Investigating drivers of N loss and NO reducers in paddy soils across China.

Denitrification plays a pivotal role in nitrogen (N) cycling in rice paddies, significantly impacting N loss and greenhouse gas emissions. Accurate quantification of net N emissions from paddy fields is therefore essential for improving fertilizer N use efficiency. However, challenges in directly measuring gaseous N hinder our understanding of microbially-mediated N loss in paddy soils at large scales. In this study, we investigated net N loss and its influencing factors in 45 paddy soils across China using membrane inlet mass spectrometry and N/Ar technique, complemented by microbial community analysis via metagenomics. Potential N loss rates varied from 0.41 to 3.58 nmol N g h, with no significant regional differences. However, soils from rice-upland rotation (1.72 ± 0.64 nmol N g h) and mono-rice cropping systems (1.41 ± 0.53 nmol N g h) exhibited higher N loss rates compared to double-rice cropping soils (1.13 ± 0.62 nmol N g h). Our results revealed a unimodal relationship between soil N loss rates and soil pH, with NO reducers and soil properties primarily regulating regional variations in N loss. Significant ecological differentiation was observed within both nosZ Clade I and Clade II, with soil pH emerging as the key factor shaping their community composition. Specifically, in rice-upland rotations, soil moisture and pH significantly influenced nosZ Clade I, while in double-rice cropping systems, soil texture and pH were the main factors affecting nosZ Clade II, thereby driving N loss. These findings enhance our understanding of N loss dynamics in paddy soil ecosystems, underscoring the critical role of NO reducers on microbial-derived N loss and highlighting the importance of developing strategies to mitigate NO emissions by balancing N loss through the manipulation of NO-reducing and NO-producing microbes.