Abstract
Reservoirs are important sites for nitrogen processing, especially those located in agricultural and urban watersheds. Nitrogen inputs promote N 2 O production and emission, but the microbial pathways controlling N 2 O have been seldom studied in reservoir water columns. We determined N 2 O concentration in the water column of 12 reservoirs during the summer stratification and winter mixing. We explored the potential microbial sources and sinks of N 2 O by quantifying key genes involved in ammonia oxidation (bacterial and archaeal amoA ) and denitrification ( nirS and nosZ ). Dissolved N 2 O 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. N 2 O concentration depended on nitrogen and oxygen availabilities, with the lowest and highest N 2 O values at suboxic conditions. Ammonia‐oxidizing archaea dominated over ammonia‐oxidizing bacteria but were not related to the dissolved N 2 O. In contrast, the abundance of the nirS gene was significantly related to N 2 O 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 N 2 O. 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 N 2 O production by denitrification in the water column of reservoirs.