Denitrification as the predominant process in nitrous oxide production in the water column of two eutrophic reservoirs

Abstract. Reservoirs are important sites for nitrogen cycling and a significant global source of the potent greenhouse gas nitrous oxide (N₂O) to the atmosphere. They receive nitrogen inputs from agriculture and urban sources, boosting the production of N₂O by nitrification, denitrification, and photochemodenitrification. However, existing estimates of N₂O production in reservoirs are uncertain because previous studies have mainly focused on N₂O in rivers or lake sediments, often overlooking the water column of lentic systems. Here, we employed stable isotope tracer incubations alongside analyses of in situ natural abundance of nitrogen pools and functional genes involved in nitrification (amoA) and denitrification (nirS), to study N₂O production in the water column of two eutrophic reservoirs with contrasting morphometries. We used ¹⁵N-NH₄+ and ¹⁵N-NO₃^- tracers to quantify rates of N₂O production, nitrification, and nitrate reduction at the beginning and the end of the stratification period. Notably, nitrate concentration decreased by up to 49 % over the two months. N₂O production from ammonium ranged from 0.02 to 48.6 nmol-N L^-1 d^-1, while N₂O from nitrate varied from 0.2 to 61.0 nmol-N L^-1 d^-1. High rates of nitrification, nitrate reduction to nitrite, and rapid nitrite turnover were observed, with total N₂O production significantly correlated with the abundance of the nirS gene. A strong positive correlation was found between δ¹⁵Ν-NO₂^- and both N₂O concentration and nirS abundance. Overall, these findings suggest that reservoirs are active sites for N₂O production and N loss, with denitrification playing a significant role in the water column.