Climate, soil, and viticultural factors differentially affect the sub-regional variations in biochemical compositions of grape berries

Terroir leaves its influence on the flavor build-up in grape berries by triggering biochemical reactions that ultimately shape the typicality of the produced wines. However, the complex relationship between terroir, carbon stable isotopic composition, and the biochemical composition of grapes remains poorly understood. To fill these gaps, grape berries were harvested at maturity from two grape cultivars, Cabernet Sauvignon and Merlot (Vitis vinifera L.), across 32 sites within the Eastern Foothills of the Helan Mountains region over two vintages. The climate and soil data for these sites were collected, and cultivation practices were surveyed. In parallel, the grape quality indicators were measured at maturity to explore their variations among sub-regions and the correlations between terroir factors and grape quality. Moreover, the carbon stable isotopes (δ¹³C) were measured to evaluate the water status of vines at different sites. Results indicated that organic acids and anthocyanins exhibited a strong vintage effect, with the cooler vintage exhibiting higher levels of these compounds relative to the warmer vintage. Specifically, the Hongsipu sub-region, with a cooler climate, consistently demonstrated higher anthocyanin concentrations than other sub-regions in both vintages of the study. Further analysis revealed that the cultivar Cabernet Sauvignon displayed a higher ratio of di-hydroxylated to tri-hydroxylated anthocyanins in the cooler year, whereas Merlot exhibited the opposite trend. The Mantel test identified several critical factors influencing anthocyanin concentration, including total soil phosphorus (Tp), total soil potassium (Tk), soil thickness (Thickness), precipitation (April-September precipitation, Pre4–9), April-September average temperature (4–9 Tave), April-September active accumulated temperature (4–9 Aa), April-June active accumulated temperature (4–6 Aa), July-September active accumulated temperature (7–9 Aa), temperature diurnal range (May, July, August, September, DR-5 and DR 7–9), trellis system, vine density, and δ¹³C. Finally, structural equation modeling (SEM) was used to analyze and summarize the relationships between terroir factors, anthocyanin concentration, and δ¹³C. The results indicated that June and September precipitation had a positive impact on anthocyanin concentrations, while soil total potassium content had a negative impact. On the other hand, the July to September temperature diurnal range, trellis system, vine perimeter, and canopy height were the main drivers of δ¹³C, which further influenced grape anthocyanin concentrations. This research contributes to a scientific foundation for grape cultivation in the Eastern Foothills of the Helan Mountains and provides valuable insights for practices in other regions.