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Cadmium contamination in paddy soils has aroused increasing concern around the world, and biochar has many positive properties, such as large specific surface areas, micro porous structure for the heavy metal immobilization in soils. However there are few studies on sulfur-iron modified biochar as well as its microbiology effects. The purpose of this study was to evaluate the Cd immobilization effects of sulfur or sulfur-iron modified biochar and its related microbial community changes in Cd-contaminated soils. SEM-EDX analysis confirmed that sulfur and iron were loaded on the raw biochar successfully. Sulfur-modified biochar (S-BC) and sulfur-iron modified biochar (SF-BC) addition increased pH value and the content of soil organic matter, and also decreased DTPA-extractable Cd. There was a negative significant correlation between organic matter content and the available Cd (P 0.05). During a 45-d incubation period, the fractions of Cd are mainly with the exchangeable (25.16-35.79%) and carbonate (22.01-25.10%) fractions. Compared with the control, the concentrations of exchangeable Cd in soil were significantly (P 0.05) decreased by 12.54%, 29.71%, 18.53% under the treatments of BC, S-BC, SF-BC respectively. The S-BC and SF-BC treatments significantly (P 0.05) increased Chao1, observed, Shannon and Simpson diversity indices compared with the control and biochar treatments. Meanwhile, the relative abundance of Proteobacteria, Bacteroidetes, and Actinobacteria increased, whereas the abundance of Acidobacteria and Germmatimonadetes decreased. Capsule: Sulfur-modified and sulfur-iron modified biochar applications decreased the available Cd and changed the microbial community.

Bauxite residue often has poor physical conditions which impede plant growth. Native plant encroachment on a bauxite residue disposal area in Central China reveals that natural regeneration may improve its physicochemical properties. Residue samples collected from three different disposal ages were assessed to evaluate residue micromorphology and three-dimensional (3D) aggregate microstructure under natural regeneration. The residue aggregates in different disposal ages were divided in two sections: macro-aggregate (2–1 mm) and micro-aggregate (0.25–0.05 mm). Residue aggregate micromorphology was determined by scanning electron microscope and energy dispersive X-ray spectroscopy, and the residue aggregate microstructure was determined by synchrotron-based X-ray micro-computed tomography (SR-μCT) and image analysis techniques. Natural regeneration may improve residue aggregate stability and form a stable aggregate structure. Calcium content increased whilst sodium content decreased significantly on the surface of residue aggregates. Under natural soil-forming processes bauxite residue porosity, specific surface area, average length of paths, and average tortuosity of paths all significantly increased. This demonstrated that natural regeneration may stimulate the formation of stable aggregate structure in residues. Further understanding should focus on particle interaction forces and agglomeration mechanisms with the addition of external ameliorations.

Bauxite residue has poor physical conditions, which hinders plant growth and causes potential environmental risks. Polymer materials have broad potential applications for holding water and improving soil aggregation. However, no attempt has been made to assess the effects of polymers on physical structure of bauxite residue. The purpose of this study was to evaluate the effects of polyacrylamide (BP), humic acid (BH), starch-acrylamide polymer (BSA) and humic acid-acrylamide polymer (BHA) on aggregate formation, stability, and pore characteristics in bauxite residue by 60-day pot experiment. Results demonstrated that 0.2% polymer addition increased the proportion of0.25 mm mechanical-stable aggregates. Under wet sieving, BP and BHA treatments increased the values of mean weight diameter (WMWD) from 0.36 mm to 0.67 mm and 0.68 mm, respectively, which may result in the increase of the percentage of organic functional groups including OCO and CC. Laser diffraction analysis and the visualized 3D surface map revealed that0.25 mm residue aggregate was more difficult to disintegrate following BHA treatment during 180-min hydraulic circulation. BP and BH treatments elevated0.5 μm pore size volumes, whilst BHA treatment increased5 μm pore size volumes and improved the porosity of bauxite residue. Polymer applications indicated that compared with polyacrylamide or humic acid, humic acid-acrylamide polymer could be regarded as an effective ameliorant due to its positive effects on both aggregate stability and pore characteristics. These findings were helpful for understanding the application potential of natural-synthetic polymers on physical conditions of bauxite residue prior to ecological reconstruction on the disposal areas.