Abstract
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 (N2) to argon ratio, thus suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. However, 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH4) 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 (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere.
Highlights
The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown
With reactive nitrogen release from other sources, anthropogenic nitrogen is predicted to result in the eutrophication of aquatic bodies including the coastal ocean[2] and intensification of seasonal hypoxia to anoxia
Nitrogen loss from anaerobic aquatic environments had long been believed to occur through heterotrophic denitrification (NO3− → NO2− → nitric oxide (NO) → N2O → N2), until anaerobic ammonium oxidation (NO2− + NH4+ → N2 + 2H2O) was discovered to be an important component of the nitrogen cycle[3,4]
Summary
The fate of the enormous amount of reactive nitrogen released to the environment by human activities in India is unknown. We show occurrence of seasonal stratification and generally low concentrations of dissolved inorganic combined nitrogen, and high molecular nitrogen (N2) to argon ratio, suggesting seasonal loss to N2 in anoxic hypolimnia of several dam-reservoirs. 15N-experiments yielded low rates of denitrification, anaerobic ammonium oxidation and dissimilatory nitrate reduction to ammonium—except in the presence of methane (CH4) that caused ~12-fold increase in denitrification. While nitritedependent 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 (N2O), through widespread coupling between methanotrophy and denitrification, potentially mitigating eutrophication and emissions of CH4 and N2O to the atmosphere
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