Estimating dissolved phosphorus 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.

Role of Phosphorus Type and Biodegradable Polymer on Phosphorus Fate and Efficacy in a Plant–Soil System

“…Thus, although P loss in runoff is a more complex, less well-defined process than leachability, involving additional factors such as soil erosion, particle size, and the affinity with which P-containing species bind to soil particles, the correlation observed in Figure 3B suggests that P leachability is a good predictor of P loss in runoff for the same treatment. However, as previously noted, 50 the methods for determining P loss in runoff are not standardized and will likely be highly sensitive to the experimental design, such as the angle of soil tilt and volume and size of water droplets used to simulate rainfall, and these results would likely vary using simulators of different design. Thus, while our data show that similar qualitative trends exist for P runoff and leachate among the 12 different treatments, it is not possible to make a quantitative determination of how much P would be lost as runoff vs leachate.…”

Characterization of nutrient runoff from perennial and annual forages following broiler litter application

“…Hydrological variability (e.g., time of runoff initiation, infiltration rates, soil moisture, and runoff volume) between replicates can result in high within‐treatment variability in concentrations and loss of nutrients (Nash et al., 2021; Srinivasan et al., 2007). In this study, rainfall duration was controlled by time to initiate runoff, which resulted in rainfall durations, total rainfall, and runoff volumes varying widely, depending on initial conditions.…”

“…These warm‐season native grasses are bunchgrasses (USDA‐NRCS., 2017) and have bare soil between bunches, which could result in high runoff volume. Runoff volume determines its erosive, transportation capacity, and the mobilization of adsorbed nutrients such as P (Nash et al., 2021; Sharpley, 1985), which could have resulted in comparable losses of sediments and nutrients from control and litter‐amended treatments under switchgrass. For litter‐amended treatments, switchgrass had lower TSS, TOC, SRP, TDP, TP, and NO 3 –N losses than gamagrass and wheat but was not different from Kernza and silphium ( p ≤ .05).…”

Impacts of excessive nutrients load in aquatic ecosystem

“…A clear distinction can be made between N and P in the timescales over which inputs of these nutrients are buffered by terrestrial ecosystems against loss, which has implications for control strategies. (Harper, 1992;Nash et al, 2021). At the river basin scale, any targets for reducing nutrient loss are best guided by site-specific information on their likely ecological impact, but this information rarely exists for rivers affected by eutrophication, and only general guidelines are available.…”