Denitrification as the dominant 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). They receive nitrogen inputs from agriculture and urban sources, fueling N2O production via nitrification, denitrification, and photochemodenitrification. However, existing estimates of N2O production in reservoirs remain uncertain because most studies have focused on N2O in rivers or lake sediments, often overlooking the water column of lentic systems. Here, we present the first integrated assessment of N2O production pathways in reservoir water columns using stable isotope tracer incubations alongside analyses of in situ natural abundance of nitrogen pools and functional genes involved in nitrification (amoA) and denitrification (nirS), across 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 production from nitrate varied from 0.2 to 61.0 nmolNL-1d-1. High rates of nitrification, nitrate reduction to nitrite, and rapid nitrite turnover were observed, with total N2O production significantly correlated with nirS gene abundance. A strong positive correlation was found between δ15N-NO2- and both N2O concentration and nirS abundance. These findings reveal that denitrification and nitrite dynamics play a central role in N2O formation within reservoir water columns, advancing understanding of nitrogen loss and greenhouse gas emissions from lentic systems.