b'Field to catchment scale carrying capacities to limit soil P risk to water quality Cassidy R1, Jordan P 2, Higgins A1, Thomas IA1,3 and Bailey JS11Agri-Environment Branch, Agri-Food and Biosciences Institute (AFBI), Newforge Lane, Belfast, BT9 5PX, Northern Ireland, UK2School of Geography and Environmental Sciences, Ulster University, Coleraine,Northern Ireland, UK3UCD Dooge Centre for Water Resources Research, School of Civil Engineering, UniversityCollege Dublin, Belfield, Dublin 4, IrelandSoils with phosphorus (P) levels above the agronomic optimum are considered a risk to water quality, particularly if located in areas likely to generate surface runoff during rainfall events. Identifying and targeting management at these areas has the potential to reduce diffuse nutrient pressures on surface waters and, for farmers, to deliver greater on-farm nutrient efficiency. To operationalise such an approach at field, farm and catchment scales a carrying capacity framework for soil P and runoff risk was developed and applied within catchments in Northern Ireland. A soil P survey of 13 sub-catchments and 7,693 fields was undertaken in the 220 km2 Upper Bann catchment, identifying fields at, below and in excess of the agronomic optimum, which is Index 2+ (21 - 25 mg Olsen P L-1). Hydrologically Sensitive Areas (HSAs) were determined as the top 25th percentile risk from a runoff routing model that used a LiDAR digital elevation model and soil hydraulic conductivity properties. Comparisons of these spatial metrics with river soluble reactive P concentrations across 13 sub-catchments indicated that, for this landscape,the carrying capacity for above agronomic optimum soil P is no greater than 15% ofsub-catchment areas and effectively no areas (<1.5%) of a catchment should have coincident high soil P and runoff risk. The opportunities to redistribute these risks were analysed on fields with below optimum soil P and where HSA runoff risk was minimal. These areas, which could be utilised to redistribute organic nutrients away from high risk areas, ranged from 0.4 % to 13.8 % of the sub-catchment areas. This limited potential, unlikely to fully reduce the P pressure to over-supplied fields, would need to be considered in conjunction with lowering on-farm P balances and in-field P interception measures.Ongoing work is extending the framework to other catchments in the region.12'