A carrying capacity framework for soil phosphorus and hydrological sensitivity from farm to catchment scales

Publication history: Accepted - 30 May 2019; Published online - 4 June 2019.Agricultural fieldswith above optimumsoil phosphorus (P) are considered to pose risks to water quality and especially when those areas are coincident with hydrologically sensitive areas (HSAs) that focus surface runoff pathways. This is a challenge tomanage in areas of agricultural intensity in surfacewater dominated catchments where water quality targets have to be met. In this study, a soil P survey of 13 sub-catchments and 7693 fields was undertaken in a 220 km2 catchment. HSAs were also determined as the top 25th percentile risk froma runoff routingmodel that used a LiDAR digital elevation model and soil hydraulic conductivity properties. Distributions of these spatial data were compared with river soluble reactive phosphorus (SRP) concentration measured fortnightly over one year. The results showed that 41% of fields exceeded the agronomic optimumfor soil P across the sub-catchments.When compared with the available water quality data, the results indicated that the high soil P carrying capacity area of the sub-catchmentswas 15%. Combining high soil P and HSA, the carrying capacity area of the sub-catchmentswas 1.5%. The opportunities to redistribute these riskswere analysed on fields with below optimum soil P and where HSA risk was also minimal. These ranged from 0.4% to 13.8% of sub-catchment areas and this limited potential, unlikely to fully reduce the P pressure to over-supplied fields, would need to be considered alongside addressing this over-supply and also with targeted HSA interception measures.This work was undertaken as a component of the “EU EAA Soil Sampling and Analysis Scheme”, funded by the Department of Agriculture, Environment and Rural Affairs (DAERA), Northern Ireland, under the European Union Exceptional Adjustment Aid Scheme.We thank catchment farmers for land access and participation. We acknowledge the contributions of AFBI scientific staffwhowere instrumental in the planning, acquisition and processing of data, Colleen Ward (AFBI Project Manager) and Peter Scott (DAERA lead). Finally we thank both anonymous reviewers for insightful comments and suggestions on the manuscript