Dissolved Phosphorus Leaching Reflects the Dynamic Interaction Between Hydrology and Soil Phosphorus Kinetics

Hydrologic processes and soil phosphorus adsorption/desorption kinetics affect subsurface nutrient transport; however, their interaction is not well understood. In this study, we investigated the effect of hydrologic variables including flow rate, preferential flow, soil–water contact time, rainfall intensity, and soil moisture on dissolved reactive phosphorus (DRP) leaching. Ten undisturbed soil columns (30 × 30 × 30 cm) were collected from an agricultural field in Indiana, USA. Seven rainfall simulations were conducted under varying rainfall intensity and soil moisture conditions to create an array of subsurface flow rates. Results showed that leachate flow rates, preferential flow, soil–water contact time, and DRP concentration varied substantially among soil columns and rainfall simulations, with both connectivity and soil adsorption/desorption kinetics controlling DRP transport. Leachate comprised of either > 90% or < 10% event water had the lowest DRP flow-weighted mean concentration (FWMC; 0.12–0.85 mg L⁻¹). This suggests that minimal and maximum soil–water interaction yielded small DRP desorption from the surface soil and large DRP adsorption in subsoils, respectively. Leachate that was comprised of a mixture of water sources tended to have the greatest DRP FWMC (0.97–3.11 mg L⁻¹) resulting in a parabolic relationship between water source/soil contact time and DRP. Rainfall infiltration and interaction with surface soil promoted DRP desorption, with subsequent matrix-derived preferential flow facilitating the transport of DRP-rich water through the subsoil. Quantifying the connection between hydrology and phosphorus kinetics provides new insights into the impact of preferential flow on DRP leaching and is essential for predicting DRP transport and developing management practices for decreasing DRP loss.