Variation in N2O emission and N2O related microbial functional genes in straw- and biochar-amended and non-amended soils

Reducing mineral fertilizer application is an important management practice for mitigating nitrous oxide (N2O) emissions. Crop straw and biochar are two farmer-friendly residues that can be used for reducing the application of mineral fertilizers. However, different effect of straw- and biochar-amended and non-amended soils on N2O emissions is not well understood. Here, we conducted a mesocosm experiment in a vegetable field in Southwestern China using four treatments: control (CT, no addition), solely mineral fertilizers at the recommended dosage (F), maize straw with mineral fertilizers (SF), and biochar with mineral fertilizers (BF), based on the same total nitrogen, phosphorus and potassium contents. Soil N2O fluxes, cumulative N2O emission, soil chemical parameters, the abundance of N2O related microbial functional genes (amoA, nirS, nirK and nosZ) and microbial community structure were monitored and analyzed. Compared to F, the SF treatment increased the cumulative N2O emission. However, no significant difference was observed in cumulative N2O emission between F and BF treatments. Covariation in cumulative N2O emission and soil dissolved organic nitrogen (DON) content indicated that heterotrophic ammonia oxidation fueled by soil DON might be the driving force for N2O production. Compared to F, both SF and BF treatments increased the amoA gene copy numbers of ammonia oxidizing bacteria (AOB) but reduced those of ammonia oxidizing archaea (AOA); nirS and nosZ gene copy numbers were also reduced under SF and BF treatments. Compared to SF, the diversity of nirK and nosZ communities under the BF treatment was increased, whereas the diversity of nirS community was decreased. Lower cumulative N2O emission under BF than that under SF may be attributed to lower N2O production from heterotrophic ammonia oxidation fueled by soil DON and increased N2O consumption mediated by nosZ-harboring denitrifiers.