Rice straw biochar mitigated more N2O emissions from fertilized paddy soil with higher water content than that derived from ex situ biowaste

Biochar could mitigate greenhouse gas emissions, especially nitrous oxide (N2O). Effects of interactions between different biochar and water content on N2O emissions from rice (Oryza sativa L.) paddy soils have not been thoroughly understood. We evaluated effects of different biochar (derived from Camellia oleifera fruit shell, FS; spent mushroom substrate made of Camellia oleifera fruit shell, MS; rice straw, RS; at the rate of 40 g kg−1) and water contents (70% and 120% water holding capacity, WHC) on N2O emissions from rice paddy soil fertilized with nitrogen (N, 0.2 g kg−1), and examined microbial functional genes associated with N2O emissions to understand the underlining mechanisms. The results showed that RS biochar was higher in pH, available N, dissolved organic N, and decreased more N2O emissions from soils with N and 120% WHC treatment relative to MS and FS biochar (by 363% and 200%, respectively). Although RS biochar potentially increased the abundance of ammonia-oxidizing archaea amoA gene (AOA), changes in functional gene abundance did not concur with decreases in N2O emissions. Instead of changes in microbial communities, the relatively higher pH as well as lower available N and dissolved organic C and N of RS biochar could have contributed to the decrease in N2O emissions compared with MS and FS biochar. Thereby, the in situ application of rice straw via biochar could be considered in the mitigation of N2O emissions from fertilized rice paddy soil instead of biochar derived from ex situ feedstock.