Ground vegetation covers increase grape yield and must quality in Mediterranean organic vineyards despite variable effects on vine water deficit and nitrogen status

In the context of a warming climate and widespread soil degradation, successful soil management practices in Mediterranean vineyards should combine environmental (e.g., soil health) and productive (yield and must quality) objectives. With this objective, we tested five soil management practices in two organic farms in Chianti Classico (Italy) across three years. Five treatments were compared: conventional tillage (CT), spontaneous vegetation (S), soil-incorporated cover crop of pigeon bean (Vicia faba L. var. minor (Peterm. em. Harz) Beck. L.) (PBI), a cover crop mixture of barley (Hordeum vulgare L.) and clover (Trifolium squarrosum L.), either mulched (BCM) or incorporated in soil (BCI). We explored the effects of soil management practices on vine stress (SPAD and stem water potential), grape production (yield per plant; number of clusters per plant; cluster weight; berries weight) and must quality (titratable acidity; malic acid; pH; sugar concentration; yeast assimilable N; potential anthocyanins and total polyphenol index). Soil variability was taken into account in the statistical analysis, by testing two sets of soil covariates. A first dataset included the “raw” electrical conductivity and gamma ray total counts. The second dataset consisted in a set of soil covariates obtained by combining data collected by the proximal sensors with the results of the chemical analyses. We found that soil management affected SPAD and stem water potential with variable effects between farms and years. Mulched cover crops showed lower vine SPAD values than tilled treatments at both farms, especially in 2019 and 2020, while spontaneous vegetation effects varied considerably across farms and were comparable to tillage. Conventional tillage also decreased vine water stress compared with S, especially at the colder site in 2020. Mulched cover crops and tillage treatments had similar vine stem water potential at the warmer site. Significantly higher grape yields were found under PBI and S (about +30% compared with the other treatments), mainly due to higher cluster weight. The most productive treatments (PBI and S) also showed higher pH and malic acid concentration but lower anthocyanins and total polyphenol index as compared with the other treatments. Conventional tillage increased yeast assimilable N in 2019 while S showed the lowest values, probably due to a drop in the abundance of N-fixing plant species. On a methodological side we found that including soil parameters as covariates, instead of ECa readings and gamma ray total counts, improved regression models for all the dependent variables studied except for juice pH. Overall, our results indicate that groundcovers induced only a moderate and temporary stress that affected grape production and quality differently. While the barley-clover mixture significantly reduced grape production irrespectively of termination type, S and PBI were associated with higher grape yields. Overall, this study demonstrated that groundcovers can be profitably introduced in vineyards also in Mediterranean climates with positive effects on yields and quality.