Cover crops mitigate nitrate leaching in cropping systems including grain legumes: Field evidence and model simulations

Grain legume-based cropping systems need to be designed holistically by taking profit of their advantages (e.g. symbiotic fixation of N2) while reducing environmental risks. In this experiment we studied the impact of the incorporation of cover crops in grain legume-based rotations on the mitigation of nitrate leaching and recycling of N for the subsequent cash crop. A cropping system experiment (2004-2010) with three 3-year rotations of different number of grain legumes (GL0, GL1 and GL2, none, one and two grain legumes, respectively) with (CC) or without (BF, bare fallow) cover crops was established at INRA Auzeville (SW France). Soil water and mineral N contents of the entire profile (0-120 cm depth) were measured three times per season at key stages. Shoot cash and cover crop biomass and biomass N concentration were measured and the N acquisition of the different crops was calculated. In addition, the STICS soil-crop dynamic model was used to simulate the amount of daily water drained and N leached under the different rotations studied. STICS performed reasonably well when simulating soil water and soil nitrate contents, crop biomass and N acquired, allowing water drainage and nitrate leaching fluxes to be modelled with confidence. Globally, simulated N leaching was low due to amount of rainfall received during the experimental period which was lower than the 30-year average. As an average of the different crop sequences the cumulative N leaching during the experimental period (i.e. 2004-2010) increased when increasing the number of grain legumes in the rotation when no cover crops were used. However, the use of cover crops reduced N leaching. Within each rotation, cash crop N uptake did not differ between the BF and CC treatments. Our study highlights the importance of a proper design of entire cropping systems, i.e. simultaneously cash crop succession and cover crop, by adapting the crop rotation and the different management practices (i.e. N fertilization, irrigation, etc.) to mitigate the environmental impact of N leaching and reduce as much as possible pre-emptive competition phenomena due to cover crops.