Long-term action of biochar in paddy soils: effect on organic carbon and functioning of microbial communities

Increasing soil carbon (C) sequestration in paddy fields is significant for ensuring food security and achieving C neutrality in China. Biochar has been widely used as a soil amendment; still, long-term effects on the mechanisms of biochar's effect on soil C accumulation and the mediating role of microorganisms are poorly understood. To address this issue, three field experiments on paddies were chosen (Changsha, Nanjing, and Jiaxing), where biochar was applied for 7 to 8 years. The treatments included control (no addition), N (N fertilizer, 120 kg ha-1), N+B1 (N and low amount of biochar, 15-24 t ha-1), and NB2 (N and high amount of biochar, 22.5-48 t ha-1), effects on soil organic C (SOC) mineralization, dissolved organic C (DOC), activities of enzymes, microbial biomass C (MBC) and community composition (based on phospholipid fatty acids (PLFAs), and C utilization efficiency (CUE) were studied. Biochar reduced cumulative CO2 emissions in Changsha (by 32-34 %), Jiaxing (3.0-27 %) (p<0.05), and in Nanjing with under NB2 treatment (by 36 %) compared to N treatment. Biochar increased soil pH (0.03-0.38 units) in Changsha and Nanjing but did not affect Jiaxing plots. Biochar increased SOC, total N, chitinase activity, MBC (by 18-28 %,) and CUE (by 24-65 %, except in Jiaxing) but decreased DOC content (by 3-14 %) and peroxidase activity. Biochar addition increased the total and bacterial PLFA contents and decreased the bacteria:fungi ratio at the three sites (except for total PLFA in Nanjing) compared to the N treatment. The correlation analysis revealed that cumulative CO2 emission was reduced under the increase of pH, MBC, SOC, and CUE, bacterial PLFA, and stimulated by DOC content and the rise of bacterial:fungi ratio. These indicated that long-term biochar amendments mainly increased the amount of C that bacteria can assimilate; the increase of MBC content and CUE could point to the stimulation of microbial C sequestration.