Colloid-facilitated transport can be important for preferential transfer of phosphorus (P) through the soil profile to groundwater and may in part explain elevated P concentrations in surface water during baseflow and particularly high flow conditions. To investigate the potential for colloidal P (P coll) mobilisation in soils, this study assessed the role of soil chemical properties and P fertilizer type on medium-sized soil P coll (200–450 nm) and its association with soil solution soluble bioavailable P (<450 nm). Hillslope soils from three agricultural catchments were sampled and untreated and treated (cattle slurry and synthetic fertilizer) subsamples were incubated. Soil supernatants were analysed for P and soil Water Dispersible Colloids (WDC) were extracted for analysis of P and P-binding materials. Soils physicochemical properties including degree of P saturation (DPS) and P sorption properties were determined. Results indicated that medium-sized P coll was mostly unreactive P associated to some extent to amorphous forms of Fe. Medium-sized P coll concentrations correlated negatively with soil maximum P sorption capacity and soluble P concentrations increased with increasing DPS. In soil with low sorption properties, cattle slurry increased soluble P concentrations by 0.008–0.013 mg l −1 and DPS but did not influence medium-sized P coll. Synthetic fertilizer increased medium-sized reactive P coll by 0.011 mg l −1 (0.088 mg kg −1 soil) and DPS in a soil with lower DPS whereas it decreased it by 0.005 mg l −1 (0.040 mg kg −1 soil) in a soil with higher DPS. Additional soil parameters (M3-Fe, M3-Al, M3-P, and DPS) should be included in soil testing, especially in Cambisol/Podzol soils, to identify critical areas where risks of P coll mobilisation are important. Further research should include the roles of finer colloidal and nanoparticulate (<200 nm) soil P fractions and soluble P to inform understanding of plant uptake and assess environmental risk.
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We thank the ACP land owners and farmers for cooperation and field sampling permission, the ACP staff especially David Ryan and Dermot Leahy for field sampling assistance, Gérard Gruau and Sen Gu from Géosciences Rennes for advices on the colloidal fractions. The authors also thank Denis Brennan, Maria Radford, Linda Moloney-Finn and Brendan Healy for laboratory support and services. The lab assistance of Shane Scannell and Shannon Devereux is greatly appreciated. Funding was provided by the Department of Agriculture, Food and the Marine through the Teagasc Agricultural Catchments Programme and by the Teagasc Walsh Fellowship Programme.
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- Soil water dispersible colloids, Mobilisation, Cattle slurry, Synthetic fertilizer, Agriculture, Groundwater pollution