ALBERT

Description: The isotopic composition of atmospheric nitrous oxide (N2O) was measured semicontinuously, at ~35 min frequency in intermittent periods of 1-6 days over one and a half years, using preconcentration coupled to a quantum cascade laser spectrometer at the suburban site of Dübendorf, Switzerland. The achieved measurement repeatability was 0.08 per mil, 0.11 per mil, and 0.10 per mil for d18O, site preference, and d15Nbulk respectively, which is better than or equal to standard flask sampling-based isotope ratio mass spectrometry performance. The observed mean diurnal cycle reflected the buildup of N2O from isotopically light sources on an isotopically heavy tropospheric background. The measurements were used to determine the source isotopic composition, which varied significantly compared to chemical and meteorological parameters monitored at the site. FLEXPART-COSMO transport modeling in combination with modified Emissions Database for Global Atmospheric Research inventory emissions was used to model N2O mole fractions at the site. Additionally, isotopic signatures were estimated for different source categories using literature data and used to simulate N2O isotopic composition over the measurement period. The model was able to capture variability in N2O mole fraction well, but simulations of isotopic composition showed little agreement with observations. In particular, measured source isotopic composition exhibited one magnitude larger variability than simulated, clearly indicating that the range of isotopic source signatures estimated from literature significantly underestimates true variability of source signatures. Source d18O signature was found to be the most sensitive tracer for urban/industry versus agricultural N2O. d15Nbulk and site preference may provide more insight into microbial and chemical emission processes than partitioning of anthropogenic source categories.

Description: Article abstract: Grasslands cover more than one fifth of total land area in Europe and contribute significantly to the total greenhouse gas budget. The impact of management and land use on the carbon cycle and carbon sequestration in grasslands has been well-studied, however effects on emissions of N2O and CH4 remain uncertain. Additionally, the majority of studies have focussed on management differences between intensively managed grasslands, with few results available for lightly managed grasslands and in particular grassland abandonment. We present N2O and CH4 flux measurements for an abandonment trajectory at low land-use intensity, comparing meadow (fertilized and cut), pasture (grazed) and abandoned (unmanaged since 1983) grassland sites located in the Austrian Alps. Mean growing season N2O fluxes were 0.07, 0.07 and −0.13 nmol m−2 s−1 and CH4 fluxes were −1.0, −0.5 and −1.6 nmol m−2 s−1 for the meadow, pasture and abandoned sites respectively. Variability for both gases at the abandoned site was dominated by 'hot moments', while 'hot spots' dominated at the managed meadow and pasture sites. Consideration of the diurnal cycle observed at the abandoned site, linear correlations within all data sets, and principal components analyses of the full data set revealed increased consumption of both N2O and CH4 with increasing temperature, but hardly any relationship between fluxes and soil moisture. Upscaled over a year, the observed fluxes correspond to enhanced non-CO2 greenhouse gas uptake of 172 g CO2-equiv. m−2 y−1 following abandonment. These results show that non-CO2 greenhouse gases form an important part of the total climate impact of land use change and grassland abandonment, such that abandoned grassland is a net sink for both CH4 and N2O.

Description: Publication abstract: Nitrous oxide is a powerful greenhouse gas whose atmospheric growth rate has increased rapidly over the past decade. Causes of N2O emission dynamics from terrestrial ecosystems and their responses to climate change remain poorly understood. Based on high-resolution isotopic measurements we found that microbial functional gene abundances and soil moisture were the main drivers of variability in baseline emissions and pathways. After fertilization and, surprisingly, under drought, denitrification dominated grassland N2O production and dynamics, contributing 70% of emissions. Drought responses of N2O emissions were supported by a clear reversible enrichment in nitrogen-bearing organic matter compared to oxygen on the surface of soil microaggregates. We observed hysteresis for both total N2O flux and denitrification contribution during rewetting, which were higher than control and drought for the same soil moisture range. These results illustrate that the magnitude of feedbacks between climate change and N2O emission pathways is sufficient to account for increases in atmospheric growth rate in the past decade. We expect these effects will be amplified in the coming decades as climatic extremes are expected to increase in severity.

Description: This dataset includes flux and isotope data for 16 monoliths from the Kaserstatt Alm subject to an experimental drought. The treatments are designated C/W/D/DF/R for Control, Wet (+25% precipitation), Drought, Drought (finishes later) and Rain (drought monolith which received rain) respectively. Further information on the experimental set up is given in the associated publication.

Description: This dataset comprises a compilation of soil bulk delta-15-N nitrogen isotopic composition that has been measured and/or published since the compilation of d15N data by Craine et al. (2015; doi:10.1007/s11104-015-2542-1; doi:10.1038/srep08280). The data was measured by the data owner / contact indicated in the dataset. All data remains the property of the listed owner but may be used for non-commercial purposes. In the case of significant use of this data for scientific research, please cite this dataset as well as the associated publication(s) and consider contacting data owners to offer co-authorship where relevant. Project: Identifying drivers of N2O emissions in a changing climate (https://www.oecd.org/agriculture/crp/fellowships/). Award: OECD Cooperative Research Program for Sustainable Agricultural and Food Systems (OECD-CRP) grant.