Flow paths and wetness conditions explain spatiotemporal variation of nitrogen retention for a temperate, humid catchment

Excess use of nitrogen (N) fertilizer has threatened the quality of drinking water and destroyed the structure and functions of aquatic ecosystems. The N retention is defined as a catchment’s capacity to withhold nitrogen from entering surface water bodies during a certain period. Previous studies have focused on N retention in response to catchment characteristics and different climates. However, little has been done to explore the spatiotemporal variation of N retention within a catchment. This study is aimed to explore the spatiotemporal variation of the N retention and the effect of the external N input heterogeneity on the retention capacity and the N export in a small temperate, humid catchment in central Germany. A spatially distributed model was built using the fully coupled surface–subsurface model HydroGeoSphere to simulate the flow and transport of nitrate. Three scenarios considering the external N input heterogeneity, which represent different land-use patterns, were created: (i) the input was uniformly distributed over the catchment, (ii) high input was associated with high retention regions (e.g. near the hillslope top), and (iii) high input was associated with low retention regions (e.g. near the stream). The results showed that N retention exhibits strong spatial heterogeneity, which is controlled by the lengths of flow paths connecting the regions and surface water body and regional wetness conditions within a catchment. N retention showed pronounced seasonality in temperate and humid climates, being higher in wetting, drying and dry periods, but lower in wet periods. The pattern is driven by distinct shifts in the dominance of deeper flow paths and fast shallow flow paths determined by the climate seasonality. Highest denitrification, lowest export fluxes and in-stream nitrate concentration were found in scenario (ii) among the scenarios. Therefore, it was suggested that agricultural activities may avoid regions near surface water bodies to improve the surface water quality. The results of this study provide insights into the nitrogen dynamics and can guide the management of surface water quality.