Effects of biochar on nitrification and denitrification-mediated NO emissions and the associated microbial community in an agricultural soil.

Nitrous oxide (NO) is a strong greenhouse gas, and it is of great significance for NO reduction to study the effects of biochar on its production pathway. In this research, the contributions and mechanisms of biochar on autotrophic nitrification (ANF), heterotrophic nitrification (HNF), and denitrification (DF) to NO emissions were studied by using N stable isotopes and high-throughput sequencing after laboratory incubation. The results showed that biochar addition at 2% (B2) significantly reduced the NO emissions from the ANF by an average of 20.6%, while adding 5% biochar (B5) had no significant effect on the ANF. Both B2 and B5 significantly reduced the NO emissions from the HNF by 15.7% and 13.2%, respectively, and reduced the NO emissions from the DF by 40.9% and 11.7%, respectively. B2 enhanced the relative contribution rate of the ANF to NO emissions by 6.3%, while B5 had little effect on it. Biochar addition significantly changed the copy numbers of the AOA and AOB, as well as the nirK, nirS, and nosZ genes, but it had no significant effect on the community composition of the AOA and had minimal effect on the AOB community. B2 significantly increased the abundance of the genus Rhodococcus of nirK type denitrifiers and had a significant effect on the relative abundance of Cupriavidus and Pseudomonas of the nosZ type denitrifiers. These results revealed that the inhibitory effects of biochar on NO emissions from nitrification might be attributed to the direct immobilization and adsorption of inorganic N by biochar and to its promotion of the genus Rhodococcus of nirK-type denitrifiers and the genera Cupriavidus and Pseudomonas of the nosZ-type denitrifiers. The soil exchangeable NH-N and NO-N concentrations were the primary factors affecting the NO emission rates. These results help to elucidate the effects and mechanisms of biochar on NO production pathways in agricultural soil.