Signifying impacts of climate warming on vulnerability to subsurface nitrate contamination in Europe

Decades of agricultural intensification across Europe have created nitrogen legacy stores that continue to threaten the functioning of aquatic ecosystems and human health. Climate change and climate extremes further aggravate the fate of nitrogen export and retention in the terrestrial system (e.g., soil, groundwater, and rivers) - the extent of which is yet not fully understood. Herein we provide a continental-wide analysis of the projected changes in timings and extent of nitrogen-vulnerable regions across European landscapes. Our assessment relies on a newly devised objective measure based on a Damkoehler number -- that encapsulates the transient nature of hydrologic transport and biogeochemical transformations [1]. We perform a century-long, spatially explicit daily hydrologic simulations forced with a suite of bias-corrected and downscale climate model (CMIP6) runs driven under different emission scenarios (SSP126, SSP370, and SSP585) till the end of the 21st Century. These simulations allow us to derive the transport dynamics of dissolved nitrogen based on a transient aspect of travel-time distributions (TTDs) using temporally resolved water storage and fluxes. On the other hand, biogeochemical processes like denitrification rates are characterized by the first-order decay coefficients that are further modulated by spatially and temporally varying environmental factors imposed by moisture content and temperature constraints. Contrasting the space-time dynamics of hydrological transport times with reactive timescales of denitrification in soil, our analysis indicates that more than two-thirds of the cultivated areas across Europe are potentially vulnerable to nitrate leaching for at least one-third of the year under the contemporary climate condition (1981-2010). Further, the climate projection-based simulation results indicate that under high emission scenarios (SSP585), arable lands in Central Europe would be more prone to nitrate leaching (times), while drier conditions in Southern Europe favor stronger denitrifications. Limiting climate warming by adhering to a low-emission scenario, such as SSP126, has the potential to decrease the vulnerability of regions to nitrate leaching (extent, duration, and load). By signifying the differentiated impacts of climate warming on nitrate leaching potential, our study contributes towards unraveling the complexity of nitrogen transport dynamics across a diverse range of European landscapes under changing climatic conditions. [1] Kumar, R., Heße, F., Rao, P.S.C., Musolff, A., Jawitz, J.W., Sarrazin, F., Samaniego, L., Fleckenstein, J.H., Rakovec, O., Thober, S. and Attinger, S., 2020. Strong hydroclimatic controls on vulnerability to subsurface nitrate contamination across Europe. Nat Commun 11, 6302 (2020). https://doi.org/10.1038/s41467-020-19955-8.