Earthworms stimulate nitrogen transformation in an acidic soil under different Cd contamination

In acidic Cd-contaminated soils, soil nitrogen conversion is inhibited and usually block nitrogen supply for plants. Earthworms are well known for improving soil properties and regulating various soil biogeochemical processes including nitrogen cycling. To figure out the effect and mechanisms of earthworms on soil nitrogen transformation in Cd-contaminated soil, ten treatments with and without A. robustus in five soil Cd concentration gradients were established. The tolerant concentration of A. robustus to Cd in the acidic soil is about 6 mg kg^-1. The potential ammonia oxidation of the acidic soils was very low, ranging from 0.05 to 0.1 µg NO₂^--N g^-1 d^-1. Although AOA was more abundant in the acidic soil than AOB, AOA was inhibited by Cd pollution, while AOB showed some increase under Cd-stress. AOA may play a dominant role in ammonia oxidation in acidic soil, but the recovery of nitrification in Cd-contaminated acidic soil was probably due to the effect of AOB. Earthworms significantly increased soil pH, DOC, ammonium and PAO, thus promoted soil ammonification and potential nitrification, but had no significant effect on soil net nitrification. Correlation analysis results demonstrate that earthworms may promote soil PAO by increasing soil pH, NH₄⁺-N content, and AOB abundance. This study could provide a theoretical basis for solving the problem of nitrogen-cycling-functional degradation and nitrogen supply in the process of phytoremediation of heavy metals-contaminated soils.