Winter cover crops affect monoculture maize yield and nitrogen leaching under irrigated Mediterranean conditions

Under semiarid Mediterranean conditions irrigated maize (Zea mays L.) has been associated with nitrate pollution of surface water and groundwater. Cover crops grown during the intercrop period of maize could reduce N leaching. A 2-yr experiment was conducted in drainage lysimeters with three cover crops: barley (Hordeum vulgare L), winter rape (Brassica rapa L.), or common vetch (Vicia sativa L.). Bare soil was used as control treatment. Maize was fertilized with 300 kg ha^-1 N in the control, and this amount was reduced after a cover crop according to the N content in the aboveground cover crop biomass. Barley and winter rape biomass had a higher N content than vetch (130-170 vs. 50 kg ha^-1). The vetch treatment did not reduce N leaching or affect maize yield. The barley and winter rape treatments reduced N leaching by 80% compared to the control (25 kg ha^-1 yr^-1) mainly due to a reduction of NO₃-N concentration in drainage. Maize yield was reduced by 2.7 Mg ha^-1 after barley and winter rape but still high (≈14 Mg ha^-1). This reduction was due to an N deficiency caused by lower soil N in spring after the cover crop and insufficient N mineralization and/or lack of synchronization with maize N uptake. To use nonlegume winter cover crops to reduce N leaching in monoculture maize it is necessary to consider that N mineralization may not be sufficient to fulfill maize N requirements and N fertilizer adjustment tools should be developed.