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 (N 2 ) or nitrous oxide (N 2 O), the latter a potent greenhouse gas. While aquatic ecosystems are a globally significant source of N 2 O to the atmosphere, the relationship between denitrification and N 2 O production is not well known. Until recently, this field of research has been limited by the technical challenges of simultaneously measuring denitrification and N 2 O 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 N 2 , and gas chromatography to directly measure N 2 and N 2 O concentrations in the hypolimnion of a stratified reservoir draining an agricultural watershed. Denitrification resulted in a consistent increase in dissolved N 2 and decrease in NO − 3 concentrations in the hypolimnion during the 5 month period of stratification, though temporal patterns in dissolved N 2 O concentrations were less consistent. Denitrification alternated between an N 2 O source and sink, with the N 2 O yield, defined as the relative production of N 2 O to N 2 via denitrification, ranging from −3.4% (i.e., net N 2 O consumption) to 19.5% (mean = −0.03%). Whereas denitrification in the hypolimnion functioned as a small N 2 O sink during the stratified period, the reservoir was an N 2 O 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 N 2 O budget.

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