The use of biochar and lime (CaO) is a common approach to mitigating soil acidification. However, little is known about how biochar and lime amendments impact N₂O emissions and potential microbial mechanisms. We conducted a 45-day microcosm incubation experiment to examine N₂O emission and associated functional guilds to biochar and lime amendment in an acidic tea plantation soil. Results show that lime and biochar treatments significantly reduced cumulative N₂O emissions by 49.69% and 63.01%, respectively, while significantly increasing cumulative CO₂ emissions by 27.51% and 19.35%, respectively. Additionally, lime and biochar treatments significantly decreased the abundances of bacterial nirK, nirS, nosZ and fungal nirK genes, while increasing that of the ammonia-oxidizing bacteria (AOB) and the complete ammonia-oxidizing bacteria (comammox) amoA genes. The stimulated or inhibitory effects of biochar on functional genes abundances were higher than lime. The N₂O emission rate was positively linked with the abundance of the fungal nirK gene but was negatively correlated with AOB and comammox amoA genes abundances. The random forest and linear regression analysis revealed that fungal denitrifiers were the most important predictors of N₂O emissions. Lime and biochar amendments reduced the alpha diversity and altered the community composition of nirK-harboring fungal denitrifiers. Ascomycota was the dominant fungal denitrifiers belonging to the families Nectriaceae, Aspergillaceae, and Chaetomiaceae, and the relative abundances of genera Chaetomium, Penicillium and Fusarium were positively correlated with N₂O emissions. Overall, our findings suggest that biochar is more effective than lime in reducing N₂O emissions, and this is likely due to the powerful effects it has on community traits of nirK-harboring fungal denitrifiers.

Soil amendments have been extensively employed for the purpose of remediating soils contaminated with cadmium (Cd). However, the potential impacts of soil amendments on soil chemical properties, soil Cd bioavailability, total Cd accumulation by rice, and rhizosphere bacterial community in Cd-contaminated paddy fields located in a tropical region is still at its infancy. In this study, a commercial MgO-CaO-SiO₂ conditioner (A), biochar (B), and a combination of the commercial MgO-CaO-SiO₂ conditioner and biochar with a ratio of 1:1 (C) were applied at two different doses [2250 kg ha⁻¹ (A150, B150, C150), 4500 kg ha⁻¹ (A300, B300, C300)] to investigate their impacts on soil Cd stabilization and total Cd uptake of rice straw and grain in a Cd-contaminated remediation field experiment. Rhizosphere bacterial community diversity and composition were also assessed using high-throughput sequencing based on 16S rRNA genes. Compared with non-amendment treatment (CK), soil pH, cation exchange capacity (CEC), organic matter (OM), total nitrogen (TN), available nitrogen (AN), and nitrate (NO₃⁻) concentrations were significantly elevated, whereas ammonium (NH₄⁺) and soil available Cd concentrations were reduced by soil amendment treatments. Meanwhile, soil amendments significantly decreased concentrations of total Cd in both rice straw and grain, with the lowest Cd concentration in the C300 treatment. Soil pH and CEC were significantly and negatively associated with soil Cd availability and rice straw and grain Cd concentrations, while NH₄⁺ concentration was positively correlated with soil available Cd concentration, and OM, TN, and NO₃⁻ concentrations were positively linked with rice grain Cd concentration. Soil amendments significantly increased bacterial Chao 1 and Shannon indexes and altered bacterial community composition in rhizosphere soil, due to changes in the composition of the community primarily influenced by variations in soil pH, CEC, and soil available Cd, NH₄⁺, available phosphorous (AP) and available Potassium (AK) concentrations. Furthermore, the abundant bacterial species (Pseudomonas) and rare bacterial species (Bacillus, Candidatus_Solibacter and Streptomyces) have been up-regulated by different soil amendments, which might be in favour of soil Cd immobilization. A structural equation model also showed that soil amendments could improve bacterial diversity and change soil pH and CEC, which were conducive to hindering the removal and conversion of Cd. Overall, these results indicate that biochar-(MgO-CaO-SiO₂) mixed amendments at high dosage exerted better performance compared with single application soil amendment A and B. The changes in soil chemical properties, available Cd content, and rhizosphere bacterial community assembly induced by soil amendments are closely correlated with the decrease in rice’s ability to accumulate Cd.

Background: Although most reported biogenic Mn oxides are hexagonal birnessites, other types of biogenic Mn oxides also commonly occur in the environment. However, sorption characteristics and underlying mechanisms of the adsorption of heavy-metal ions on these biogenic Mn oxides are still rarely addressed. In this study, the sorption mechanisms of Cu(II) on a low valence biogenic Mn oxide, poorly crystallized bixbyite-like Mn₂O₃(α-Mn₂O₃), were investigated.Results: The maximum adsorption capacity of Cu(II) onto this biogenic Mn oxide at pH 6.00 was 796 mmol/kg (0.45 mol Cu mol⁻¹Mn). The complex structure of adsorbed Cu(II) was constrained using Cu extended X-ray absorption fine structure (EXAFS) analysis, combined with structural parameters of the biogenic Mn oxide with alternately arranged regular and distorted MnO₆octahedra obtained through multiple-FEFF fitting of Mn EXAFS data. The sorbed Cu(II) was found to coordinate with the biogenic Mn oxide particle edges as inner-sphere complexes. At a relatively low Cu²⁺loading (233 mmol/kg, pH 6.00), Cu(II) adsorbed onto the biogenic Mn oxide with two types of coordinated complexes, i.e., (1) coordinated with one regular/distorted MnO₆octahedron as a monodentate-mononuclear complex and (2) with two adjacent MnO₆octahedra as a bidentate-binuclear complex. While, at a relatively high Cu²⁺loading (787 mmol/kg, pH 6.00), only one type of coordinated complex was constrained, the adsorbed Cu(II) coordinated with one regular/distorted MnO₆octahedron as a monodentate-mononuclear complex.Conclusions: This research extends further insight into the bacterial Mn(II) oxidation in the environment and serves as a good reference for understanding the interactions between metal ions and biogenic low valence Mn oxides, which are still poorly explored either theoretically or practically.

Cadmium (Cd) pollution of paddy soils poses a serious threat to food safety and human health. Calcium- and magnesium-aluminum layered double hydroxides (CaAl-/MgAl-LDHs) have recently been deployed to remediate Cd-contaminated soils; nevertheless, the responses of soil properties and microbial communities to CaAl-/MgAl-LDHs are not well elucidated. Herein, a field trial was implemented to evaluate the impacts of CaAl-/MgAl-LDHs and conventional amendments (lime (LM) and biochar (BC)) on soil Cd availability and bacterial community structure, together with the consequent impacts on rice grain Cd enrichment. Compared with CK without amendments, CaAl-LDHs, MgAl-LDHs, LM, and BC amendments significantly raised rice yield-related agronomic traits, with the greatest yield increment observed in the CaAl-LDHs treatment. The four amendments enhanced soil pH, cation exchange capacity (CEC) and available phosphorus (AP), while reducing DTPA-extractable Cd concentration. Notably, CaAl-LDHs and MgAl-LDHs treatments outperformed LM and BC in suppressing DTPA-extractable Cd at rice-grain filling and maturity stages. The four amendments substantially decreased rice Cd concentration, with the lowest values observed in CaAl-LDHs and MgAl-LDHs treatments. Furthermore, the four amendments boosted bacterial α-diversity and reconstructed bacterial community structure by elevating the phyla Thermodesulfobacteriota, Bacteroidota, and Firmicutes. Based on redundancy and Pearson correlation analyses, soil pH, CEC, available Cd, ExCa, and AP were identified as critical factors governing bacterial community structure. Partial least squares path model suggested that soil amendments modified pH and CEC, which influenced bacterial communities and available soil Cd, thereby suppressing rice Cd accumulation. These findings suggest that bacterial communities and soil chemical properties play critical roles in Cd immobilization upon amendment addition, and support the application potential of CaAl-/MgAl-LDHs to achieve safe rice production in Cd-polluted soils.