Microbial metagenomics of vineyard soils and wine terroir

Aims: The aims of this study were to (i) characterize bacterial and fungal communities in selected Australian vineyard soils and (ii) determine if the soil microbiome composition and diversity varied between different zones within a vineyard. 

Rationale: The soil on which vines are grown has been suggested to impart a unique quality to the grapes and wine due to the physiological responses of the vines to soil type, topography and climatic conditions, in addition to their viticultural management. The influence of bacteria and fungi in wine fermentation is well known but little is known about the effect of soil microbes, other than microbial pathogens, on grape composition or their role in vintage or site (terroir) impacts on grape composition. 

Methods and Results: We investigated the potential relationships between soil microbiome composition measured using a metagenomic approach (16S rRNA and ITS region amplicon and metagenomic sequencing) and inherent spatial variation in grape metabolite composition, specifically the concentration of the ‘impact aroma compound’ rotundone in Shiraz grapes (Vitis vinifera L.) grown in vineyards in the Grampians region of Victoria and in the Adelaide Hills in South Australia. Results from the metagenomics analysis of surface soil samples collected from the previously identified ‘rotundone zones’ in a vineyard indicated marked differences in the genetic diversity and composition of the soil bacterial and fungal microbiomes of these zones. Soils from the high rotundone zone exhibited higher diversity of bacteria, but lower diversity of fungi, compared to the soils in the Low rotundone zone. In addition, the network analysis of the microbial community in the High rotundone zone soils appeared well structured, especially with respect to the bacterial community, compared to that in the Low rotundone zone soils. A few specific taxa/groups of microorganisms e.g. Acidobacteria-GP4 and GP7, Rhizobiales, Burkholdiales, Gaiellales, Alphaproteobacteria and the Nectriaceae and Tremellaceae families of fungi, were associated with the rotundone-based variation. Short-term mulching effects did not seem to mask the rotundone zone-based variation. Predictive functional profiling using 16S rRNA marker gene sequences, FAPROTAX-based analysis indicated differences in functional categories such as manganese oxidation, methylotrophy, methanotrophy, oxidation of sulfur compounds. These findings suggest that the observed taxonomic variation needs to be translated into functional aspects of soil microbiome before mechanistic links to rotundone concentrations can be established.

Conclusions:  

Distinct differences in soil bacterial and fungal community composition and structure in different zones within the same vineyard are associated with different propensities for grape berry rotundone concentration. Also, high rotundone zone soil exhibited a well-connected microbial community network by comparison with the Low rotundone zone soil.

Significance and impact of the Study: These findings of a systematic rotundone zone-based variation in soil microbiomes paves the way to bring together understanding of microbial ecology and viticultural management for improved grape composition and wine flavour (terroir).