrations than REE hyperaccumulators, Pteris ensiformis and Cibotium barometz still probably have the function as phytostabilizers in urban areas, as REEs can be enriched in their roots beyond the normal levels of plants. The enrichment of REEs in ferns is influenced by the availability of various nutrients (K, Ca, Fe, and P), which probably can be associated with different growth processes. The four fern species show LREE enrichment, moderate Eu anomalies and different Ce anomalies. It is difficult to absorb and transfer Ce to the aboveground parts of Blechnum orientale and Cibotium barometz. The study also identified selective enrichment of Ce in Pteris ensiformis, which has potential for comprehensive extraction of REEs when combined with other REE hyperaccumulators. REE fractionations are probably determined by the specific characteristics of different fern parts. Overall, these findings provide insights for addressing potential environmental problems related to IADs and offer guidelines for phytoremediation technology in addressing high REE levels in urban areas.

Rhizospheric Precipitation of Manganese by Phosphate: A Novel Strategy to Enhance Mn Tolerance in the Hyperaccumulator Phytolacca americana

Manganese (Mn) exclusion in the Mn hyperaccumulator pokeweed (Phytolacca americana L.) was investigated. Hydroponic experiments were carried out to observe the responses of pokeweeds continually exposed to high levels of Mn. In this study, crystals were observed to appear firstly on the root hair, and soon after, more crystals appeared on the root surface, and crystals of Mn phosphate were observed to appear on the root surface in a time sequence negatively correlated with the number of leaves treated with 5 mM Mn. Crystals were identified via phase analysis of X-ray diffraction and element analysis, and these white insoluble crystals were identified using XRD to be Mn phosphate, with the molecular formula (Mn,Fe)3(PO4)2·4H2O. The nutrient solution pH increased from 4.5 to about 5.6 before the crystals appeared. Mn phosphate crystals appeared in all solutions except those without phosphate and emerged earlier in the solutions containing no Fe. Compared with control group, pokeweed accumulated much more Mn in the leaves when treated without phosphate or Fe. The present study suggests that pokeweed can exclude Mn by means of rhizosphere precipitation by phosphate to form Mn phosphate crystals that accumulate on the root surface. Although the detailed mechanism requires further investigation, this study provides the first direct evidence of a novel strategy to inhibit Mn uptake in the roots of a hyperaccumulator in a P-enriched environment.

Pollution and health risk assessment of rare earth elements in Citrus sinensis growing soil in mining area of southern China

Background

Analyzing the pollution and health risk of rare earth elements (REEs) in crop-growing soils around rare earth deposits can facilitate the improvement of REE mining-influenced area. In this study, pollution status, fraction and anomaly, plant accumulation characteristics, and potential risks of REEs (including heavy and light rare earth elements, HREEs and LREEs) in C. sinensis planting soil near ion-adsorption deposits in southern Ganzhou were analyzed. The influence of the soil environment on REEs in soil and fruit of C. sinensis was also explored.

Methods

The geo-accumulation index (Igeo) and ecological risk index(RI) were used to analyze the pollution potential and ecological risks of REEs in soils, respectively. Health risk index and translocation factor (TF) were applied to analyze the accumulation and health risks of REEs in fruit of C. sinensis. The influence of soil factors on REEs in soil and fruit of C. sinensis were determined via correlation and redundancy analysis.

Results

Comparison with background values and assessment of Igeo and RI indicated that the soil was polluted by REEs, albeit at varying degrees. Fractionation between LREEs and HREEs occurred, along with significant positive Ce anomaly and negative Eu anomaly. With TF values < 1, our results suggest that C. sinensis has a weak ability to accumulate REEs in its fruit. The concentrations of REEs in fruit differed between LREEs and HREEs, with content of HREE in fruit ordered as Jiading > Anxi > Wuyang and of LREE in fruit higher in Wuyang. Correlation and redundancy analysis indicated that K2O, Fe2O3 and TOC are important soil factors influencing REE accumulation by C. sinensis, with K2O positively related and Fe2O3 and TOC negatively related to the accumulation process.

Effects of composite materials and revegetation on soil nutrients, chemical and microbial properties in rare earth tailings

The mining of ionic rare earth elements in Ganzhou left large area of barren tailings with severe vegetation destruction in pressing needs of remediation. However, the remediating effects of soil additives combined with revegetation on the preservation of nutrients in the tailings and microbial communities were rarely studied. For this purpose, pilot experiments were implemented in a field, with the control group (CK) only cultivating plants without adding materials, and three treatments including peanut straw biochar composite (T1), phosphorus‑magnesium composite (T2) and modified zeolite composite (T3) along with the cultivation of Medicago sativa L., Paspalum vaginatum Sw. and Lolium perenne L. Soil pH and organic matter in CK significantly decreased from 4.90 to 4.17 and from 6.62 g/kg to 3.87 g/kg after six months, respectively (p ≤ 0.05), while all the treatments could effectively buffer soil acidification (over 5.74) and delay the loss of soil organic matter. Soil cation exchange capacity was still below the detection limit in all the groups except T2. The results of rainfall runoff monitoring indicated that compared with CK, only T2 could significantly reduce the runoff loss of soil NO₃^- and SO₄^2- by 45.61 %-75.78 % and 64.03 %-76.12 %, respectively (p ≤ 0.05). Compared with CK, the bacterial diversity in T2 and T3 significantly increased 21.18 % and 28.15 %, respectively (p ≤ 0.05), while T1 didn't change the bacterial or fungal diversity (p > 0.05). Co-occurrence network analysis showed that compared with CK, the whole microbial communities interacted more closely in the three treatments. Functional prediction of the microbial communities revealed all the treatments were dominated by carbon transforming bacteria and saprotrophic fungi except T2. This study demonstrated that the composite materials combined with revegetation couldn't retain soil nitrogen compounds and sulfate in rare earth tailings in the long term.

Abundance, spatial variation, and sources of rare earth elements in soils around ion-adsorbed rare earth mining areas

Rare earth elements (REEs) concentrated in soils have attracted increasing attention about their impact on soil health as emerging contaminants. However, the sources of REEs enriched in soils are diverse and need to be further investigated. Here, surface soil samples were collected from southern Jiangxi Province, China. REEs contents and soil physicochemical properties were determined, and cerium (Ce) and europium (Eu) anomalies were calculated. Moreover, we established a model to further identify the main sources of REEs accumulation in the studied soils. Results show that the abundance of soil REEs reveals larger spatial variation, suggesting spatially heterogeneous distribution of REEs. The median content of light REEs in soils (154.5 mg kg^-1) of the study area was higher than that of heavy REEs and yttrium (35.8 mg kg^-1). In addition, most of the soil samples present negative Ce anomalies and all the soil samples present negative Eu anomalies implying the combined effect of weathering and potential exogenous inputs on soil REEs. Positive matrix factorization modeling reveals that soil REEs content is primarily influenced by soil parent materials. Potential anthropogenic sources include mining-related leachate, traffic exhaust, and industrial dust. These results demonstrate that the identification of sources of soil REEs is an important starting point for targeted REEs sources management and regulation of excessive and potentially harmful REEs levels in the soil.

Biogeochemical dynamics of nutrients and rare earth elements (REEs) during natural succession from biocrusts to pioneer plants in REE mine tailings in southern China

The exploitation of ion-adsorption rare earth element (REE) deposits has resulted in large quantities of abandoned mine tailings, which pose significant risks to the surrounding environment. However, the natural evolutional patterns at early successional stages and related biogeochemical dynamics (e.g. nutrient and REE cycling) on such mine tailings remains poorly understood. To this end, a chronosequence of REE mine tailings abandoned for up to 15 years was investigated in a post-mining site in south China. Our results showed that biocrusts were the earliest colonizers on these tailings, reaching a peak of 10% of surface coverage after 10 years of abandonment. Later on, after 15 years, the biocrusts began to be replaced by pioneer plants (e.g. Miscanthus sinensis), suggesting a rather rapid succession. This ecological succession was accompanied by obvious changes in soil nutrients and microbial community structure. Compared to bulk soils, both the biocrusts and rhizospheric soils favored an accumulation of nutrients (e.g. P, S, N, C). Notably, the autotrophic bacteria (e.g. Chloroflexi and Cyanobacteria) with C and N fixation abilities were preferentially enriched in biocrusts, while heterotrophic plant growth promoting bacteria (e.g. Pseudoocardiaceae and Acidobacteriales) were mainly present in the rhizosphere. Moreover, the biocrusts showed a remarkably high concentration of REEs (up to 1820 mg kg^-1), while the rhizospheric soils tended to decrease REE concentrations (~400 mg kg^-1) in comparison with bulk soils, indicating that the REEs could be redistributed by biological processes. Principal component analysis and mantel tests showed that the concentrations of nutrients and REEs were the most important factors affecting the microbial communities in biocrusts, rhizospheric and bulk soils. In sum, based on the observation of nutrient accumulation and pollutant (i.e. REE) dynamics in the initial successional stages, this work provides a feasible theoretical basis for future restoration practices on REE mine tailings.