Predicting environmental soil phosphorus limits for dissolved reactive phosphorus loss

The dependence of runoff dissolved reactive phosphorus (DRP) loss on soil test P or rapid estimations of degree of P saturation (DPS) often varies with soil types. It is not clear whether the soil-specific nature of runoff DRP versus DPS is due to the different sorption characteristics of individual soils or the inability of these rapid DPS estimates to accurately reflect the actual soil P saturation status. This study aimed to assess environmental measures of soil P that could serve as reliable predictors of runoff DRP concentration by using soils collected from Ontario, Canada, that cover a range of chemical and physical properties. A P sorption study was conducted using the Langmuir equation Qs+Q0Qmax=kC1+kC to describe amount of P sorbed or desorbed by the soil (Qs, mg/kg) versus equilibrium P concentration (C, mg/L) in solution, where Qmax is P sorption maximum (mg/kg), k represents P sorption strength (L/mg), and Q0 (mg/kg) is the P sorbed to soil prior to analysis. Runoff DRP concentration increased linearly with increasing DPSsorp (i.e. the ratio of (Q0 + QD)/Qmax) following a common slope value amongst soil types, while the P buffering capacity (PBC0) at C = C0 yielded a common change point, below which runoff DRP concentration decreased greatly with increasing PBC0 compared to that above the change point, where C0 and QD represent the equilibrium P concentration and amount of P desorbed, respectively. Both DPSsorp and PBC0 showed great promises as indicators of runoff DRP concentration.