Decreasing trends of nitrogen export to waters in tile-drained landscapes are linked to a shifting water balance

In this study, time series over 14 years of climate, stream flow, land management, nitrate-nitrogen (NO₃^--N) concentrations and losses were analyzed to identify potential drivers for temporal trends at two tile-drained catchments under cropland use in northeastern Germany. Mean annual NO₃^--N concentrations were 9.7 (drainage plot) and 6.8 mg l^-1 (ditch catchment), while mean annual NO₃^--N losses amounted to 22 and 20 kg ha^-1, respectively. Results indicated decreasing trends for discharge, NO₃^--N concentrations and losses, and N surpluses at both the drainage plot and the ditch catchment scale. However, a significant downward trend was only detected for flow-weighted mean annual NO₃^--N concentrations of the ditch water. A significant positive relationship between annual discharge and mean annual NO₃^--N concentrations of the ditch underlines the importance of the hydrologic conditions on the NO₃^--N concentrations. No direct relationships were found between NO₃^--N concentrations and N surpluses. We conclude that the decreasing NO₃^--N concentrations could be primarily attributed to decreasing discharge rates. The possible impact of reduced N surpluses was overridden by the hydrologic conditions in the catchment. The statistically significant downward trend of flow-weighted mean annual NO₃^--N concentrations in the ditch water suggests, however, a positive effect of reduced N surpluses on stream water quality. Our analysis has further shown that effects of land management aiming at reducing N surpluses might only become visible with a delay of years or even decades.