Mechanism of Cd Passivation by Biochar in Contaminated Soil Is Influenced by Nitrogen Fertiliser Application

ABSTRACT Biochar passivation is an important safety utilisation method for Cd‐polluted soil, but the passivation effect is mostly affected by fertiliser application. A 49‐day experiment was conducted to examine the efficiency of applying biochar in passivating soil Cd with different strategies of nitrogen fertiliser application. Two Cd concentrations and three types of nitrogen fertilisers were used. Additionally, the impact on the ammonia‐oxidising microorganisms and microbial community was assessed. The findings showed that over time and in the presence of biochar, the amount of exchangeable Cd in the soil gradually decreased. The Cd‐contaminated soil treated with (NH 4 ) 2 SO 4 and CO(NH 2 ) 2 significantly reduced the pH of the soil due to the weak acidity of the fertiliser, promoted the recovery of ammonia‐oxidising bacteria (AOB) and significantly increased nitrification. Therefore, the concentration of NO 3 − –N increased significantly, which weakened the passivation effect of biochar. The exchangeable Cd concentration in the soil treated with Ca(NO 3 ) 2 was significantly lower than that in the soil treated with (NH 4 ) 2 SO 4 and CO(NH 2 ) 2 , and this change inhibited Cd mobilisation. Different types of nitrogen fertiliser application altered the enzymatic activity and microbial community of the soil. (NH 4 ) 2 SO 4 treatment increased urease activity. However, a high level of nitrogen fertiliser application weakened the passivation effect of biochar on Cd pollution, resulting in a larger percentage of exchangeable Cd when exposed to high Cd compared to low Cd. Therefore, applying a low level of Ca(NO 3 ) 2 represents a feasible strategy by which biochar can eliminate potential Cd toxicity on both soil biological and chemical processes.