Contribution of Ammonium-Induced Nitrifier Denitrification to N 2 O in Paddy Fields.

Paddy fields are one of the most important sources of nitrous oxide (N 2 O), but biogeochemical N 2 O production mechanisms in the soil profile remain unclear. Our study used incubation, dual-isotope ( 15 N- 18 O) labeling methods, and molecular techniques to elucidate N 2 O production characteristics and mechanisms in the soil profile (0-60 cm) during summer fallow, rice cropping, and winter fallow periods. The results pointed out that biotic processes dominated N 2 O production (72.2-100%) and N 2 O from the tillage layer accounted for 91.0-98.5% of total N 2 O in the soil profile. Heterotrophic denitrification (HD) was the main process generating N 2 O, contributing between 53.4 and 96.6%, the remainder being due to ammonia oxidation pathways, which was further confirmed by metagenomics and quantitative polymerase chain reaction (qPCR) assays. Nitrifier denitrification (ND) was an important N 2 O production source, contributing 0-46.6% of total N 2 O production, which showed similar trends with N 2 O emissions. Among physicochemical and biological factors, ammonium content and the ratio of total organic matter to nitrate were the main driving factors affecting the contribution ratios of the ammonia oxidation pathways and HD pathway, respectively. Moisture content and pH affect norC -carrying Spirochetes and thus the N 2 O production rate. These findings confirm the importance of ND to N 2 O production and help to elucidate the impact of anthropogenic activities, including tillage, fertilization, and irrigation, on N 2 O production.