Isotopic tracing of post-drought N2O emission pathways

Increasing summer droughts and shifts in precipitation patterns due to a warming climate are predicted to affect the European Alps in the coming decades. In addition to climate change and drought, alpine grasslands are also subject to rapid land use change. These processes affect biogeochemical cycles in soils, such as the nitrogen (N) cycle, thus altering emissions of nitrous oxide (N2O) an important greenhouse gas and strong stratospheric ozone-depleting substance. N2O is released from two major microbial pathways in soil: nitrification and denitrification. These pathways are poorly understood, although they determine N2O emissions as well as many of the other environmental impacts associated with nitrogen fertiliser use. It is unknown how nitrification, denitrification and N2O emissions will change in the coming decades under the influence of climate and land use change in alpine grasslands. This study will investigate the impact of drought on N2O emissions at contrasting grassland sites: a mountain meadow and an abandoned grassland in Stubaital (Tyrol) and a heavily fertilised meadow in Gumpenstein (Styria). We hypothesise that we will see drier soils, reduced microbial activity, and thus less N2O emissions under drought. When soils are subject to rewetting, N2O emissions will show a strong peak due to increased denitrification. We expect overall higher emissions at the fertilised site in Styria, but a similar response pattern and timing at both sites. We furthermore expect that in a future warmer climate with elevated atmospheric CO2 concentrations drying and rewetting effects on N2O emissions will become more pronounced. To investigate the complex N cycle in soils, we will optimise and implement several new techniques using stable isotope tracers naturally occurring atoms weighing slightly more than usual, allowing detection and quantification of reactions. We will measure N2O stable isotopic composition using a newly developed laser system that allows fast and precise monitoring in the field. Measuring N2O not just at the surface, but at multiple depths in the soil, will allow us to see how the impacts of drought change in the soil column. We will also use a unique technique known as NanoSIMS to examine the reactions of nitrogen on the nanoscale less than a thousandth of a millimeter within soil particles. These ground-breaking results will be used to make a computer model of the N cycle and N2O emissions from nitrification and denitrification, thus allowing us to predict emissions across Europe in coming decades. This study will be the first field-based, multi-faceted investigation of drought, climate and land use change impacts on N2O emissions, providing critical information to plan future management and use of alpine grasslands and reduce emissions of this strong and harmful greenhouse gas.

The NitroTrace project was an interdisciplinary effort aimed at understanding sources and sinks of the important greenhouse gas nitrous oxide (N2O) from grasslands in a changing climate. The primary human-related source of N2O is emission from agricultural soils following nitrogen fertilization. N2O raises particular environmental concerns because emissions are accelerating, meaning that the concentration in the atmosphere is rapidly increasing. In NitroTrace, we used isotopic measurements as unique "fingerprints" to trace the pathways leading to N2O production in changing climates. This approach aimed to help find effective ways to reduce these emissions. To achieve this, we directly connected chambers, used for measuring soil gas emissions, with an isotope spectrometer to measure N2O isotopic composition. Our initial findings highlighted the unexpectedly significant role of the "denitrification" pathway in drought-affected soils, with implications for nitrogen management in dry regions. Further measurements, still being analysed, will show the combined impact of increased temperature, elevated CO2 and drought on nitrogen cycling and N2O emissions from grasslands. We developed two new modelling tools within NitroTrace. The "TimeFRAME" tool employs a Bayesian approach to consider various sources of uncertainty, allowing us to quantify trace gas production and consumption pathways based on isotopic data. The "IsoTONE" model uses global soil nitrogen isotopic composition as a proxy for N2O emissions. Using IsoTONE, we were able to show that both climate warming and the shift of nitrogen fertilization towards tropical and sub-tropical regions are contributing to the global increase in N2O emissions.