Methane stimulates massive nitrogen loss from freshwater reservoirs in India

The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. Here we show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N 2 ) to argon ratio, thus suggesting seasonal loss to N 2 in anoxic hypolimnia of several dam-reservoirs. However, 15 N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH 4 ) that caused ~12-fold increase in denitrification. While nitrite-dependent anaerobic methanotrophs belonging to the NC10 phylum were present, previously considered aerobic methanotrophs were far more abundant (up to 13.9%) in anoxic hypolimnion. Methane accumulation in anoxic freshwater systems seems to facilitate rapid loss of reactive nitrogen, with generally low production of nitrous oxide (N 2 O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH 4 and N 2 O to the atmosphere. The fate of anthropogenic nitrogen (N) remains understudied in South Asian water bodies despite its impact on water chemistry and quality. Here the authors show that N loss in Indian freshwater reservoirs is tightly coupled to methanotrophy, which has helped curb eutrophication and greenhouse gas emissions.