Release of soil phosphorus during runoff as affected by ionic strength and temperature

Dissolved reactive phosphorus (DRP) from two cultivated clay soil samples (Vertic Cambisols) was extracted under conditions simulating the variation in the properties of surface runoff water in the field. DRP was extracted at three temperatures (5, 15 and 25 degrees C), and at different ionic strengths by using deionized water and CaCl2 solutions (0.00005-0.005 M) as extractants. The solution-to-soil ratio varied from 50 to 2000 l kg(-1). Sorption to and desorption from the soils were studied at different temperatures and ionic strengths by determining quantity-intensity (Q/I) plots at the solution-to-soil ratio of 50 l kg(-1), and the results were fitted to a modified Langmuir equation: Q = Q(max)I/(1/K + I) - Q(0) where Q is P sorbed or desorbed, Q(max) = maximum P sorption, I = P concentration in the equilibrium solution, K = sorption/desorption equilibrium constant, and Q(0) = instantly labile P. The desorption of DRP was depressed by increases in the CaCl2 concentration of the extractant and promoted by widening of the solution-to-soil ratio. At the solution-to-soil ratio of 50 l kg(-1), the increase in the temperature from 5 to 25 degrees C raised the DRP release to water from 12.6 to 20.7 mg kg(-1) in the Aurajoki soil and from 1.8 to 3.4 mg kg(-1) in the Jokioinen soil. In the Aurajoki soil, the constant Q(0) of the Langmuir equation responded to the changes of ionic strength and temperature in the same way as did DRP extracted at wide solution-to-soil ratios. However, the P release capacity of both soils was underestimated by the constant Q(0).