Impacts of fertilization on grassland productivity and water quality across the European Alps under current and warming climate: insights from a mechanistic model

Alpine grasslands sustain local economy by providing fodder for livestock. Intensive fertilization is common to enhance their yields, thus creating negative externalities on water quality that are difficult to evaluate without reliable estimates of nutrient fluxes. We apply a mechanistic ecosystem model, seamlessly integrating land-surface energy balance, soil hydrology, vegetation dynamics, and soil biogeochemistry, aiming at assessing the grassland response to fertilization. We simulate the major water, carbon, nutrient, and energy fluxes of nine grassland plots across the broad European Alpine region. We provide an interdisciplinary model evaluation by confirming its performance against observed variables from different datasets. Subsequently, we apply the model to test the influence of fertilization practices on grassland yields and nitrate (inline-formula $M1inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg57a4663cbf0d11bf294d99bb32c9ae29 bg-18-1917-2021-ie00001.svg25pt16ptbg-18-1917-2021-ie00001.png ) losses through leaching under both current and modified climate scenarios.

Despite the generally low inline-formula $M2inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimg4c315b3ea451cf26923ad12993612b33 bg-18-1917-2021-ie00002.svg25pt16ptbg-18-1917-2021-ie00002.png concentration in groundwater recharge, the variability across sites is remarkable, which is mostly (but not exclusively) dictated by elevation. In high-Alpine sites, short growing seasons lead to less efficient nitrogen (N) uptake for biomass production. This combined with lower evapotranspiration rates results in higher amounts of drainage and inline-formula $M3inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimga186e28964d6ae507e65dbc91f8b1f71 bg-18-1917-2021-ie00003.svg25pt16ptbg-18-1917-2021-ie00003.png leaching to groundwater. Scenarios with increased temperature lead to a longer growing season characterized by higher biomass production and, consequently, to a reduction of water leakage and N leaching. While the intersite variability is maintained, climate change impacts are stronger on sites at higher elevations.

The local soil hydrology has a crucial role in driving the inline-formula $M4inlinescrollmathmlchem{\mathrm{normal\; NO}}_{\mathrm{normal\; 3}}^{-}$ 25pt16ptsvg-formulamathimge16cba38499a6a16cb1a10e488ec56da bg-18-1917-2021-ie00004.svg25pt16ptbg-18-1917-2021-ie00004.png use efficiency. The commonly applied fixed threshold limit on fertilizer N input is suboptimal. We suggest that major hydrological and soil property differences across sites should be considered in the delineation of best practices or regulations for management. Using distributed maps informed with key soil and climatic attributes or systematically implementing integrated ecosystem models as shown here can contribute to achieving more sustainable practices.