Long-term effects of biochar addition and straw return on N2O fluxes and the related functional gene abundances under wheat-maize rotation system in the North China Plain

To evaluate long-term effects of biochar addition and straw return (SR) on N2O fluxes in wheat-maize rotation system, a 2-year field experiment following a 7-year biochar addition and SR was investigated in an intensively managed agricultural soil in the North China Plain (NCP). Four treatments were included: 1) no biochar addition (CK); 2) biochar treatment with 4.5 t ha−1 yr−1 (C1); 3) biochar treatment with 9.0 t ha−1 yr−1 (C2); and 4) all the wheat/maize straw return (SR). The results showed the annual cumulative N2O emissions from C1 treatment were increased by 15.9–16.5%, while C2 treatment suppressed the annual cumulative N2O emissions by 22.8–26.3%. In comparison, straw return suppressed N2O emissions by 13.4–43.6% in the wheat season but increased N2O emissions remarkably by 45.3–53.9% in the maize season. Biochar addition enhanced the copies of AOA, AOB, nirK, nirS, and nosZ genes, while straw return decreased the copies of AOB, nirK, nirS, and nosZ genes at the high N2O emission period in the maize season. The production of N2O in the maize season was mainly driven by the AOB and nosZ genes, and by AOB gene in the wheat season. These results suggest that application of 9.0 t ha−1 yr−1 biochar is a more optimum agricultural strategy for reducing N2O emission in the wheat-maize system.