Colloidal phosphorus transfer processes in the soil-groundwater-surface water continuum

Abstract

To better understand the critical processes controlling where and when phosphorus (P) can be transferred to groundwater (GW) and later delivered to catchment streams, this study investigated highly mobile soil colloidal P (Pcoll) and other P fractions in the soil-groundwater-surface water continuum. ​​​​​​​Three experiments were undertaken. Firstly an investigation into soil chemical and fertilizer influences on soluble (<450 nm) and total medium-sized colloidal P (TPcoll; 200-450 nm) at the batch scale; secondly an investigation into static and dynamic physical controls on water flow at the soil profile scale, and implications for P transfer to shallow GW; and finally, at the hillslope scale, an investigation into meteorological influences on the below-ground delivery of TPcoll and total fine, nano-particulate and truly dissolved P (TFPcoll; <200 nm) fractions to streams. Soil Fe content and degree of P saturation increased soil TPcoll and soluble P, respectively. Synthetic fertilizer influenced TPcoll while cattle slurry did not; the latter increased soluble P. The downslope and midslope zones had similar soil hydraulic properties and subsurface water flow dynamics. Inter-annually, higher total rainfall increased water flow peaks and shallower GW level mobilised soil P which further increased GW P concentrations. The Al-rich catchment with lower soil (macro)porosity and higher bedrock hydraulic conductivity was dominated by fine reactive P (FRPcoll) in GW and in the stream. The Fe-rich catchment with higher soil (macro)porosity and lower bedrock hydraulic conductivity was dominated by coarser particulate P in GW and by FRPcoll in the stream. Rainfall event characteristics controlled the dominating hydrological flowpaths and mobilised P sources which led to contrasting contribution of TFPcoll in a receiving stream between rainfall events, even though the GW P contribution was similar. These Pcoll investigations could contribute to a better identification of the critical areas and critical times where and when TPcoll and TFPcoll can be transferred to GW and later delivered to the stream. These findings can be used to better target costeffective mitigation measures to further reduce diffuse P pollution and support sustainable food production.

Date of Award	Feb 2021
Original language	English
Sponsors	Teagasc - Irish Agriculture and Food Development Authority
Supervisor	Phil Jordan (Supervisor) & Joerg Arnscheidt (Supervisor)