Nitrous oxide (N₂O) is now the third largest contributor to radiative forcing of the long-lived greenhouse gases. Human inputs of nitrogen (N), mainly as fertilizers, have stimulated the microbial N cycle transformations that are the dominant source of N₂O. The partitioning between the anthropogenic and natural N₂O sources is relatively well constrained at 7 and 11 Tg N/yr, respectively. However, large uncertainties remain in the relative contributions of terrestrial versus aquatic environments and with regards to the underlying biogeochemical controls on microbial N₂O production. Such uncertainties present challenges for those devising and implementing N₂O emissions policies, and make it difficult to interpret prehistoric atmospheric N₂O fluctuations and predict the response of N₂O production rates to future climate change.

Aquatic N₂O fluxes are often highly variable through time and space. The variation is likely modulated by fluctuations in the biogeochemical conditions, which may affect microbial N₂O production pathways differentially. N₂O can be produced by several processes, such as microbial nitrification (ammonia oxidation), denitrification, and nitrifier-denitrification, and multiple groups of microorganisms are involved. Yet, the exact environmental controls on temporal/spatial variations in net N₂O production and the balance between the different pathways are still poorly constrained. It is highly probable that changes in the microbial processes that generate N₂O are closely linked to seasonal changes in water productivity, organic matter remineralization rates, and in turn water-column redox-conditions.

In this study we propose using incubation-based stable N-isotope tracer methods and natural N isotope measurements in dissolved N₂O to identify and quantify specific N₂O production pathways in two aquatic environments with strong seasonal N cycle dynamics: eutrophic Lake Lugano in southern Switzerland and the highly productive Benguela Upwelling region along the coast of southwestern Africa. Our main goals will be to shed light on the dynamics and controls on N₂O production in two comparable environments. Within the frame of one postdoctoral project we propose to address the following research questions:

• How much do ammonia oxidation, nitrifier-denitrification, and denitrification, respectively, contribute to N₂O formation in Lake Lugano and the Benguela Upwelling?
• Which biogeochemical factors control nitrifier-denitrification rates, and are there systematic differences between the marine and freshwater environment?
• How do the concentrations and stable isotopic signatures of N₂O, NO₂^-, NO₃^-, and NH₄⁺ observed in Lake Lugano and the Benguela Upwelling zone relate to the rates of the studied pathways?

In both environments, key variables, such as water pH, chlorophyll a, oxygen (O₂), and dissolved inorganic N concentrations, will be measured, and some of these parameters will be manipulated in incubations to study their impacts on N₂O production rates. Combining natural abundance N₂O isotope measurements and N₂O turnover measurements, and integrating the results in a 1-D geochemical model (for Lake Lugano) we attempt to gain information on the N and O isotope effects that are associated with N₂O cycling processes. Measurements of the isotope effects and isotopic signatures associated with N₂O production by natural microbial communities are valuable biogeochemical information because they allow detection of N cycle transformation processes in environments where direct rate measurements cannot be made. Our study is designed to capture the temporal dynamics associated with N₂O production in both aquatic environments. The University of Basel has been invited to join two research cruises (R/V Meteor) to the Benguela Upwelling zone that are scheduled for the seasonal maximum in upwelling intensity (September 2013) and the minimum (February 2014). The upcoming cruises will provide a unique opportunity to identify seasonal change in biological N₂O production mechanisms and rates, as well as the physical parameters that affect sea-to-air fluxes of N₂O.

The research proposed here will result in a detailed characterization of seasonal cycles in N₂O biogeochemistry. The study in Lugano has been designed to produce clear information for water resource managers describing the aspects of water quality that most influence N₂O emissions in nutrient-impacted lakes. The Benguela cruises will provide maximum and minimum N₂O flux constraints on this geochemically important upwelling region. Both studies will be used to provide new information about the controls on aquatic N₂O that are needed to accurately model the global dynamics of this powerful greenhouse gas.