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

Denitrifying nitrous oxide (N 2 O) emissions in agroecosystems result from variations in microbial composition and soil properties. However, the microbial mechanisms of differential N 2 O 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 N 2 O 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 N 2 O production ( narG, nir S, and nirK ) to the gene for N 2 O reduction ( nosZ ) did not significantly differ between the two soils. However, the soils showed obvious discrepancies in denitrifying bacterial communities. High accumulation of N 2 O was verified by the isolates of Rhodanobacter , which is predominant in BF due to its truncated denitrifying genes and lack of N 2 O reduction capacity. The dominance of complete denitrifiers such as Castellaniella in FF led to a rapid reduction in N 2 O and reduced N 2 O accumulation, as demonstrated when its corresponding isolate was inoculated into both studied soils. Therefore, the different phenotypes of N 2 O metabolism of the distinct denitrifiers maintained in the two soils caused their differing N 2 O accumulation. This knowledge could guide the regulation of the denitrifying bacterial community and the phenotypes of N 2 O metabolism in agricultural soils to reduce N 2 O emissions.