Abstract
Abstract. Nitrous oxide (N2O) 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 N2O budget has been widely neglected. Furthermore, the biogeochemical controls on the production rates and the microbial pathways that produce benthic N2O in lakes are essentially unknown. In this study, benthic N2O fluxes and the contributions of the microbial pathways that produce N2O were assessed using 15N label flow-through sediment incubations in the eutrophic, monomictic south basin of Lake Lugano in Switzerland. The sediments were a significant source of N2O throughout the year, with production rates ranging between 140 and 2605 nmol N2O 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 N2O 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 N2O flux. A marked portion (up to 15%) of the total NO3− consumed by denitrification was reduced only to N2O, 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 N2O. This study provides evidence that lake sediments are a significant source of N2O to the overlying water and may produce large N2O fluxes to the atmosphere during seasonal mixing events.
Highlights
Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential that is ∼ 300 times higher than that of CO2 over a 100 yr time horizon (Forster et al, 2007)
This study demonstrates that the sediments of the Lake Lugano south basin are a net source of N2O to the water column
NO−3 reduction by denitrification was found to be the primary source of this N2O, while N2O production during ammonia oxidation was minimal or not observed at all, even when oxic conditions prevailed at the sediment/water interface
Summary
Nitrous oxide (N2O) is a potent greenhouse gas with a global warming potential that is ∼ 300 times higher than that of CO2 over a 100 yr time horizon (Forster et al, 2007). The atmospheric concentration has increased from 270 ppb in 1750 (Forster et al, 2007) to 323 ppb in 2011 (AGAGE, 2012), but there are still large uncertainties with regard to the relative contributions of the major sources and sinks of N2O (Forster et al, 2007). Anthropogenic fixed nitrogen loading can have multiple detrimental environmental effects, such as eutrophication, acidification, and the reduction of biodiversity (Galloway et al, 2003). In these environments, microbial processes taking place in redox transition zones play an important role in removing fixed N. N2O is produced during other N transformation reactions (Galloway et al, 2003) such as nitrification (ammonia oxidation) and nitrifier denitrification (Fig. 1)
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