Effect of Organic Soil Amendments and Vineyard Topographic Position on the Chemical Composition of Syrah, Trincadeira, Alicante Bouschet, and Antão Vaz Grapes (Vitis vinifera L.) in the Alentejo Wine Region

Climate change and unsustainable agricultural practices are triggering land degradation in semi-arid Mediterranean regions. Organic amendments, such as mulching materials, have shown promising potential to mitigate these impacts by improving soil chemical, physical, and biological properties, while enhancing grapevine growth and productivity. This study evaluated the effects of wheat straw mulch (M) and wheat straw combined with biochar (MB), together with vineyard topography (bottom vs. top), on grape chemical and phenolic composition in four Vitis vinifera L. cultivars (Syrah, Trincadeira, Alicante Bouschet, and Antão Vaz) grown in the Alentejo wine region. Grapes were sampled separately at top and bottom topographic positions, and classical and phenolic parameters were analyzed. The application of M and MB significantly modified must composition, mainly through changes in nitrogen and sugar levels across topographic positions. Only MB exhibited stronger effects, enhancing must quality, while MB and M reduced bottom–top variability. Similar patterns and positional effects were observed for phenolic and color parameters. Both organic treatments lowered total monomeric anthocyanin concentrations, although positional differences with wheat straw mulch were found. The results highlight that combining soil management with topography and variety response can optimize grape phenolic composition and promote sustainable viticulture through targeted, site-specific mulching strategies.