Stronger link of nosZI than nosZII to the higher total N2O consumption in anoxic paddy surface soils

Rice paddy surface soils emit substantial amounts of atmospheric nitrous oxide (N2O). However, the soil can also act as sites for the abiotic and biological consumption of some of the N2O produced in the soil profile, thus, reducing net N2O emissions. Determining the proportion of N2O consumed in the soil, as well as the amount of N2O microbially reduced to nitrogen (N 2 ) is critical for developing N2O mitigating strategies in paddy soils. However, few studies have focused on these processes. Furthermore, the association of microorganisms containing nosZl and nosZll genes with variations in biological N2O consumption potential remains largely unexplored. Here, moisture-controlled (60% gravimetric water content, GWC) and completely anoxic (helium [He] system) microcosm experiments were conducted on paddy surface soils (0-5 cm deep) treated with N2O. During a 96-h incubation period, the N2O cumulative emissions were monitored, the total N2O consumption and N-2 production were quantified, and the population sizes of nosZI- and nosZII-containing microorganisms were analyzed. The results showed that 97.87%-99.99% of the N2O that accumulated in the 5 cm soil profile were consumed before emission, and the N-2 increase in these soils accounted for 64.50%-82.44% of the total N2O consumption, indicating that the total N2O consumption potential and biological N2O sink capacity of paddy surface soils were considerable under experimental conditions. Furthermore, the production of N-2 from N2O seemed to be positively related to soil pH, with less N-2 being produced in more acidic soil. In addition, nosZl gene abundance had a strong positive correlation with N-2 production, while nosZII had a much weaker correlation. Considering the positive correlation between N-2 production and soil dissolved organic carbon (DOC) consumption and available potassium content (AK), it is speculated that complete denitrification by nosZI-N2O reducers dominate the N2O biological sink capacity of the paddy surface soils investigated in this study.