Estimating dissolved p losses from legacy sources in pastures - the limits of soil tests and small-scale rainfall simulators.

Abstract

A legacy of using P fertilizers on grazed pastures has been enhanced soil fertility and an associated, increased risk of P loss in runoff. Rainfall simulation has been extensively used to develop relationships between soil test P (STP) and dissolved P (DP) in runoff as part of modelling efforts scrutinizing the impact of legacy P. This review examines the applicability of rainfall simulation to draw inferences related to legacy P. Using available literature, we propose a mixing layer model with chemical transfer to describe DP mobilization from pasture soils where readily-available P in the mixing layer is rapidly exhausted and contact time controls DP concentrations responsible for subsequent DP mobilization. That conceptual model is shown to be consistent with field monitoring data and then used to assess the likely effect of rainfall simulation protocols on DP mobilization, highlighting the influence of soil preparation, scale and measurement duration, and, most importantly, hydrology that can facilitate the physical transport of P into, and out of, surface flow. We conclude that rainfall simulation experimental protocols can have severe limitations for developing relationships between DP in runoff and STP, that are subsequently used to estimate legacy P contributions to downstream water resources. This article is protected by copyright. All rights reserved.