Indicator to predict the movement of phosphorus from soil to subsurface flow

The movement of phosphorus (P) in subsurface flow can contribute to losses from agricultural land. This study aims to identify a soil P threshold above which P loss is likely to accelerate as a function of soil and management type. Lysimeters (25 cm i.d. by 30 cm deep) were taken of four soils from agricultural watersheds in Pennsylvania and New York. The soils had a range of Mehlich-3 extractable P (7-517 mg of P kg(-1)) in surface soil (0-7.5 cm for grassland and 0-23 cm for cultivated/arable) and reactive P, filtered <0.45 microm (RP(<0.45 microm) in subsurface flow (0.007-1.53 mg of P L(-1)). The loss of P from lysimeters increased greatly when Mehlich-3 extractable P was in excess of a mean concentration of 280 mg kg(-1), the degree of saturation of P sorption sites exceeded 38%, and the corrected P sorption strength (corrected for desorption) derived from the monolayer Langmuir equation was less than 0.07 L of P mg(-1). Of these variables, P sorption strength was most consistently related to RP(<0.45 microm) in subsurface flow across a range of soil managements. Use of the corrected Langmuir sorption strength parameter to estimate subsurface flow RP(<0.45 microm) derived from four U.S. soils was tested on four soils of different physiochemical characteristics (one calcareous) from the U.K. The U.K. soils showed similar concentration trends to the U.S. soils, with elevated RP(<0.45 microm) below a sorption strength of 0.07 L of P mg(-1). We propose that the P sorption strength derived from the monolayer Langmuir equation may be used to estimate the potential for P loss in subsurface flow, when simpler environmental tests such as P saturation derived from acid ammonium oxalate extraction are unclear or unsuitable.