Nitrogen loss through anaerobic ammonium oxidation mediated by Mn(IV)-oxide reduction from agricultural drainage ditches into Jiuli River, Taihu Lake Basin

Abstract Up to date, no great breakthrough has been made in the research of anaerobic ammonium oxidation mediated by Mn(IV)-oxide reduction (termed Mnammox). Recently, the Feammox process has become a hot research topic in the study of nitrogen loss from soils. Interestingly, in this study, an alternative pathway of N loss was proposed in terrestrial ecosystems. Mnammox could produce NO2−, NO3−, and N2 as end products. Here, our study demonstrated the occurrence of Mnammox, and direct evidence for Mnammox in agricultural drainage ditch soils with microbial Mn(IV) and Fe(III) reduction was obtained using the 15NH4+ isotopic tracing technique. The extent and rate of 30N2 and 29N2 production and Mn(IV) reduction were enhanced when amended with 15NH4+ and were further promoted when amended with 15NH4++MnO2. Moreover, although the Fe(III) reduction rate was stimulated with the addition of 15NH4+, the Fe(III) reduction rate greatly decreased when MnO2 was added. Mnammox rates ranged from 0.40 to 0.79 mg N kg−1 d−1, and an estimated 6.57–18.25 kg ha−1 year−1 N loss was associated with Mnammox in the examined soils. We revealed that the Mnammox reaction may be more efficient than the Feammox reaction, and the Feammox rates found in previous studies may have been overestimated. Overall, for the first time, this work provided key evidence for the existence of Mnammox in terrestrial ecosystems and suggested that Mnammox could be an important pathway for nitrogen loss in agricultural drainage ditch soils.