Tea biochar-immobilized Ralstonia Bcul-1 increases nitrate nitrogen content and reduces the bioavailability of cadmium and chromium in a fertilized vegetable soil
Pyrolytic biochar (PL-BC, pyrochar) and hydrothermal biochar (HT-BC, hydrochar) derived from branches and leaves of tea plants had different pH, electrical conductivity (EC), total carbon nitrogen content, BET surface area, total pore volume, average pore diameter, and functional groups. HT-BC had a larger specific surface area and more functional groups than PL-BC. Ralstonia Bcul-1 ( R -B) was the dominant and functional bacteria in a fertilized vegetable soil supplemented with TBB - immobilized R -B (TBB + R -B). R -B vitality was more closely related to BET surface area, total pore volume, and functional groups of tea-based biochar (TBB: PL-BC and HT-BC). R -B was able to maintain high oxidase activity. R -B and TBB + R -B can increase the activities of urease and peroxidase in vegetable soil playing an essential role in the biotransformation of ammonium nitrogen (NH 4 + -N) and nitrate nitrogen (NO 3 − -N). TBB was able to simultaneously increase the content of NO 3 − -N and NH 4 + -N, and TBB + R -B also significantly increased NO 3 − -N content but decreased NH 4 + -N content in a fertilized vegetable soil. These results indicated that R -B promoted nitrification in the soil, i.e. conversion of NH 4 + -N into NO 3 − -N, by enhancing the activities of urease and peroxidase. R -B had high adsorption capacity for cadmium (Cd) and chromium (Cr) (Cd&Cr: Cd and Cr). Moreover, TBB + R -B was able to convert weak acid extractable and reducible Cd&Cr into a more stable residual fraction and oxidizable Cd&Cr. The overall effect of the treatments was to reduce plant uptake of Cd&Cr by cabbage. TBB + R -B significantly promoted R -B growth, changed inorganic nitrogen speciation, increased NO 3 − -N supply, reduced Cd&Cr bioavailability, and decreased plant tissue Cd&Cr content.
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