Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir

Abstract

Nitrogen loading from developed watersheds to aquatic ecosystems can stimulate microbial denitrification, a process that reduces nitrate (NO−3) to dinitrogen (N2) or nitrous oxide (N2O), the latter a potent greenhouse gas. While aquatic ecosystems are a globally significant source of N2O to the atmosphere, the relationship between denitrification and N2O production is not well known. Until recently, this field of research has been limited by the technical challenges of simultaneously measuring denitrification and N2O production or consumption in situ at the ecosystem scale. Here we use membrane inlet mass spectrometry, an analytical method providing precise and accurate measurements of dissolved N2, and gas chromatography to directly measure N2 and N2O concentrations in the hypolimnion of a stratified reservoir draining an agricultural watershed. Denitrification resulted in a consistent increase in dissolved N2 and decrease in NO−3 concentrations in the hypolimnion during the 5 month period of stratification, though temporal patterns in dissolved N2O concentrations were less consistent. Denitrification alternated between an N2O source and sink, with the N2O yield, defined as the relative production of N2O to N2 via denitrification, ranging from −3.4% (i.e., net N2O consumption) to 19.5% (mean = −0.03%). Whereas denitrification in the hypolimnion functioned as a small N2O sink during the stratified period, the reservoir was an N2O source on an annual time scale. Additional studies across reservoirs of different sizes, trophic status, and ages are needed to resolve the role of reservoirs in the global N2O budget.