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Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir
Citation:
Beaulieu, J., R. Smolenski, C. Nietch, A. Townsend-Small, M. Elovitz, AND J. Schubauer-Berigan. Denitrification alternates between a source and sink of nitrous oxide in the hypolimnion of a thermally stratified reservoir. LIMNOLOGY AND OCEANOGRAPHY. Association for the Sciences of Limnology and Oceanography, Waco, TX, 59(2):495-506, (2014).
Impact/Purpose:
To inform the public
Description:
Nitrogen loading from developed watersheds to aquatic ecosystems can stimulate microbial denitrification, a process which reduces nitrate (NO3-) 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/consumption in situ at the ecosystem scale. Here we use membrane inlet mass spectrometry, an analytic 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. We show that denitrification in the hypoxic hypolimnion was sufficient to remove >90% of the entrained NO3- during a 5 month stratification event and that denitrification alternated between an N2O source and sink, influencing N2O exchange at the air-water interface. The N2O yield, defined as the relative production of N2O to N2 via denitrification, ranged from -3.4% (i.e., net N2O consumption) to 19.5% (mean=-0.03) and was related to algal blooms, NO3- availability, and dissolved oxygen concentration. While denitrification in the hypolimnion functioned as a small N2O sink during the study, the reservoir was a large 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.
