Catch Crops to Mitigate Phosphorus Leaching Under Cold Climate.

Nutrient leaching from agricultural land contributes greatly to the eutrophication of recipient water bodies. Catch crops (cover crops) used for reducing nitrogen leaching are proposed to mitigate phosphorus (P) leaching too. However, in regions with winter frost, catch crops may act as a source of P losses after plant cell lysis. A field study on two clay soils in south Sweden in 2009-2012 examined eight potential P catch crops; chicory (Cichorium intybus L.), cocksfoot (Dactylis glomerata L.), perennial ryegrass (Lolium perenne L.), red clover (Trifolium pretense L.), phacelia (Phacelia tanacetifolia L.), white mustard (Sinapis alba L.), and radish (Raphanus sativus L.) with the latter represented by both R. oleiformis and R. longipinnatus. Soil columns with catch crops and controls were collected in late autumn and used in leaching experiments. The columns were undergone seven repeated freezing-thawing cycles (FTCs) in the laboratory and irrigated with 70 mm simulated rainfall both before and after FTCs. The P content in leachate was analyzed after each irrigation. Leaching of total-P increased after FTCs compared with before (p=0.045) and varied significantly with year (p<0.0001) and species (p=0.037). Chicory and white mustard had lower total-P leaching than the control, while radish (R. oleiformis) and red clover had highest total-P leaching of all species. Ryegrass and radish (R. oleiformis) were the most sensitive species to frost in terms of causing the largest increase in total-P concentrations compared with the control. Radish (R. longipinnatus), cocksfoot, and chicory were the least sensitive crops and caused no significant increase in P leaching after FTCs. It was concluded that the effect of catch crops on P leaching differs between years and that perennial ryegrass, commonly grown as a catch crop in the Nordic countries, tends to leach more P than other potential catch crops after exposure to severe frost.