Denitrification contributes to NO emission in paddy soils.

Denitrification is vital to nitrogen removal and NO release in ecosystems; in this regard, paddy soils exhibit strong denitrifying ability. However, the underlying mechanism of NO emission from denitrification in paddy soils is yet to be elucidated. In this study, the potential NO emission rate, enzymatic activity for NO production and reduction, gene abundance, and community composition during denitrification were investigated using the N isotope tracer technique combined with slurry incubation, enzymatic activity detection, quantitative polymerase chain reaction (qPCR), and metagenomic sequencing. Results of incubation experiments showed that the average potential NO emission rates were 0.51 ± 0.20 μmol⋅N⋅kg⋅h, which constituted 2.16 ± 0.85% of the denitrification end-products. The enzymatic activity for NO production was 2.77-8.94 times than that for NO reduction, indicating an imbalance between NO production and reduction. The gene abundance ratio of to Z from qPCR results further supported the imbalance. Results of metagenomic analysis showed that, although Proteobacteria was the common phylum for denitrification genes, other dominant community compositions varied for different denitrification genes. Gammaproteobacteria and other phyla containing the B gene without Z genes, including Actinobacteria, Planctomycetes, Desulfobacterota, Cyanobacteria, Acidobacteria, Bacteroidetes, and Myxococcus, may contribute to NO emission from paddy soils. Our results suggest that denitrification is highly modular, with different microbial communities collaborating to complete the denitrification process, thus resulting in an emission estimation of 13.67 ± 5.44 g NO⋅m⋅yr in surface paddy soils.