How do runoff mechanisms influence the form of phosporus lost from grassland soils?

Soil pore structure influences hydraulic conductivity and saturation regimes. The influence of soil structure, and mechanisms of overland flow on the form of phosphorus (P) loss is not fully understood. Models of P transport assume either saturation excess (SE) or infiltration excess (IE) overland flow as the triggering mechanism. SE runoff may lead to higher dissolved P losses, due to a) lower applied energy hence less physical separation of particulates, and/or b) greater mobilization of P from sorbed reserves during antecedent saturation. Conversely, IE runoff may lead to greater particulate P loss due to increased physical separation. Structure, and contingent runoff mechanisms, is therefore likely to impact not only gross P losses, but also forms of P delivered to surface water. The influence of soil structure, and type of overland flow (SE or IE) on forms of P loss from grasslands is being examined through a runoff trial. Soil was collected, air dried, and sieved. Subsequently, soil was packed into boxes at two different bulk densities, to reflect good and poor soil structures. Perennial ryegrass was sown and rooting was allowed to establish over a 6 month period to encourage structural development. After the priming period, the runoff boxes will be saturated to two different levels. A simulated rainfall event will be imposed, and runoff water will be collected. The resulting dissolved and particulate P concentrations will be measured. Soil pore structure will be assessed at the conclusion of the trial in 2025. Intact soil cores will be extracted from each box and soil water retention curves will be measured.