Predictors of Dissolved Reactive Phosphorus Concentration in Surface Runoff Water From Major Ontario Soils.

The dependence of runoff dissolved reactive phosphorus (DRP) loss on soil test P (STP) or routine estimations of degree of P saturation (DPS) often varies with soil types. It is not clear whether the soil specific nature of the relationships between runoff DRP and DPS is due to the different sorption characteristics of individual soils, or the inability of these relatively quick methods of estimating DPS to accurately reflect the actual P saturation status of the soil. This study aimed to develop universal predictors of DRP concentration in surface runoff from a wide range of Ontario soils. The soil samples (0- to 20-cm depth) were collected from six soil series in Ontario, with 10 sites each to provide a wide range of STP values. Rainfall simulation studies were conducted following the USEPA National P Research Project protocol. A P sorption study was conducted using the equation () to describe the relationship between equilibrium P concentration (C, mg L^-1) in solution and the amount of P sorbed or desorbed by soil (Q_s, mg kg^-1), where Q_max is P sorption maximum (mg kg^-1), k represents P sorption strength (L mg^-1), and Q₀ (mg kg^-1) is the pre-existing P that was already sorbed to the soil prior to analysis. Within each soil type, runoff DRP concentration increased linearly with increasing DPS_sorp (i.e. the ratio of (Q₀ + Q_D)/Q_max) following a common slope value, while the P buffering capacity (PBC₀) at C = C₀ yielded a common change point below which runoff DRP concentration decreased greatly with increasing PBC₀ than above; where C₀ and Q_D represent the equilibrium P concentration and amount of P desorbed into a 0.03 M KCl solution during shaking, respectively. Both DPS_sorp and PBC₀ showed great promise as universal indicators of runoff DRP concentration for these Ontario soils.