Depth trends of soil organic matter C:N and 15N natural abundance controlled by association with minerals

Anaerobic ammonium oxidation (anammox) is believed to be an important sink for fixed inorganic nitrogen in terrestrial and aquatic ecosystems, and many studies have reported that macroscale oxic–anoxic interfaces, such as riparian zones, were hotspots of anammox reaction. However, no research has linked microscale interfaces with the anammox process in natural environments. This study provides evidence for the presence of anammox bacteria and potential anammox activity on the suspended sediment (SPS) in the oxic water of the Yellow River. The anammox bacteria in the overlying water were mainly attached to SPS. The abundance of anammox bacteria in the overlying water was positively correlated with SPS concentration (R ² = 0.97, P < 0.01), with abundance ranging from 9.5 × 10² to 1.5 × 10⁴ hydrazine synthase gene copies per g of SPS. Phylogenic analysis of anammox bacteria revealed that the SPS phase was dominated by Candidatus Brocadia. Candidatus Scalindua genera was detected in this study with a conductivity of 1100 μS cm^?1. Moreover, \(^{15} {\text{NH}}_{4}^{ + }\)-amended anaerobic incubation of the overlying water showed that the average potential anammox activity was 0.076 nmol-N L^?1 day^?1. The ¹⁵N labeling simulation experiments demonstrated the occurrence of anammox in the oxic water of the Yellow River. This study suggests that the anammox process at the SPS–water interface might be a non-negligible pathway for the loss of fixed nitrogen in natural freshwaters, but this remains to be determined in further studies.