Nitrous oxide (isotope) biogeochemistry
Nitrous oxide (N2O) is an important contributor to radiative forcing of the long-living greenhouse gases, but relative large uncertainties remain with regards to the contribution of ocean and lacustrine versus terrestrial environments to global N2O emissions, because aquatic N2O fluxes are often highly variable through time and space. There are multiple microbial pathways that produce N2O and they may respond differently to changing biogeochemical conditions. Sources of N2O include microbial nitrification (ammonia oxidation), denitrification, and nitrifier-denitrification.
The goal of our research is to verify to which extent the microbial processes that generate N2O are closely linked to changes in water productivity, organic matter remineralization rates, and water-column redox-conditions. Towards this goal we measure N2O fluxes, and N2O production rates (using 15N-tracer techniques) in various environments, combined with natural abundance level N and O isotope analyses. Active cycling of N2O imparts a wide range of isotopic signatures, which reflect the balance of underlying N-transformation pathways. Therefore, as with NOx, stable N and O isotope analysis of N2O (including information on the intramolecular distribution of 15N, i.e., site preference), can provide important information about where, and by what mechanism N2O is produced and/or consumed. A particular focus of recent work in our group is to understand biogeochemical controls on nitrifier-denitrification, as well as the relationship between the O-isotope composition of N2O and nitrite, another important intermediate within the N cycle. Study sites include lakes in Southern Switzerland (Lake Lugano and Lake Cadagno), as well as the Benguela upwelling area off the Namibian coast. More recently, in collaboration with the Swiss Federal Institute for Forest, Snow and Landscape Research WSL and the ETH Zurich we are also investigating the controls on N2O emissions from restored river floodplain soils.