Large-scale assessment of optimal emergence and destruction dates for cover crops to reduce nitrate leaching in temperate conditions using the STICS soil-crop model

Cover crops can be an efficient agricultural practice to reduce nitrate leaching during the fallow period and decrease nitrate pollution of aquifers. For maximum reduction of nitrate leaching, cover-crop management must be adapted to the pedoclimatic context. We used the soil-crop model STICS to assess optimal emergence dates in summer and destruction dates in autumn and winter for three species of cover crops (mustard, ryegrass and vetch) at a large scale in France. We first calibrated and validated STICS's predictions of soil and crop dynamics under a wide range of conditions, including bare soil and cover crops. Since predictions were sufficiently accurate, we then predicted the impact of several emergence and destruction dates of cover crops on nitrate leaching in 24 contrasting climatic sites over 20 years. These sites represented a wide range of conditions found in France and much of Europe. Using generalized linear model (GLM), we extrapolated to the vulnerable nitrate zones in France the optimal dates predicted for the 24 sites. We defined optimal emergence date as the date allowing the higher nitrate leaching decrease in drained water and optimal destruction dates as those for which a high reduction of nitrate leaching was predicted and that also avoided negative effects on the subsequent main crop and water resources. STICS accurately predicted mineral N and water dynamics from both calibration and validation datasets, despite lower accuracy for vetch simulations. Over the 20-year simulations, optimal dates varied, depending mainly on site, cover crop and year. Optimal emergence dates for cover crops were earlier for vetch (late July) than for ryegrass (early August) and mustard (late August) and earlier in the north (cold and rainy) than in the south of France (warm and dry). Optimal destruction dates, as defined in this paper, were frequently between October and December. Mean optimal dates by site and species were strongly correlated with climatic characteristics of the 24 sites tested, which enabled us to extrapolate them to a larger scale and identify areas with similar optimal emergence and destruction dates according to cover-crop species. Later optimal emergence dates occurred in southeastern France in late August and September, while the earliest dates occurred in late July and early August in the north and east of France. The extrapolated optimal destruction dates occurred in autumn (October-December), with later destruction dates located in zones with higher temperature in the western coastal zones of the Atlantic Ocean and in southeastern France. (C) 2015 Elsevier B.V. All rights reserved.