Colloid-facilitated phosphorus (P) delivery from agricultural soils in different hydrological pathways was investigated using a series of laboratory and field experiments. A soil colloidal P test was developed that yields information on the propensity of different soils to release P attached to soil colloids. The relationship between turbidity of soil extracts and total phosphorus (TP) was significant (r2 = 0.996, p < 0.001) across a range of agricultural soils, and a strong positive relationship (r2 = 0.86, p < 0.001) was found between "colloidal P" (H2O–CaCl2 extracts) and turbidity. Linear regression of the proportion of fine clay (<2 µm) for each soil type evaluated against the (H2O–CaCl2) colloidal P fraction gave a weak but positive relationship (r2 = 0.38, p = 0.082). The relative contribution of different particle-size fractions in transporting P in agricultural runoff from grassland soils was evaluated using a randomized plot experiment. A significant difference (p = 0.05) in both TP and reactive phosphorus (RP) in subsurface flow was recorded for different particle-size fractions, with most TP transferred either in association with the 2-µm fraction or with the 0.001-µm or smaller fractions. Total P concentrations in runoff were higher from plots receiving P amendments compared with the zero-P plots; however, these differences were only significant for the >0.45-µm particle-size fractions (p = 0.05), and may be evidence of surface applications of organic and inorganic fertilizers being transferred through the soil either as intact organic colloids or attached to mineral particles. Our results highlight the potential for drainage water to mobilize colloids and associated P during rainfall events.
First demonstration of the importance of colloids as vehicles of P transport in subsurface hydrological pathways. Previous supposition was that only surface pathways were critical. All co-authors are external. Work derived from collaborative project with IGER. Heathwaite/Haygarth were PIs. Heathwaite led this paper. RAE_import_type : Journal article RAE_uoa_type : Earth Systems and Environmental Sciences