Human interference with the nitrogen cycle has caused a tremendous increase in reactive nitrogen in the biosphere. As one consequence, high nitrate concentrations in our freshwaters threaten the health of aquatic ecosystems and drinking water quality. Catchments, as the main unit for water quality management, encompass numerous hydrological and biogeochemical processes that shape nitrogen retention and its export to downstream receiving water bodies. Hence, we need a good understanding of these processes across spatiotemporal scales to most efficiently address nitrate pollution. In my dissertation, I disentangled the complex interplay of catchment processes that shape the signal of discharge and nitrate concentrations measured at the outlet of a heterogeneous mesoscale (460 km²) catchment. To do so, I analyzed nitrate retention and export at different spatial scales, i.e., at the sub-catchment level and in comparison to neighboring catchments, and at different temporal scales, i.e., on the long term, during the severe drought 2018-2019, seasonally and during individual runoff events. Results highlight the important role of changing hydro-meteorological conditions and sub-catchment-specific transit times to understand and better foresee nitrate export dynamics at the catchment outlet. Moreover, results from this dissertation allow for more targeted suggestions for future research and water quality management.