Quantifying changes and trends of NO3 concentrations and concentration-discharge relationships in a complex, heavily managed, drought-sensitive river system

Stream nitrate nitrogen (NO3) concentrations and concentration-discharge (C-Q) relationships are key indicators of water quality. However, their long-term variability and response to climate change in large-scale catchments are still poorly understood. Here, we report a synoptic survey and long-term observations (2000-2021) in the Spree catchment (10,105 km2), Germany, to identify the spatial-temporal patterns in stream NO3 concentrations and C-Q relationships, as well as the underlying environmental drivers. We found that in the context of gradual reduction of nitrate inputs, the stream NO3 concentrations had a decreasing trend, but showed different mag-nitudes at different spatial scales (-0.01-0.48 mg L-1 yr-1). In the upstream parts of the catchment -with a high proportion of farmland -high levels of stream NO3 concentration remained with a risk of eutrophication due to the large nitrogen legacy. Especially in winter, stream NO3 concentrations were much higher due to the groundwater export with high NO3 concentrations, a decreased dilution effect of rainfall and vegetation uptake became clear. Consequently, the large nitrogen legacy in the upper catchment resulted in NO3 concentrations not significantly changing with streamflow and showing chemostatic behavior. This was different from NO3 dy-namics in the mid-and lower-catchment areas, which were positively related to streamflow, showing a che-modynamic behavior. Further, we found the export patterns (i.e. enrichment vs. dilution) of stream NO3 concentrations strongly correlated with the export regimes (i.e. chemostatic vs. chemodynamic), which were also affected by drought conditions. This is probably due to the decrease in stream depth which would enhance benthic removal leading to a decrease in stream NO3 concentrations. Reduced hydrological connectivity leads to higher spatial heterogeneity of residual NO3 in soil and drainage water, which will result in chemodynamic behavior of stream NO3 concentrations. After rewetting, the export of these high NO3 concentration in soils led to elevated stream NO3 concentrations, resulting in a positive correlation with streamflow. Our work revealed significant heterogeneity in stream NO3 concentrations and C-Q relationships at different spatial-temporal scales in large catchments. Critically, current stream NO3 concentrations and C-Q relationships are likely to respond strongly to future drought, leading to challenges for future land and water management.