Phosphorus leaching and runoff risks from non-calcareous sandy soils with a low sorption capacity and high...

Sustainable phosphorus (P) management includes producing food within environmental boundaries for water quality. In regions where environmental boundaries are crossed, it is beneficial to identify P loss hotspots and implement mitigation measures. In this study, we assessed the risk of P losses to shallow groundwater and surface water from agricultural fields on non-calcareous sandy soils with an exceptionally low P sorption capacity and high hydrological connectivity due to shallow groundwater levels and the presence of open trenches. Specifically, we investigated P quantity-intensity relationships in soils from two agricultural fields and monitored groundwater levels and P concentrations in both groundwater and water fluxes from open trenches. The results showed that non-calcareous soils with low sorption capacities reach high P saturation degrees when fertilized to an agronomic optimum based on a P quantity measure. This leads to high reactive P concentrations in soil solution that can be transported to surface water via interflow, overland flow, and land drainage. In these situations, open trenches are a significant P loss pathway because they directly connect the P-saturated topsoil to surface water, leading to P losses ranging from 1.3 to 7.5 kg P ha^-1 year^-1. Effective mitigation measures include reducing dissolved P losses by reducing the soil P status of fields to environmental soil P intensity thresholds through negative P balances and reducing particulate P losses by implementing erosion control measures. However, because inlet water substantially contributes to the total water discharge, within-catchment mitigation measures may need to be complemented by upstream mitigation measures.