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 modeling 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 was 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 important, 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.
Highlights
Excessive P concentrations in streams and water impoundments are a feature of many agricultural catchments where P amendments are applied to intensively grazed pastures (Grayson et al, 2001; McDowell et al, 2019; Pierson et al, 2001; Sharpley et al, 2007, 2012)
Such tools are often informed by empirical relationships between soil test P (STP) and P concentrations developed from rainfall simulation studies (Vadas et al, 2005)
Given the potential importance of legacy P, we investigate the veracity of using rainfall simulation to study processes controlling dissolved P (DP) in runoff from pastures and to develop relationships (e.g., STP vs. P concentration) commonly used to estimate the contribution of legacy P to those losses
Summary
Excessive P concentrations in streams and water impoundments are a feature of many agricultural catchments where P amendments are applied to intensively grazed pastures (Grayson et al, 2001; McDowell et al, 2019; Pierson et al, 2001; Sharpley et al, 2007, 2012). In the absence of better alternatives, various index systems (Melland et al, 2004; Sharpley et al, 2017) and hydrological modeling paired with an estimate of P export potential (Davies et al, 2006; Donigian et al, 1984; Novotny et al, 1978) have been used Such tools are often informed by empirical relationships between soil test P (STP) and P concentrations developed from rainfall simulation studies (Vadas et al, 2005). Despite STP at the Darnum site increasing from 17 to 57 mg Olsen P kg−1 (0-to-100-mm deep soil sample) between 1994 and 2000, there appeared to be no relationship between STP and the systematic P concentrations in runoff water (Figure 1d) Such findings are consistent with other modeling of that area (Government of Victoria, Department of Primary Industries, unpublished data, 2011) and with field-scale studies elsewhere (Owens & Shipitalo, 2006). A mixing layer model with chemical transfer is described for DP mobilization from the grazing soils (i.e., pasture) and used to evaluate the influence of experimental protocols in small-scale rainfall simulation studies on inferences related to P source
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