Studies have been made, by 15N-tracer technique on nitrogen loss resulting from adding amorphous manganese oxide to NH4+-N medium under anaerobic conditions. The fact that the total nitrogen recovery was decreased and that 15NO2, 15N2O, 15N14NO, 15NO, 15N2and 15N14N were emitted has proved that, like amorphous iron oxide, amorphous manganese oxide can also act as an electron acceptor in the oxidation of NH4+-N under anaerobic conditions and give rise to nitrogen loss. This once again illustrates another mechanism by which the loss of ammonium nitrogen in paddy soils is brought about by amorphous iron and manganese oxides. The quantity of nitrogen loss by amorphous manganese oxide increased with an increase in the amount of amorphous manganese oxide added and lessened with time of its aging. The nitrogen loss resulting from amorphous manganese oxide was less than that from amorphous iron oxide. And the nitrogen loss by cooperation of amorphous manganese oxide and microorganisms (soil suspension ) was lar Studies have been made, by 15N-tracer technique on nitrogen loss resulting from adding amorphous manganese oxide to NH4 + -N medium under anaerobic conditions. The fact that the total nitrogen recovery was decreased and that 15NO2, 15N2O, 15N14NO, 15NO, 15N2 and 15N14N were emitted has proved that, like amorphous iron oxide, amorphous manganese oxide can also act as an electron acceptor in the oxidation of NH4 + -N under anaerobic conditions and give rise to nitrogen loss. This once again illustrates another mechanism by which the loss of ammonium nitrogen The amount of nitrogen loss by amorphous manganese oxide increased with an increase in the amount of amorphous manganese oxide added and lessened with time of its aging. The nitrogen loss caused from amorphous manganese oxide was less than that from amorphous iron oxide. And the nitrogen loss by cooperation of amorphous manganese oxide and microorganisms (soil su was lar