Contrasting the spatial management of nitrogen and phosphorus for improved water quality: Modelling studies in New Zealand and France

• We modelled critical source areas of N and P loss at critical source areas and catchment scales.
• Mitigation N loss from critical source areas was achieved as more grassland was incorporated into the catchment.
• Mitigation P losses occurred by decreasing erosion and P solubility.
• Mitigating P loss from critical source areas was 6–7 times more cost-effective than catchment-wide strategies.
• We recommend managing critical source areas to improve water quality and minimise the impact on farm profitability.

Critical source areas (CSAs) define areas of a farm or catchment that emit the majority of water quality contaminants but account for a minority of the area at a field, farm or catchment scale. Using process based modelling we tested the hypothesis that the definition and management of CSAs would decrease losses of phosphorus (P) in two New Zealand catchments and nitrogen (N) in a French catchment. In the New Zealand catchment, CSAs of P loss were isolated to small areas within fields commensurate with surface flow pathways, while in the French catchment, CSAs for N loss were influenced by factors (inputs and land use) relevant at a field (or multiple field) scale. Scenarios were tested that examined the management of CSAs versus whole field management for P, and decreasing N loss within CSAs by increasing the proportion of grassland fields and changing their location relative to cropland. The results showed that N losses decreased by up to 25% as more grassland was incorporated into the catchment, especially in wet areas near valley bottoms due to a longer growth period and better utilisation/storage of N than cropland. For P, focusing mitigation on CSAs decreased catchment losses to a similar degree as mitigations applied across the whole catchment, but was on average 6–7 times more cost-effective. Therefore, the definition and management of CSAs at an appropriate scale is recommended to improve water quality and minimise the impact on farm profitability.