Competition for nitrogen in an unfertilized intercropping system: The case of an association of grapevine and grass cover in a Mediterranean climate

Cover cropping is currently increasing in vineyards as it provides solutions to some of the problems encountered in vine growing. However, its development is still hampered in Mediterranean regions because of fears of severe competition for water. Recent studies have shown that soil resources other than water may also be restrictive, and particularly nitrogen. Over a three-year period, the effect of introducing a cover crop was studied with respect to the temporal and spatial changes to nitrogen dynamics in a Mediterranean vineyard. The experiment compared the impact of three different types of soil cover management on nitrogen dynamics, and particularly on soil nitrogen mineralization which is the principal source of inorganic nitrogen in situations with no application of N fertilizers which are frequent in viticulture. This experiment provided evidence that the presence of an intercrop significantly reduced nitrogen accumulation in aerial parts of grapevine during the year due to competition for soil resources. This reduction varied markedly between years and treatments, and was more pronounced during dry years. The competition for nitrogen was direct as intercrop deprived grapevine of soil nitrogen beneath the inter-row and caused grapevine uptake to be higher beneath the row. In deep soils, a grapevine can adapt its root system in order to access deeper water resources, but it then partially abandons the mineralization zone containing most inorganic nitrogen. Competition for nitrogen was less marked with a temporary cover crop than with a permanent one, because of the shorter period of uptake with the former and the time needed for an annual cover crop to develop its root system each year. Intercrop also competed indirectly for nitrogen with grapevine as it took up soil water and made inorganic nitrogen less mobile and accessible to grapevine. Intercrop markedly decreased soil nitrogen mineralization. Although it did not significantly affect organic matter characteristics or soil temperature, it clearly modified the water regime. Indeed, under either temporary or perennial cover crops, the upper soil layers dried more rapidly than when there was only evaporation from bare soil. Consequently, nitrogen mineralization decreased faster with intercropped treatments and halted prematurely during the summer. The earliness of the reduction in nitrogen accumulation in intercropped vineyards also suggested that a lower level of nitrogen transfer to perennial reserves was involved. Indeed, grass cover grows and competes for nitrogen during the autumn which is a favourable period for nitrogen accumulation in wood reserves. Although better water infiltration was observed in the presence of a cover crop (notably in the autumn), the favourable conditions for nitrogen mineralization were propitious for grass cover growth and uptake. Consequently, intercrop reduced grapevine growth of the year but also the potential growth for the next year by decreasing grapevine nitrogen perennial reserves. (Résumé d'auteur