Biochar with large specific surface area recruits N2O-reducing microbes and mitigate N2O emission

Bacteria and archaea colonizing on biochar have been reported to possess nitrogen-metabolizing abilities. A larger specific surface area of biochar may enhance the activities of nitrous oxide (N2O)-reducing microbes, thereby mitigating N2O emission; however, the underlying mechanisms remain unclear. A 56-day incubation assay was performed with five treatments: no addition, urea only, and addition of three types of biochars (with different specific surface areas: 1193, 2023, and 2773 m2 g−1) combined with urea. N2O emission increased with the specific surface area of biochar up to 2023 m2 g−1 and decreased thereafter by 37% as compared with the urea only addition. By increasing soil pH, C/N ratio, nitrogen availability, and cation exchange capacity, the biochar with the largest specific surface area decreased soil N2O emission by affecting the diversity, abundance, and composition of total bacteria and N2O-producing microbial communities. A larger specific surface area of biochar correlated with a higher abundance of nitrogen-fixing (nifH), -nitrifying (amoA), and -denitrifying (nirK, nirS, and nosZ) genes. An increased abundance of ammonia-oxidizing bacteria and archaea, in the biochar with a smaller specific surface area, resulted in higher N2O emission. As the abundance of nosZ increased, the addition of the biochar with the largest specific surface area resulted in a higher ratio of nosZ/(amoA + nirS + nirK), leading to decreased N2O emission. Furthermore, the abundance of nifH, amoA, nirK, and nosZ on biochar (extraction from soil after 56-day incubation) was positively correlated with that in soil. Thus, the relative specific surface area of biochar should be taken into consideration when using it in agriculture, as our results show that biochars with larger specific surface areas decrease N2O emission by recruiting N2O-reducing microbes and upregulating the abundance of nitrogen-fixing, -nitrifying, and -denitrifying genes.