Abstract
Since biomagnification takes place in food chains, cadmium (Cd) contamination in paddy fields has aroused substantial concern. The remediation of soil cadmium by amendment addition has been commonly implemented in agriculture. However, for sustainable development, this technology still has unclear microbial and metabolic effects. In the present study, a 3-month in situ field-scale experiment was performed to clarify how five compound amendments with different pH values affect the Cd concentration, soil microbes and metabolic activities related to Cd transformation. Compared with the control treatment CK1 (Cd-polluted soil), the five amendments noticeably decreased the content of soil-available Cd, which was lowest in alkaline treatments (approximately 0.2 mg/kg). In the acidic treatment (E), the Shannon diversity was reduced in both June and September, whereas the abundant genera related to ammonia oxidation, namely, Nitrospira and Nitrosospira, were enriched. Among the Cd transformation genes identified, genes participating in Cd2+ transport, reducedglutathione (GSH) metabolism, and nitrogen and sulfur metabolism showed considerable variations. Genes participating in nitrogen fixation, sulfate dissimilation and Cd2+/Zn2+-exporting ATPases and relevant microbes (e.g., sulfur-reducing and nitrogen-fixing bacteria) exhibited their maximal abundances in alkaline treatments A and D. Genes participating in GSH metabolism and sulfate assimilation, which affected the intracellular precipitation of CdS and Cd complexation by GSH, exhibited their maximal abundances in the E and D treatments. As suggested from the results achieved here, of two critical metabolisms (N and S), nitrogen fixation more significantly accounted for soil Cd reduction. To successfully combine remediation and microbial effects, soil parameters and components should be rigorously considered to minimize additional effects on the soil ecosystem.
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