High Resolution, Spatiotemporal Monitoring of N2O Hotspots on Inorganically Fertilized Wheat Field

Very few high-resolution field campaigns combining a temporal and spatial component have been carried out to quantify N2O emissions in agricultural settings. Traditionally, large land areas are assumed to emit N2O more or less homogenously, which can for instance be seen in the IPCC guidelines for national estimates of agricultural N2O emissions. On a temporal scale, this means we are missing information about annual variations and the consequences of farming processes. In the spatial plane, we lack knowledge about the influence of topographical variations and related variations in plant, soil, and microbial properties. It is well known that the processes controlling the turnover of N and N2O emissions in soil systems are extremely complex, but there have been instrumental limitations to studying the often short-lived processes directly in the field, causing an important data gap. We present a month-long intensive study investigating spatial and temporal variation of N2O surface fluxes along a gentle topographic slope in an active agricultural field. It is part of a larger campaign, where we have carried out a weekly to biweekly 1½ year N2O and NO3- monitoring program on two semi-permanently installed field sites located in inorganically fertilized agricultural systems. We here show N2O and related NO3- data from a rain event in the spring 2021, occurring following the application of inorganic N-fertilizer to the field site which was cropped to winter wheat. Measurements were carried out on a weekly to daily basis during a four-week period to elucidate the importance of contrasting soil moisture conditions along the elevation gradient. We used the state of the art Ultra Transportable Greenhouse Gas Analyzer LGR OA-ICOS 909-004 N2O/CH4 to obtain in situ, real time fluxes. We observed significant fluctuations in N2O emissions along the slope with several prominent hotspots developing and changing position along the slope during the month-long study of the rain event. Our results demonstrate the urgent need for high resolution field studies of N2O emissions in agricultural settings, in order to include temporal and spatial hotspots in accurate modelling of regional and global scale N2O emissions.