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

Abstract

Aerobic ammonia oxidation often couples with anaerobic ammonium oxidation (anammox) in natural ecosystems; however, their in situ activities and relative contributions are still unclear. We evaluated in situ activities of both processes in the water-soil interface, bulk, and rhizosphere regions of paddy soil by 15N stable isotope. At the interface, we only detected aerobic ammonia oxidation activities (RA, 6.06\xa0mmol\xa0L−1 d−1), and RA was significantly higher than in the other two regions. In the bulk soil, the RA and anammox activities (RAn) were 0.87 and 1.84\xa0mmol\xa0L−1 d−1, respectively. In the rhizosphere soil, the RA and RAn were 0.92 and 1.17\xa0mmol\xa0L−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. RA and RAn 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 RA and RAn, while dissolved organic carbon (DOC) and total organic carbon (TOC) was also correlated with RAn. 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.