How to Achieve a 50% Reduction in Nutrient Losses From Agricultural Catchments Under Different Climate Trajectories?

Under persistent eutrophication of European water bodies and a changing climate, there is an increasing need to evaluate best-management practices for reducing nutrient losses from agricultural catchments. In this study, we set up a daily discharge and water quality model in Hydrological Predictions of the Environment for two agricultural catchments representative for common cropping systems in Europe's humid continental regions to forecast the impacts of future climate trajectories on nutrient loads. The model predicted a slight increase in inorganic nitrogen (IN) and total phosphorus (TP) loads under RCP2.6, likely due to precipitation-driven mobilization. Under RCP4.5 and RCP8.5, the IN loads were forecasted to decrease from 16% to 26% and 21%-50% respectively, most likely due to temperature-driven increases in crop uptake and evapotranspiration. No distinct trends in TP loads were observed. A 50% decrease in nutrient loads, as targeted by the European Green Deal, was backcasted using a combination of management scenarios, including (a) a 20% reduction in mineral fertilizer application, (b) introducing cover crops (CC), and (c) stream mitigation (SM) by introducing floodplains. Target TP load reductions could only be achieved by SM, which likely results from secondary mobilization of sources within agricultural streams during high discharge events. Target IN load reductions were backcasted with a combination of SM, fertilizer reduction, and CC, wherein the required measures depended strongly on the climatic trajectory. Overall, this study successfully demonstrated a modeling approach for evaluating best-management practices under diverging climate change trajectories, tailored to the catchment characteristics and specific nutrient reduction targets. The European Union has set a target to reduce nutrient losses from agricultural areas by 50% by 2030 to improve the quality of its water bodies. However, we argue that climate change will have a strong impact on nutrient dynamics, implying that the required management actions for improving water quality need to adapt depending on the climate trajectory. In this study, we simulated the future losses of two major nutrients, inorganic nitrogen (IN) and total phosphorus (TP), for two Swedish agricultural streams representative of major crop-growing regions. We also modeled best management practices to reach the targeted 50% reduction in nutrient losses. The model predicted that IN loads will decrease under moderate and severe climate change pathways, but increase under the mild climate change pathway. We found that targeted reductions in TP loads could only be achieved through SM. Targeted reductions in IN loads could be achieved by combining SM with a 20% reduction in mineral fertilizer and/or protecting the soil with cover crops (CC) in winter. This study demonstrated how to apply water quality models for identifying the required management actions to reduce future nutrient losses from agricultural catchments. Inorganic nitrogen (IN) loads were forecasted to decrease under future climate change, while no distinct trends in total phosphorus (TP) were observed Target reductions in TP loads rely on more widespread implementation of stream mitigation (SM) to tackle secondary pollution sources Target reductions in IN loads can be achieved by combining SM with fertilizer reductions and/or cover crops