ABSTRACT Runoff of phosphorus (P) from agricultural fields is a leading contributor to impairment of freshwater quality. The influence of soil structure, and of mechanisms of runoff generation on forms of P losses is not fully understood. This study investigated P mobilisation from grassland soils in a controlled laboratory rainfall simulation. A 60‐min rainfall event was applied to soils subject to saturation excess and infiltration excess runoff generation mechanisms, at high and low soil bulk density scenarios. Runoff samples were collected at 10‐min intervals to quantify patterns of total, total dissolved, particulate, and dissolved reactive P loss. Results indicate that the influence of runoff generation mechanism on P losses was contingent upon soil bulk density. Particulate P was consistently the dominant form of P loss across all runoff and bulk density scenarios. Flow‐weighted mean concentrations were greatest under saturation excess runoff at high bulk density. For dissolved P, the highest flow‐weighted mean concentrations were observed under infiltration excess runoff at high bulk density. These results highlight that soil bulk density typically exerts a greater influence on the magnitude of P losses than does the runoff generation mechanism. Management of soil structure in hydrologically sensitive areas may therefore help to mitigate P loss. Furthermore, catchment‐scale models of nutrient loss may benefit from including soil bulk density as a parameter.
Roche et al. (Wed,) studied this question.