NoLaMa "No laughing matter - N2O cycling in lacustrine environments"

Nitrous oxide (N 2 O, also known as laughing gas) has become the third most important anthropogenic greenhouse gas, after CO 2 and methane. Oceanic N 2 O emissions to the atmosphere represent up to 35 % of the global natural sources. Freshwater N 2 O emission are less well constrained, due to high spatial and temporal variability of aquatic N 2 O fluxes from inland waters. The exact biogeochemical controls on N 2 O cycling are still poorly constrained. In order to understand changes in the magnitude of N 2 O fluxes from aquatic ecosystems in response to fluctuating biogeochemical conditions (i.e., redox state, dissolved nitrogen, and organic substrates), it is imperative to determine the individual contributions of the microbial (ammonium oxidation/nitrification, nitrifier-denitrification, and denitrification) and abiotic N 2 O production pathways and their sensitivity to changing environmental conditions. The potential niche overlap of denitrifiers and nitrifiers along oxygen gradients, or between ammonium oxidizing bacteria and archaea in freshwater, make it difficult to distinguish between the different N 2 O sources and their process-specific controls. For example, an increasing number of studies suggest that denitrification, mostly known as N 2 O sink in anoxic waters, is a largely overlooked N 2 O source in the suboxic water masses overlaying open ocean oxygen minimum zones. At the same time, despite the canonical view that N 2 O reduction is an anaerobic process, high abundances of N 2 O reduction genes and transcripts have been found in marine oxic waters indicating a potential unknown N 2 O sink in surface waters. Whether such an aerobic N 2 O sink exists also in lakes remained unaddressed. As for nitrification in surface/subsurface lake waters, the role of dissolved organic N compounds (i.e. urea and cyanate) as potential substrate and precursor in N 2 O production is uncertain. The aim is to identify and quantify specific N 2 O production and consumption pathways in lacustrine environments, and to provide insight into the key microbial players, pathways, dynamics and environmental controls on N 2 O cycling. We will shed light on the blurring redox boundaries and environmental controls (e.g., nutrient availability) that modulate net N 2 O production in a lacustrine environment, where autotrophic denitrification is the dominating N loss pathway (Lake Lugano, Switzerland). The main objectives of the proposed project are to: 1) Identify the seasonal and vertical variability of N 2 O consumption, on the relative importance of N 2 O production processes in the water column, the abundance of process marker genes/transcripts, and the N 2 O producing/consuming microbial community composition in the studied lake. 2) Assess the sensitivity of N 2 O production and reduction processes to changes in dissolved nitrogen substrate and oxygen availability. 3) Determine the importance of underappreciated organic N-sources such as urea and cyanate for biotic and abiotic N 2 O production in the lake water column. We will use 15 N/ 18 O-tracer incubation experiments for potential-rate estimates along with natural abundance isotope measurements of N 2 O, NO 3 - , NO 2 - , and NH 4 + , which will provide an integrated signal of overlapping processes that affect the stable isotope pools of these molecules over short and longer time periods. We will combine isotope-biogeochemical results with data on abundance and community composition of N 2 O producers and consumers. The results that we expect to come out of the proposed project will provide essential knowledge about the mechanistic regulation of different N 2 O production pathways in aquatic environments, and will thus help to improve and validate existing and future global models predicting lacustrine N 2 O emissions.