Soil moisture and activity of nitrite- and nitrous oxide-reducing microbes enhanced nitrous oxide emissions in fallow paddy soils

Although cumulative N 2 O emissions are greater in the winter fallow season than in the rice-growing period, the mechanisms by which the emissions affect fallow paddy fields remain unclear. We aimed to identify N 2 O flux characteristics and illustrate how key nirS -, nirK - and nosZ -containing denitrifiers affect N 2 O emission levels in acidic fallow paddy soil. Five water-filled pore space (WFPS) levels were set at 25%, 50%, 75%, 100% and 125%, respectively. During the 48-h-long, high-flux incubation period, the N 2 O flux was the highest in soil samples with 75% WFPS, followed by those with 100% WFPS. The size of nirS- containing denitrifier community was more sensitive to the shifts in soil moisture and showed a stronger correlation with N 2 O flux than that of nirK- containing denitrifiers, whereas higher N 2 O concentrations induced an increase in the levels of nosZ- containing bacteria. After incubation for 48 h, nirK - and nosZ -denitrifying bacterial composition varied remarkably under 50%, 75%, and 100% WFPS treatments. However, the composition of nirS -containing denitrifying bacterial community gradually varied with an increase in soil moisture from 25% to 100% WFPS. Certain dominant OTUs of nirK- nirS- and nosZ -containing denitrifiers were highly abundant, especially under treatments of 50%, 75% and 100% WFPS, which were closely associated with the N 2 O flux. Thus, nirK , nirS and nosZ- containing denitrifiers respond to soil moisture differently, and enriched species might mainly be involved in controlling N 2 O flux in fallow paddy soils via denitrification, while the abundance of nirS -containing denitrifiers might affect N 2 O emission levels more significantly than that of nirK -containing denitrifiers.