Unraveling nitrogen loss in paddy soils: A study of anaerobic nitrogen transformation in response to various irrigation practice

Soil nitrogen (N) transformation processes, encompassing denitrification, anaerobic ammonium oxidation (anammox), and anaerobic ammonium oxidation coupled with iron reduction (Feammox), constitute the primary mechanisms of soil dinitrogen (N₂) loss. Despite the significance of these processes, there is a notable gap in research regarding the assessment of managed fertilization and irrigation impacts on anaerobic N transformations in paddy soil, crucial for achieving sustainable soil fertility management. This study addressed the gap by investigating the contributions of soil denitrification, anammox, and Feammox to N₂ loss in paddy soil across varying soil depths, employing different fertilization and irrigation practices by utilizing N stable isotope technique for comprehensive insights. The results showed that anaerobic N transformation processes decreased with increasing soil depth under alternate wetting and drying (AWD) irrigation, but increased with the increasing soil depth under conventional continuous flooding (CF) irrigation. The denitrification and anammox rates varied from 0.41 to 2.12 mg N kg^-1 d^-1 and 0.062-0.394 mg N kg^-1 d^-1, respectively, which accounted for 84.3-88.1% and 11.8-15.7% of the total soil N₂ loss. Significant correlations were found among denitrification rate and anammox rate (r = 0.986, p < 0.01), Fe (Ⅲ) reduction rate and denitrification rate (r = 0.527, p < 0.05), and Fe(Ⅲ) reduction rate and anammox rate (r = 0.622, p < 0.05). Moreover, nitrogen loss was more pronounced in the surface layer of the paddy soil compared to the deep layer. The study revealed that denitrification predominantly contributed to N loss in the surface soil, while Feammox emerged as a significant N loss pathway at depths ranging from 20 to 40 cm, accounting for up to 26.1% of the N loss. It was concluded that fertilization, irrigation, and soil depth significantly influenced anaerobic N transformation processes. In addition, the CF irrigation practice is best option to reduce N loss under managed fertilization. Furthermore, the role of microbial communities and their response to varying soil depths, fertilization practices, and irrigation methods could enhance our understanding on nitrogen loss pathways should be explored in future study.