Wheat straw and microbial inoculants have an additive effect on N2O emissions by changing microbial functional groups

Straw-decomposing microbial inoculants (MIs) have been increasingly applied to straw-amended soils. However, the interactive effects and underlying microbial mechanisms of straw and decomposing MIs on nitrous oxide (N₂O) emissions remain unclear. Here, a pot experiment with an Aquic Inceptisol was conducted to determine soil N₂O emissions and the abundance and composition of microbial functional genes under six amendments: wheat straw (W), Bacillus subtilis MI (B), Streptomyces rochei MI (S), a combination of straw and B. subtilis MI (WB), a combination of straw and S. rochei MI (WS), and a control without wheat straw or MI (C). Compared with the control, the amendments of straw, decomposing MIs, and their combinations decreased soil N₂O emissions by 43%, 22–30%, and 46–60%, respectively. Mechanistically, the positive relationship between ammonia-oxidizing bacteria (AOB) and soil potential nitrification rate (PNR), along with decreased AOB abundance (−36%) following straw and decomposing MI amendments, further suggested that AOB predominated soil nitrification and was responsible for the suppressed nitrification. In addition, straw amendment increased nirK gene abundance (by 48%) and potential denitrification rate (by 11%), while the increase in nosZI gene abundance (+25%) involved in N₂O consumption contributed to the decrease in N₂O emissions. Overall, the additive effect of straw and decomposing MIs on soil N₂O emissions was associated with two amendment-induced changes in the abundance and composition of AOB and straw-stimulated the abundance of the nosZI gene. Our results revealed the potential for mitigating N₂O emissions following the amendments of straw and MI by influencing soil N₂O-related microbial groups.