The use of isotopic exchange kinetics to assess phosphorus availability in overland flow and subsurface drainage waters

The loss of phosphorus (P) on overland now and subsurface drainage from soils receiving long-term applications of fertilizer and manure has been linked to the accelerated eutrophication of fresh waters. This loss is initiated by the release of P from soil to solution, which for overland now can be estimated by water extraction and for subsurface drainage waters by 0.01 M CaCl2 extraction. Although this release is rapid, the information available on the kinetics of P loss is insufficient to support improved soil P management. In this study, an isotopic exchange kinetics (IEK) approach was used to assess the effect of two solutions (water and 0.01 M CaCl2) and different soil-to-solution ratios on soil isotopically exchangeable P (Et). Results are described by a compartmental analysis that quantified the amount of P isotopically exchangeable within 1 min (E-1min), 30 min (E-30min), 24 hr (E-24hr), and between 24 hr and 3 months (E24hr-3mo). The quantity of P in each compartment was then compared with the concentration of P in overland now and subsurface drainage waters. Isotopically exchangeable soil P within 1 min (1:5 soil to solution ratio) was correlated most closely with the concentration of P in overland flow (r(2) = 0.84 with water) and subsurface drainage waters (r(2) = 0.93 with 0.01 M CaCl2). For overland now, a significant correlation was maintained at a soil-to-solution ratio of 1:100 for more exchangeable P pools than at other ratios (1:5 or 1:10). Similarly, the relationship between isotopically exchangeable P in 0.01 M CaCl2 (but not Et in water) and subsurface drainage waters P was maintained for exchange times up to 24 hr, reflecting the greater contact period of subsurface drainage water with soil compared with overland flow. The results suggest that the concentration of P in overland now and subsurface drainage waters is dependent on the rapid or short-term (1 min and < 24 hr) kinetics of soil exchangeable P and that the IEK approach can explain, and with further work may predict, the amount of P that can potentially move from agricultural soils to overland now and subsurface drainage waters.