Inorganic nitrogen stimulates methane oxidation in coastal lagoon sediments

Methane (CH4) oxidation is a critical process to reduce CH4 emissions from aquatic environments to the atmosphere. Considering the continuous increase in nitrogen in rivers, lakes, and lagoons from human sources, we re-evaluated the still controversial potential effect of inorganic nitrogen on CH4 oxidation. Here, we approached three shallow coastal lagoons that represent great environmental heterogeneity and used slurry sediments as a model system. The addition of ammonium chloride (NH4Cl) and potassium nitrate (KNO3) significantly stimulated CH4 oxidation in the sediments of all studied lagoons, indicating the potential limitation of nitrogen for the growth of CH4 oxidizing bacteria. Our findings contrast to some previous reports, where ammonium and nitrate inhibited CH4 oxidation in sediments. Indeed, our experiment was performed in a more realistic range in relation to natural concentrations of inorganic nitrogen in aquatic systems (0.5 to 1 mM) and was opposed to extreme concentrations previously used (2 to 50 mM). Our results point to the need to further assess the connection between nitrogen inputs and CH4 budgets in aquatic sediments, considering the potential fuel for CH4 oxidation that may affect the global greenhouse gas balance