Distinct denitrifying phenotypes of predominant bacteria mould nitrous oxide metabolism in upland agricultural soils

Denitrifying nitrous oxide (N2O) emissions in agroecosystems result from variations in microbial composition and soil properties. However, the microbial mechanisms of differential N2O emissions in agricultural soils are less understood. Microcosm experiments of two types of Chinese farmland soil were conducted with nitrate (250 mg/kg) and a combination of glucose (1000 mg/kg) and nitrate, and a case with no addition was used as the control. The results show that N2O accumulation in black soil (BF) was significantly higher than that in fluvo-aquic soil (FF) independent of carbon and nitrogen supply. The abundance of denitrifying genes was significantly higher in FF, but the ratios of genes responsible for N2O production (narG, nirS, and nirK) to the gene for N2O reduction (nosZ) did not significantly differ between the two soils. However, the soils showed obvious discrepancies in denitrifying bacterial communities. High accumulation of N2O was verified by the isolates of Rhodanobacter, which is predominant in BF due to its truncated denitrifying genes and lack of N2O reduction capacity. The dominance of complete denitrifiers such as Castellaniella in FF led to a rapid reduction in N2O and reduced N2O accumulation, as demonstrated when its corresponding isolate was inoculated into both studied soils. Therefore, the different phenotypes of N2O metabolism of the distinct denitrifiers maintained in the two soils caused their differing N2O accumulation. This knowledge could guide the regulation of the denitrifying bacterial community and the phenotypes of N2O metabolism in agricultural soils to reduce N2O emissions.