Microbial resistance stability of rice straw biochar with vermiculite modification: A novel insight into persistent free radicals and pore structure

Biochar is considered a promising material for carbon sequestration owing to its high stability. However, the effects of biochar properties, especially pore structure and persistent free radical (PFR) content, on its microbial resistance stability (MRS) remain unclear. Incubation experiments were conducted to investigate the anaerobic and aerobic MRS of rice straw biochar produced at 300–700ºC, with and without vermiculite modification, and to evaluate the relationship between key biochar properties and MRS. The results showed that PFR signal intensity in biochar was much higher under anaerobic conditions than that under aerobic conditions, increasing with production temperatures of 300 to 600ºC, but sharply decreasing at 700ºC. Anaerobic MRS of biochar produced at 700°C was lower than that of biochar produced at 300–600ºC, as indicated by higher CH4 emission. Redundancy analysis revealed a negative correlation between PFR concentration and cumulative CH4 emission with a correlation coefficient of –0.467, and a positive correlation between cumulative CH4 emission and pore volume and specific surface area with correlation coefficients of 0.405 and 0.281, respectively. Biochar dissolved organic carbon content and pH were the two most important factors affecting aerobic MRS as reflected by their high correlation coefficients (0.933 and –0.848) with cumulative CO2 emission. Vermiculite modification enhanced MRS and accordingly lowered the global warming potential of biochar produced at low temperature (300–500°C) by 4.59–20.2% and 15.3–42.9% under anaerobic and aerobic conditions, respectively. However, it had a contrasting effect on biochar produced at 700°C. Biochar would be more effective at mitigating global warming in paddy fields than in dry lands.