Significant but overlooked manganese oxides mediate anaerobic ammonium oxidation processes in the littoral zone

Abstract The littoral zone of lakes is recognized as hotspots of biogeochemical cycles. While the coupling of Fe(III) reduction and ammonium oxidation is acknowledged as an important nitrogen loss pathway, the role of anaerobic ammonium oxidation mediated by Mn oxides (Mnammox) remains uncertain. In this study, we conducted a 15 NH 4 + isotopic incubation experiment and applied Mn oxides amended treatment to investigate the role of Mnammox in the lake littoral zone with respect to the N biogeochemical cycle. Our findings confirmed the widespread presence of Mnammox in the littoral zone of Baiyangdian Lake, the largest freshwater lake in the North China Plain. The sediment exhibited an average 30 N 2 production rate of 19.91 μ g kg −1 d −1 , accounting for approximately 2.0% of total nitrogen loss in situ. Significant nitrate production was also observed with a rate of 0.26 mg N kg −1 d −1 , highlighting the role of Mnammox as a potential source of nitrate under anaerobic conditions. Furthermore, we identified a key threshold of 3.15 g kg −1 dry weight sediment MnO 2 amendment, roughly 20 times the in situ MnO 2 content, which most significantly enhanced the Mnammox process. Sediment Mn oxides content, moisture content, and pH were the main stimulators for the Mnammox process, with unique microbial groups. This work underscores the importance of Mnammox in the lake littoral zone, illuminating the Mn–N coupling that drives multi‐element cycles in this critical environment.