Root architectures differentiate the composition of organic carbon in bauxite residue during natural vegetation

Migration and transformation behaviors of potentially toxic elements and the underlying mechanisms in bauxite residue: Insight from various revegetation strategies

Revegetation is a promising strategy for large-scale bauxite residue disposal and management, potentially influencing the geochemical stability of potentially toxic elements (PTEs) through rhizosphere processes. However, the geochemical behaviors of PTEs and the underlying mechanisms during bauxite residue revegetation remain unclear. This study examined the migration and transformation behaviors of PTEs and their underlying mechanisms in the bauxite residue-vegetation-leachate system under various revegetation strategies, including single and co-planting of perennial ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.), over a 100-day microcosm experiment. The results showed significant decreases in pH, EC, Na, Al, and Cr levels in the leachate under various revegetation strategies, with slight increases in Cu, V, As, and Pb. Over time, the pH, EC, Na, Cr, Cu, V, Pb, and As levels in the leachate decreased, while those of Al, Fe, Mn, and Zn increased. The mean pH, EC, and concentrations of Na, Al, Fe, and Cr in the leachate of the revegetated treatments decreased by 6%-8%, 21%-33%, 2%-4%, 19%-27%, 7%-22%, and 15%-26%, respectively, while the mean concentrations of Mn, V, Zn, and As increased by 47%-134%, 26%-46%, 39%-47%, and 3%-10%, respectively, compared to the unamended treatment. Co-planting generally exhibited a greater impact on leachate components compared to single planting. Available contents of Al, Cr, and Pb decreased by 81%-83%, 57%-77%, and 55%-72%, respectively, while those of other PTEs increased in the revegetated bauxite residue. Co-planting significantly reduced the availability of PTEs compared to single planting. Except for Na and Mn, the bioaccumulation and transportation factors of PTEs in both vegetation species remained below 1 under various revegetation strategies. The migration and transformation behaviors of PTEs in the bauxite residue-vegetation-leachate system were mainly influenced by pH and nutrient levels. These findings provide new insights into the migration and transformation behaviors of PTEs during bauxite residue revegetation.

Restoration-mediated protein substances preferentially drive underlying bauxite residue macroaggregate formation during the simulated ecological reconstruction process

Constructing a restoration strategy from bauxite residue to Technosols is a cost-effective and sustainable strategy for addressing the ecological and environmental issues caused by high alkalinity, salinity, and fine-grained bauxite residues. However, the quantitative contribution of restoration strategies on the upper bauxite residue-derived Technosols to the underlying untreated bauxite residue in the short term remains poorly understood. This study investigated the mediating mechanisms of vegetation and microbial metabolic effects on the alkalinity, nutrient content, and structure of the underlying bauxite residue (20-50 cm) through a simulated ecological reconstruction of the bauxite residue stockpile. Results indicated that implementing plant restoration strategies resulted in the content of polyphenolic compounds, lipids, tannins, and carbohydrates in bauxite residue dissolved organic matter (DOM) increased significantly from 52.5, 8.2, 3.3, and 2.0 % to 54.4, 10.4, 5.6, and 2.8 %, respectively, while the content of condensed aromatics, unsaturated hydrocarbons, and proteins/amino sugars decreased significantly from 15.5, 12.0, and 6.5 % to 12.1, 9.7, and 5.1 %, respectively. The newly produced molecules were concentrated in regions with low O/C and high H/C ratios, suggesting that short-term vegetation restoration strategies facilitate the transformation of substrate DOM towards easily decomposable and highly bioavailable substances. This led to the migration of the newly produced molecules to the underlying bauxite residue, and as a result, the protein and soluble microbial products of the underlying bauxite residue increased significantly, as well as the pH, exchangeable Na, and < 0.054 mm particles decreased from 10.2, 44.2 cmol kg-1, and 28.1 % to 9.7, 27.1 cmol kg-1, and 19.4 %, respectively, available nitrogen, urease, and 1-2 mm particles increased from 7.3 mg kg-1, 0.2 U mg-1, and 14.5 % to 7.6 mg kg-1, 0.3 U kg-1, and 21.7 %, respectively. Results of the structural equation model further confirmed that plant biomass, proteins/amino sugars, and condensed aromatics in the upper Technosol were the main factors controlling the aggregate formation of the underlying bauxite residue by mediating the protein-dominated biogenic organic matter produced by microbial metabolism.

Characteristics of soil organic carbon fractions and influencing factors in different understory mosses in karst urban parks

The impact of different vegetation types on soil organic carbon (SOC) is a key focus in global warming research. Bryophytes, commonly found in karst urban forests, significantly contribute to carbon accumulation in surface soil. However, the changes in soil organic carbon fractions under moss and the influencing factors remain unclear. To address this knowledge gap, the study examined the organic carbon content, soil physicochemical properties, and associated environmental factors in both moss-covered soil and bare soil under six different forest species within an urban park. The results showed that the SOC contents under moss cover in evergreen coniferous forest (127.28 g/kg), bamboo forest (144.70 g/kg), deciduous broad-leaved forest (87.63 g/kg), and evergreen shrub (109.28 g/kg) were significantly higher compared to bare soil. Moss cover also had a significant impact on soil readily oxidizable carbon (ROC), particulate organic carbon (POC), mineral-associated organic carbon (MOC), and heavy fraction organic carbon (HFOC) (P < 0.01). The soil under moss had a higher content of stable organic carbon fraction, which is conducive to the stability of the soil organic carbon pool. The interaction between moss cover and stand type had the most significant effect on soil organic carbon, especially in bamboo forests. Canopy density, moss biomass, and soil moisture were the main environmental factors affecting the content of soil organic carbon and its fractions, while soil organic carbon content was mainly affected by soil nitrogen and phosphorus. This study establishes a theoretical framework for investigating the carbon cycle in karst urban underforest ecosystems, offering a scientific basis for the management and preservation of urban green space ecosystems. Future studies should include bryophytes in the assessment of dynamic factors affecting the soil carbon pool under forest cover and further explore the function and ecological significance of bryophytes in understory ecosystems.

Seed endophytes and rhizosphere microbiome of Imperata cylindrica, a pioneer plant of abandoned mine lands

Some plant-associated microorganisms could improve host plants biotic and abiotic stress tolerance. Imperata cylindrica is a dominant pioneer plant in some abandoned mine lands with higher concentrations of heavy metal (HM). To discover the specific microbiome of I. cylindrica in this extreme environment and evaluate its role, the microbiome of I. cylindrica's seeds and rhizosphere soils from HM heavily contaminated (H) and lightly contaminated (L) sites were studied. It was found that HM-contamination significantly reduced the richness of endophytic bacteria in seeds, but increased the abundance of resistant species, such as Massilia sp. and Duganella sp. Spearman's rank correlation coefficient analysis showed that both Massilia sp. and Duganella sp. showed a significant positive correlation with Zn concentration, indicating that it may have a strong tolerance to Zn. A comparison of the microbiome of rhizosphere soils (RS) and adjacent bare soils (BS) of site H showed that I. cylindrica colonization significantly increased the diversity of fungi in rhizosphere soil and the abundance of Ascomycota associated with soil nutrient cycling. Spearman's rank correlation coefficient analysis showed that Ascomycota was positively correlated with the total nitrogen. Combined with the fact that the total nitrogen content of RS was significantly higher than that of BS, we suppose that Ascomycota may enhance the nitrogen fixation of I. cylindrica, thereby promoting its growth in such an extreme environment. In conclusion, the concentration of HM and nutrient contents in the soil significantly affected the microbial community of rhizosphere soils and seeds of I. cylindrica, in turn, the different microbiomes further affected soil HM concentration and nutrient contents. The survival of I. cylindrica in HM severely contaminated environment may mainly be through recruiting more microorganisms that can enhance its nutrition supply.

Enhancement of nitrogen on core taxa recruitment by Penicillium oxalicum stimulated microbially-driven soil formation in bauxite residue

Microbially-driven soil formation process is an emerging technology for the ecological rehabilitation of alkaline tailings. However, the dominant microorganisms and their specific roles in soil formation processes remain unknown. Herein, a 1-year field-scale experiment was applied to demonstrate the effect of nitrogen input on the structure and function of the microbiome in alkaline bauxite residue. Results showed that the contents of nutrient components were increased with Penicillium oxalicum (P. oxalicum) incorporation, as indicated by the increasing of carbon and nitrogen mineralization and enzyme metabolic efficiency. Specifically, the increasing enzyme metabolic efficiency was associated with nitrogen input, which shaped the microbial nutrient acquisition strategy. Subsequently, we evidenced that P. oxalicum played a significant role in shaping the assemblages of core bacterial taxa and influencing ecological functioning through intra- and cross-kingdom network analysis. Furthermore, a recruitment experiment indicated that nitrogen enhanced the enrichment of core microbiota (Nitrosomonas, Bacillus, Pseudomonas, and Saccharomyces) and may provide benefits to fungal community bio-diversity and microbial network stability. Collectively, these results demonstrated nitrogen-based coexistence patterns among P. oxalicum and microbiome and revealed P. oxalicum-mediated nutrient dynamics and ecophysiological adaptations in alkaline microhabitats. It will aid in promoting soil formation and ecological rehabilitation of bauxite residue. ENVIRONMENT IMPLICATION: Bauxite residue is a highly alkaline solid waste generated during the Bayer process for producing alumina. Attempting to transform bauxite residue into a stable soil-like substrate using low-cost microbial resources is a highly promising engineering. However, the dominant microorganisms and their specific roles in soil formation processes remain unknown. In this study, we evidenced the nitrogen-based coexistence patterns among Penicillium oxalicum and microbiome and revealed Penicillium oxalicum-mediated nutrient dynamics and ecophysiological adaptations in alkaline microhabitats. This study can improve the understanding of core microbes' assemblies that affect the microbiome physiological traits in soil formation processes.

Ecological restoration affects the dynamic response of alkaline minerals dissolution in bauxite residue

Regulating alkalinity is the key process to eliminating environmental risk and implementing sustainable management of bauxite residue. Nevertheless, continuous release of free alkali from the solid phase (mainly sodalite and cancrinite) is a major challenge for long-term stability of alkalinity in amended bauxite residue. In order to understand the dissolution behavior of sodalite and cancrinite, their dissolution kinetics under simulated pH conditions of 8, 9 and 10 were investigated. Additionally, PHREEQC software and shrinking core model (SCM) were employed to analyze the release pattern of saline ions. The results revealed that the ratio of Na/Si and Na/Al values exhibited greater stability in sodalite than in cancrinite. The dissolution of elemental Na, Si, and Al in sodalite and cancrinite was matched with non-chemometric characteristics. The kinetic calculations by the shrinking core model (SCM) suggested that both sodalite and cancrinite exhibited slow dissolution kinetics, and their dissolution processes belong to internal diffusion control and external diffusion control, respectively. pH controlled the dissolution kinetic rates of sodalite and cancrinite mainly by changing their coupled dissolution-precipitation processes. More importantly, these findings can predict the change of alkaline components accurately, thus facilitating the implementation of efficient alkalinity regulation strategies for the ecological restoration of bauxite residue disposal areas.

Assessing the potential of red mud and dehydrated mineral mud mixtures as soil matrix for revegetation

The disposal of red mud (RM) and dehydrated mineral mud (DM) presents a significant challenge for the global alumina industry. This study proposes a novel disposal method for RM and DM, which uses mixtures of RM and DM as a soil matrix for revegetation in the mining area. RM mixed with DM effectively alleviated its salinity and alkalinity. X-ray diffraction analysis indicated that reduction of salinity and alkalinity may be due to the release of chemical alkali from sodalite and cancrinite. Applications of ferric chloride (FeCl3), gypsum, and organic fertilizer (OF) improved the physicochemical properties of the RM-DM mixtures. FeCl3 significantly reduced available Cd, As, Cr, and Pb content in the RM-DM, while OF significantly increased the cation exchange capacity, microbial carbon and nitrogen, and aggregate stability (p < 0.05). Micro-computed tomography and nuclear magnetic resonance analysis showed that amendment with OF and FeCl3 increased the porosity, pore diameter, and hydraulic conductivity in the RM-DM mixture. The RM-DM mixtures had low leaching of toxic elements, indicating low environmental risk. Ryegrass grew well in the RM-DM mixture at a ratio of 1:3. OF and FeCl3 significantly increased the ryegrass biomass (p < 0.05). These results suggested that RM-DM amended with OF and FeCl3 has a potential application in the revegetation of areas after bauxite mining.