Soil chemical and fertilizer influences on soluble and medium-sized colloidal phosphorus in agricultural soils.

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 (Pcoll) mobilisation in soils, this study assessed the role of soil chemical properties and P fertilizer type on medium-sized soil Pcoll (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 Pcoll was mostly unreactive P associated to some extent to amorphous forms of Fe. Medium-sized Pcoll 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 Pcoll. Synthetic fertilizer increased medium-sized reactive Pcoll 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 Pcoll 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.