Runoff and drainage trade-offs from cover crops exposed to freeze–thaw events

Winter cover crops are widely promoted for multiple ecosystem services although nutrient leaching from the plant biomass and subsequent transport of those nutrients is possible after freeze–thaw events. The objective of this study was to determine the effect of freeze–thaw conditions on both runoff and drainage or leachate water quality from two common cover crops. Rainfall simulations were performed on packed columns (20 cm diam.; 56 cm length) that included triplicate combinations of cereal rye (Secale cereale L.), forage radish (Raphanus sativus L. var. longipinnatus), and bare soil and a heavy, light, and no freezing treatment. The two cover crops reduced runoff total suspended solids concentrations and nitrate leaching compared with a bare soil control, confirming many cover cropping benefits. The heavy freezing conditions (including two nights of <–15 °C) resulted in significantly higher total phosphorus and nitrate concentrations in both runoff and leachate from the cereal rye, which had been killed, compared with unfrozen cereal rye. Some of the conditions tested in this lab-scale study could be considered extreme (e.g., 10.1 cm h⁻¹ rainfall × 2 h, −15.7 °C) but these results nonetheless help demonstrate the importance of effective conservation practices for both surface and subsurface flows as well as for multiple water quality parameters in the face of increasing weather variability.