Influencia del uso de cubiertas vegetales sobre la fertilidad del suelo, la expresión vegetativa de la vid y la composición de la uva y del vino en un viñedo de Tempranillo (Vitis vinifera L.) de la D.O.C. Rioja stars

Resum

Tillage is the most commonly used soil management technique in vineyards with Mediterranean climate. In order to improve some of the properties of soils as well as the productive and qualitative characteristics of the vines, alternative techniques such as cover crops are employed. Therefore, the main objective of this Thesis was to study the effect of two cover crops, with different agronomic performance, on the soil nutrients availability, the productive and vegetative parameters of the vine and the grape and wine composition, comparing these results to tillage. In this context, an experiment with three different soil management treatments in the inter-row of the vineyard, was carried out for four years (2009-2012). Treatments were: tillage, a cereal cover crop (barley, Hordeum vulgare L.), and, a leguminous cover crop (clover, Trifolium resupinatum L.). The vineyard was a cv. Tempranillo (Vitis vinifera L.), planted in 1999 with a 2.70 x 1.30 m spacing and a VSP (double cordon Royat) trellis system. Soil was classified as Oxyaquic Xerorthent, its pH was basic and the organic matter content was low (< 1 %), texture was loamy for the topsoil and sandy loam in deeper horizons. Results showed differences in soil moisture between treatments for short periods of time from July until the end of August, especially in the clover cover crop treatment at 60 cm deep. The water consumption rate was higher for both cover crops treatments with respect to tillage, although rainfall recharged the soil profile similarly in the three treatments. Regarding soil nutrients availability, the nitrogen fixation capacity of the leguminous crop marked a difference and, in fact, the soil nitrogen availability, in its nitrate form, which is the one that the vine assimilates primarily, was higher in the clover crop treatment from the second year of the experiment with respect to tillage. However, the nitrate form from the soil N was lower for the barley cover crop. This reduction was linked to the nitrogen content of the cover crop aerial biomass. Furthermore, the reintroduction of the cover crop remains in the soil after withering favoured a greater availability of potassium, magnesium and sodium in the soil surface in the fourth year of trial and for the clover treatment when compared to the other two treatments. Cover crops did not affect the phosphorus and calcium soil availability. Although no significant differences were observed in the total biomass production of the two cover crops, from 2010 onwards the concentration of nitrogen, potassium, magnesium, calcium, copper, boron and sodium was higher for clover than for barley cover crop, which reversely had a higher concentration of iron and zinc. However, the different competition of the cover crops for nutrients only had a significant effect over the leaf nitrogen content in the barley treatment in 2011 and 2012. Regarding the vegetative development, the shoot longitudinal development in the barley treatment decreased at bloom in the third year of trial, and the total leaf weight was lower in both cover crops. Cover crops did not significantly change the vineyard production either; although the vine vigour was lower in the barley treatment during the last two years, with lower values of shoot and pruning weights with respect to the other two treatments. Thanks to the vigour reduction, a better balance between vigour and production was achieved with the barley treatment and, therefore, Ravaz index values were within the optimal range for cv. Tempranillo in the A.O.C. Rioja. Regarding the cover crops impact on the grape characteristics, only a berry weight reduction in 2010 and an increase of phenolic compounds in 2012 were observed for the barley cover crop. Levels of Yeast Assimilable Nitrogen (YAN) in the musts changed in the cover crop treatments with respect to tillage; in the clover treatment YAN values were higher in the fourth year of the trial whilst they were lower for barley. Similarly, the presence of cover crops modified the amino acid profile of musts. Total titratable acidity as well as the tartaric acid content and the total polyphenol index showed higher values in the barley than in the other two treatments; as well as the concentrations of biogenic amines and most of the amino acids studied showed lower content for barley treatment. Organoleptically, tasters did not appreciate significant differences between wines, although they showed a preference for those made from the tillage treatment grapes. With all these results in mind, it can be concluded that under the conditions of our experiment, cover crops can be an appropiate alternative to tillage as, in the long term, they can regulate both, the vine vegetative development and its yield, therefore improving the quality of grape and wine and, at the same time, their use does not imply an excessive competition with the vine for both water and nutrients.