Soil and water conservation dilemmas associated with the use of green cover in steep vineyards

Vineyards are customarily managed using tillage to eliminate competition from other plants. If the vineyards are on a slope, however, this process leads to considerable soil loss. The impacts of the practice on organic matter and the associated nutrient depletion can pose a threat to soil and water conservation. This paper considers three active rain fed vineyards in central Spain. Over two years, during which the erosivity of the rains was very low, we measured erosion rates between 35 and 179 g m−2 y−1 using erosion plots of 2 m2. The associated sediment had an organic matter Enrichment Ratio ranging from 1.4 to 2. Values similar to these were found for nitrogen and phosphorus. These findings indicate that alternative soil management practices, such as the use of vegetation cover, should be considered. A permanent cover consisting of Brachypodium distachyon was tested, as were covers of barley, rye and spontaneous vegetation cut in the spring. These tests resulted in reduced soil loss. The treatments exhibited erosion rates between 2 and 32 g m−2 y−1. We also noted a decrease in the runoff coefficient. The average runoff coefficients for the study period were 5% for tilled soil, 0.9% for permanent covers and 1.4% for cut covers. We note that the infiltration benefits of tillage were only temporary because on an annual scale, the change in the soil structure led to sealing and increased runoff and erosion. Nevertheless, the moisture in the soil during the vineyardʹs vegetative period was higher in traditionally tilled treatments. Although the transpiration through the permanent cover was offset by the ability of the cover to favor infiltration, the cover competed actively for water and resulted, on average, in 40% lower grape yields, with variations among the different vineyards. A similar decrease in yield (44%) was noted for the cover crop consisting of cut spontaneous vegetation. The cut cover crops of rye and barley did not significantly reduce grape yield.