Winter Phosphorus Release from Cover Crops and Linkages with Runoff Chemistry

Abstract

Cover crops (CC) have both agronomic and environmental benefits but also have the potential to increase losses of dissolved reactive P after freeze-thaw cycles (FTC). This field study, conducted over one nongrowing season (NGS) in Ontario, Canada, characterized water-extractable P (WEP) content in different CC species and compared observed changes in plant WEP content with changes in P content in soil, surface runoff, and shallow groundwater (5-25 cm). Five plots (0.4 ha) of cereal rye (Secale cereal L.), oilseed radish (Raphanus sativus L. var. oleoferus Metzg Stokes), oat (Avena sativa L.), and hairy vetch (Vicia villosa Roth) were established after winter wheat (Triticum aestivum L.) harvest. Throughout the NGS (October-April), CC shoot tissues and surface soil were routinely sampled for WEP analyses, and groundwater and runoff water samples were collected after rain and snowmelt. Responses to FTC varied among CC species, with P released from frost-intolerant species but not frost-tolerant species. Although CC released P, the top 5 cm of soil contained greater WEP than plants at all times, and the changing WEP content in CC over the NGS was not reflected in soil or water P concentrations. These results suggest that the degree of frost exposure should be considered in the selection of CC species in cold regions; however, in temperate regions with snow cover that insulates the soil surface from heavy frost, P release from vegetation may not lead to increased P loss in runoff.