Spatial distribution and co-occurrence of aerobic ammonia oxidation and anaerobic ammonium oxidation activities in the water-soil interface, bulk, and rhizosphere regions of paddy soil

Purpose Aerobic ammonia oxidation often couples with anaerobic ammonium oxidation (anammox) in natural ecosystems; however, their in situ activities and relative contributions are still unclear. Methods We evaluated in situ activities of both processes in the water-soil interface, bulk, and rhizosphere regions of paddy soil by 15 N stable isotope. Results At the interface, we only detected aerobic ammonia oxidation activities (R A , 6.06 mmol L −1 d −1 ), and R A was significantly higher than in the other two regions. In the bulk soil, the R A and anammox activities (R An ) were 0.87 and 1.84 mmol L −1 d −1 , respectively. In the rhizosphere soil, the R A and R An were 0.92 and 1.17 mmol L −1 d −1 , respectively. Aerobic ammonia oxidation contributed 7.0%, 5.9%, and 3.9% to the total ammonium consumption in the interface, bulk, and rhizosphere regions, respectively, while anammox contributed 13.3% and 5.3% in the bulk and rhizosphere regions, respectively. R A and R An were positively correlated with transcript abundances of amoA and hzsB genes, respectively, rather than gene abundances. Moreover, dissolved oxygen (DO) was the most critical factor statistically influencing R A and R An , while dissolved organic carbon (DOC) and total organic carbon (TOC) was also correlated with R An . Conclusion Overall, we demonstrated that in situ aerobic ammonia oxidation in the interface and rhizosphere regions was more active than in the bulk soil, whereas in situ anammox was more active in the bulk soil. This study provides an increased understanding of the in situ distribution and contributions of aerobic ammonia oxidation and anammox co-occurring in complex ecosystems.