Year-round N2O production by benthic NOx reduction in a monomictic south-alpine lake

Nitrous oxide (N2 O) is a potent greenhouse gas, generated through microbial nitrogen (N) turnover processes, such as nitrification, nitrifier denitrification, and denitrification. Previous studies quantifying natural sources have mainly focused on soils and the ocean, but the potential role of terrestrial water bodies in the global N2 O budget has been widely neglected. Furthermore, the biogeochemical controls on the production rates and the microbial pathways that produce benthic N2 O in lakes are essentially unknown. In this study, benthic N2 O fluxes and the contributions of the microbial pathways that produce N2 O were assessed using 15 N label flow-through sediment incubations in the eutrophic, monomictic south basin of Lake Lugano in Switzerland. The sediments were a significant source of N2 O throughout the year, with production rates ranging between 140 and 2605 nmol N2 O h-1 m-2 , and the highest observed rates coinciding with periods of water column stratification and stably anoxic conditions in the overlying bottom water. Nitrate (NO3- ) reduction via denitrification was found to be the major N2 O production pathway in the sediments under both oxygen-depleted and oxygen-replete conditions in the overlying water, while ammonium oxidation did not contribute significantly to the benthic N2 O flux. A marked portion (up to 15%) of the total NO3- consumed by denitrification was reduced only to N2 O, without complete denitrification to N2 . These fluxes were highest when the bottom water had stabilized to a low-oxygen state, in contrast with the notion that stable anoxia is particularly conducive to complete denitrification without accumulation of N2 O. This study provides evidence that lake sediments are a significant source of N2 O to the overlying water and may produce large N2 O fluxes to the atmosphere during seasonal mixing events.