The influence of landscape spatial arrangement on nitrogen and phosphorus export in agricultural catchments

Nitrogen and Phosphorus losses from rural landscapes can cause eutrophication of inland and coastal waters. Here, we assessed the influence of the spatial configuration of nutrient sources on nitrate (NO₃^-) and total phosphorus (TP) exports at the catchment scale. We analyzed NO₃^- and TP in 19 headwater catchments (1 -14 km²) fortnightly during 17 months. The study catchments had similar soil type, climate, and farming systems but differed in their landscape composition (% agricultural land-use) and spatial configuration (field distance to streams & intersection with flow accumulation zones). We propose a landscape configuration index (LCI) that describes the spatial configuration of nutrient sources with regards to their hydrological distance to streams and flow accumulation zones. The LCI has two parameters that we calibrated to maximize rank correlation with median concentrations of TP and NO₃^-. Results showed that NO₃^- exports were correlated with %agricultural land-use whereas TP exports were better explained by the spatial configuration of agricultural fields. For a given landscape composition, landscape spatial configuration was highly heterogeneous at small scales (<10 km²) but became homogeneous at larger scales (>50 km²). This could explain why relationships between landscape composition and water quality parameters influenced by landscape spatial configuration break down below a certain size threshold.