Effects of pepper straw biochar on karst limestone soil organic carbon mineralization

【Objective】To explore the direct effect of pepper straw biochar addition on soil organic carbon(SOC) mineralization and organic carbon pool in limestone soil in karst areas on a short-term time scale, and provide a scientific basis for evaluation of the ecological environmental effect of pepper straw biochar returned to cropland in limestone soil in karst areas in southwestern China.【Method】Conducted the sealed incubation experiment in a jar at constant temperature and humidity, no biochar treatment was as control(CK), set 5 levels of pepper straw biochar addition, 0.1%, 0.5%, 1.0%, 2.0% and 4.0%, measured the CO₂ release during mineralization of SOC within the incubation period of 63 d with the method of NaOH solution absorption, and tested the change of SOC of various forms at the end of incubation.【Result】After 63 d of incubation, the cumulative amount of mineralization of SOC in limestone soil with 0-4.0% additives was 473.05±78.60-673.74±102.66 mg C/kg;4.0% addition could increase the cumulative amount of mineralization. At different levels of addition, SOC mineralization could be fitted by the two-compartment first order kinetic model. Under 0.1%-0.5% and 1.0%-4.0% treatments, easily biodegradable SOC mineralization rate constant(k _a) was 0.021±0.001-0.034±0.004/d and 0.248±0.021-0.343±0.033/d, the effect on mineralization of easily biodegradable SOC was suppressing and facilitating, respec-tively. All addition levels could promote mineralization of hard-to-degrade SOC. The addition level of 1.0%-4.0% could significantly increase reserves of labile SOC(C_a) and microbial biomass carbon(MBC) content in the soil(P<0.05, the same below), the scope ranges were 238.19±20.72-937.48±71.75 mg/kg and 368.22±12.19-449.52±18.91 g/kg respectively. The addition levels of 2.0% and 4.0% significantly increased the content of readily oxidized carbon(ROC) in soil, the value being 2849.97±184.21 and 3163.92±107.16 mg/kg respectively. Biochar addition had no significant effect on water soluble organic carbon(WSOC) in soil(P>0.05, the same below). According to the process of pepper straw biochar addition, MBC was of extremely significantly positive correlation with C_a, k_a, constant of the mineralization rate of recalcitrant SOC(k_s) and ROC(P<0.01, the same below), of extremely significantly negative correlation with reserves of recalcitrant SOC(C_s), and no significant correlation to WSOC.【Conclusion】The effect of pepper straw biochar on the mineralization rate of limestone soil in karst areas is related to the level of addition;1.0%-4.0% addition can enhance the SOC mineralization rate while increase C_a and the content of MBC and ROC, but has no effect on WSOC content. The addition level of 4.0% can increase the cumulative emissions of mineralization during the incubation period of 63 d. To reduce soil carbon emissions, it is recommend the level of pepper straw biochar addition should be lower than 4.0% in improving limestone soil in karst areas in southwestern China.