Phosphorus sorption and desorption as affected by long-term cover cropping at two soil surface depths

Abstract

Phosphorus (P) loss from agricultural land is a persistent environmental challenge, and a better understanding of the impact of continuous cover crops (CCs) growth on soil P sorption and desorption characteristics is needed to inform mitigation strategies. This study investigated the impact of CC species on soil P pools, sorption characteristics, and dissolved reactive P (DRP) after 9 yr. Soil samples were collected at 0-to-2- and 2-to-4-cm soil depths from a silty clay loam Mollisol. Treatments included cereal rye (Secale cereal L.; CR), annual ryegrass (Lolium multiflorum, AR), oats/radish (Avena sativa L./Raphanus sativus L.; OR), and no CC (CN). A sorption experiment was done with varying P concentrations for 24 h equilibration, and sorption parameters were estimated using the Langmuir model. The DRP was estimated using sequential soil extraction by 0.01 M CaCl₂ for 5 h. Long-term CC significantly decreased P sorption maximum but increased binding energy relative to CN. Annual ryegrass significantly decreased soil water extractable P, Mehlich 3 P, and degree of P saturation relative to OR and CN at the 0-to-2-cm depth. Annual ryegrass and CR significantly decreased desorbed DRP by an average of 42 and 45% relative to CN and OR, respectively, at the 0-to-2-cm depth. These results demonstrated that long-term grass species decreased the concentrations of labile P pools and desorbed DRP at the soil runoff interaction zone. Therefore, planting of AR and CR should be promoted in fields susceptible to runoff DRP losses.