P inputs determine denitrifier abundance explaining dissolved nitrous oxide in reservoirs

Abstract

Reservoirs are important sites for nitrogen processing, especially those located in agricultural and urban watersheds. Nitrogen inputs promote N₂O production and emission, but the microbial pathways controlling N₂O have been seldom studied in reservoir water columns. We determined N₂O concentration in the water column of 12 reservoirs during the summer stratification and winter mixing. We explored the potential microbial sources and sinks of N₂O by quantifying key genes involved in ammonia oxidation (bacterial and archaeal amoA) and denitrification (nirS and nosZ). Dissolved N₂O varied up to three orders of magnitude (4.7–2441.2 nmol L⁻¹) across systems, from undersaturated to supersaturated values (37%–24,174%) depending on reservoirs and depths. N₂O concentration depended on nitrogen and oxygen availabilities, with the lowest and highest N₂O values at suboxic conditions. Ammonia-oxidizing archaea dominated over ammonia-oxidizing bacteria but were not related to the dissolved N₂O. In contrast, the abundance of the nirS gene was significantly related to N₂O concentration, and three orders of magnitude higher than amoA abundance. Denitrifying bacteria appeared consistently in the water column of all reservoirs. The nirS and nosZ genes appeared in oxic and suboxic waters, but they were more abundant in suboxic waters. The nitrate concentration, and nirS and nosZ relative abundances explained the dissolved N₂O. Besides, nirS abundance was related positively with total phosphorus and cumulative chlorophyll a, a proxy for fresh organic matter. Therefore, P inputs, not just N inputs, promoted N₂O production by denitrification in the water column of reservoirs.