Agrifood Research Reports 53

Runoff phosphorus (P) associated with eroded soil contributes to eutrophication to some extent. The present work examines two methods for estimating the potential bioavailability of particulate P (PP) in runoff, and studies the concentrations and losses of different P forms in surface and subsurface runoff from arable soils. The potential bioavailability of PP was approximated by extraction with (i) anion exchange resin (AER-PP), giving a measure of desorbable PP under aerobic conditions, and with (ii) bicarbonate-dithionite (BD-PP), which dissolves redox-labile PP. Both methods were applied in runoff analysis without sediment preconcentration. In turbid runoff, AER extracted about 50% of the PP pool utilized by Selenastrum capricornutum in 3-week biotests. The AER-PP yields correlated well (R = 0.94) with the algal uptake of PP. The BD extraction solubilized 5–6 times the amount of PP extracted by AER. Of the total P of rock phosphates (Ca-P) and P-amended synthetic Al and Fe(III) oxides, BD extracted 0.1% (Ca-P), 7% (Al-P), and 72% (Fe-P), respectively. Concentrations of AER-PP and BD-PP in runoff were closely related to the concentration of sediment-associated P (R = 0.77–0.96), and the estimates of annual surface runoff losses of BD-PP and AER-PP were derived from the relationships between PP and these P forms. The annual AER-PP losses in surface runoff from three field sites during 4 years ranged from 13 to 270 g ha and the annual BD-PP losses from 94 to 1340 g ha. Since DRP losses at the same time equaled 29–510 g ha, runoff PP was considered to make at least as marked a contribution to bioavailable P losses as DRP. Soil P status affected the losses of all bioavailable P forms, but major fluctuations in the transport of AER-PP and BD-PP were due to the annual variation in soil losses. Therefore, erosion control was regarded as a necessary part of P loss abatement at all sites of this study. At one of the sites, drainflow P losses were 3–4 times those via the surface pathway, drainflow being the major runoff pathway. The particles carried by surface and subsurface runoff were enriched in clay-sized particles and Cs by a factor of about 2 as compared to topsoil, and contained about as much total P as did the bulk Ap horizon soil. The great drainflow losses of potentially bioavailable (topsoil-derived) PP show that soil dispersion and subsurface transport may be the major factors contributing to eutrophying P losses from subdrained clayey soils.