Dissolved phosphorus leaching reflects the dynamic interaction between hydrology and soil phosphorus kinetics

Authors: MUMBI, ROSE - Purdue University; Williams, Mark; FORD, WILLIAM - University Of Kentucky; Penn, Chad; CAMERATO, JAMES - Purdue University

Submitted to: Hydrological Processes
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/22/2025
Publication Date: 10/5/2025
Citation: Mumbi, R.C., Williams, M.R., Ford, W.I., Camerato, J.J., Penn, C.J. 2025. Dissolved phosphorus leaching reflects the dynamic interaction between hydrology and soil phosphorus kinetics. Hydrological Processes. https://doi.org/10.1002/hyp.70285.
DOI: https://doi.org/10.1002/hyp.70285

Interpretive Summary: Water bypassing the soil profile through cracks and earthworm burrows through preferential flow pathways is hypothesized to deliver phosphorus-rich water directly to subsurface tile drains. In this study, we collected undisturbed soil cores (1 ft3) to study how rainfall intensity, soil moisture, and direct connectivity through an artificial macropore impacted dissolved reactive phosphorus (DRP) transport through the soil. Neither rainfall intensity nor soil moisture significantly impacted DRP losses because of the substantial hydrologic variability among the soil cores. Results showed that both the water source (rainfall vs. soil water) and the ability of the soil to adsorb or desorb phosphorus as a function of how fast the water was moving through the profile controlled DRP losses. Water moving through the soil too rapidly in preferential flow had little time to desorb phosphorus from the soil and, as a result, DRP concentrations were low. Water moving too slow through the soil matrix had enough time for the soil to re-adsorb phosphorus also resulting in low DRP concentrations. When conditions were just right, enough contact between the infiltrating rainfall and the surface soil followed by rapid flow bypassing the adsorption capacity of deeper soil layers, high DRP concentrations were observed. Determining the mechanisms and processes impacting DRP transport can help improve computer simulation models, but more importantly be used to better understand how agricultural practices such as drainage water management, tillage, and nutrient management affect nutrient loss.

Technical Abstract: Preferential flow decreases subsurface residence time causing faster and less attenuated water and nutrient transport. While it is ubiquitous in soils, the effect of rainfall intensity and soil moisture on preferential flow and phosphorus transport is not well understood. In this study, we investigate the effect of rainfall intensity, soil moisture, and the degree of hydrological connectivity on dissolved reactive phosphorus (DRP) losses in subsurface leachate. Ten undisturbed lysimeters (30×30×30 cm) were collected from a drained agricultural field in Indiana, USA. A series of seven rainfall simulations were conducted under varying rainfall intensity (n=30 events), soil moisture (n=28), and connectivity (n=12) conditions. Stable water isotopes were used to separate leachate into event and pre-event water components. Results showed that the effect of rainfall intensity and soil moisture on DRP FWMC varied among lysimeters, with both connectivity and soil adsorption/desorption kinetics controlling subsurface DRP transport. Leachate that was comprised of either >90% event water or <10% event water had the lowest DRP FWMC (0.12-0.85 mg L-1) suggesting 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 (0.97-3.14 mg L-1) resulting in a parabolic relationship between water source/soil contact time and DRP FWMC. Results showed that rainfall infiltration and mixing with surface soil water promoted DRP desorption, with subsequent matrix-derived preferential flow facilitating the transport of DRP-rich water through the subsurface. Quantifying the connection between soil P kinetics and hydrologic connectivity provides new insights into the impacts of preferential flow dynamics on DRP loss and is essential for predicting subsurface DRP transport and developing management practices for decreasing the risk of DRP loss.