Abstract
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•Mutual responses between soil nitrification and Cd are firstly investigated.•N inputs did not cause soil acidification and Cd mobilization in Cd-contaminated soil.•Cd inhibited soil nitrification through eliminating AOB.•Biochar promoted AOB recovery and restored nitrification of Cd-contaminated soil.•Biochar could resist the re-acidification and Cd mobilization.
Inorganic nitrogen (N) inputs increase soil nitrification, acidification and trace metal toxicity e.g. cadmium (Cd). Biochar (B) has been widely used for metal immobilization. However, little is known about how the combination of N fertilizers with biochar (N-B) changes soil Cd availability through altering nitrification process. Here, (NH4)2SO4 or CO(NH2)2 was applied in combination with biochar to an acidic, artificially enriched Cd contaminated soil. Not as we expected, available Cd did not increase following (NH4)2SO4 or CO(NH2)2 addition. Nitrification and acidification of Cd contaminated soils were greatly inhibited, accompanied by elimination of ammonia-oxidizing bacteria (AOB). Exchangeable H+ of Cd contaminated soils was significantly lower than that of uncontaminated soils, thus inhibiting Cd itself from mobilization. N-B addition nearly halved soil available Cd and significantly increased nitrification by promoting AOB recovery. However, the restored nitrification did not cause soil acidification, due to the high buffering and slow liming effects of biochar. Available Cd continuously decreased with decreasing soil acidity and exchangeable Al. This study firstly demonstrated a feedback between soil nitrification and Cd after N application, and how biochar modified the feedback. Biochar, therefore, provides a feasible strategy for eliminating potential Cd toxicity on both soil biological and chemical processes.