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By Cristina Lazcano, Charlotte Decock and Stewart Wilson

In no other crop is the importance of environmental conditions for the quality of the final product appreciated as much as in winegrapes. The effects of the soil on wine quality, when combined with climate, topography and grapevine variety are encompassed in the well-known concept of terroir. Inherent soil properties such as the depth, texture mineralogy, nutrient content, infiltration and water holding capacity, are known to have an impact on the grape chemical composition, therefore affecting wine quality.

Vineyards arguably occupy a broader range of soil types than any other crop. In the old world, where winegrape production dates back to the Roman empire, vineyards were often established on marginal land such as steep slopes, reserving the most fertile soils for cultivation of cereals, and other food crops. On the other hand, extensive availability of land in the new world offered deep, alluvial, fertile soils for vineyard establishment, and the opportunity for large scale intensive production and mechanization.

Winegrapes have generally lower demand for water and nutrients than other crops; nevertheless, they frequently receive intensive management including tillage and the use herbicides to eliminate competing vegetation, being highly susceptible to soil degradation. Vineyards on slopes and marginal lands especially are at elevated risk for soil degradation, causing declines in soil quality and fertility, reduced water infiltration and storage, impaired quality of ground and surface water, diminished air quality, and risks associated with climate change mitigation and adaptation. The permanent loss of soil could not only significantly and permanently damage the wine industry, but also contribute to overall global environmental degradation. Soils are known to store more carbon than the planet’s atmosphere and vegetation combined. Thus, sequestering carbon by storing it in soils, constitutes one of the most promising solutions for climate change.

There is general consensus of the urgency to protect soils, not only as non-renewable resources but also as natural living bodies that provide essential ecosystem services to support crops and environmental health. This “continued capacity of soil to function as a vital living ecosystem that sustains plants, animals, and humans,” has been defined as Soil Health by the United States Natural Resources Conservation Services (NRCS), a term embraced by academia, industry, governmental and non-governmental organizations alike.

Soil health is defined by a series of physical, chemical and biological properties. Several of these properties are also important components of the terroir, usually static soil properties that are not directly affected by management. However, the concept of soil health particularly emphasizes dynamic properties of soil and the support of functions which are, in most cases, directly and strongly affected by management. Because crop quality is one of the outcomes of a healthy soil, we postulate that a healthy vineyard soil must optimally express its terroir. While the concept of terroir and its relationship with crop quality is well understood and deeply engrained in the context of wine production, the concept of soil health has recently gained acceptance by growers worldwide and there is an urgent need to reconcile these two concepts.

In particular, the role of soil biota and the soil microbiome for wine grape production needs to be carefully evaluated. The soil microbiome regulates many important ecological processes such as decomposition and nutrient cycling, being ultimately responsible for carbon sequestration and crop nutrition. Therefore, the soil microbiome is essential for soil health. In addition, several recent studies show that the soil serves as a reservoir of microorganisms that, through wind in dust particles, rain drops, or people during harvesting, end up colonizing the grapes and are carried over to the fermentation stage, influencing wine quality. Because the soil microbiome varies depending on the soil type, scientists have recently proposed the concept of the microbial terroir.

Nevertheless, research on the microbial terroir is still in its infancy and we lack a good mechanistic understanding of the connections between soil microorganisms and grape and wine quality. Among some of the open questions is how soil management practices can be tailored to support soil biota and therefore soil health, while contributing to grape quality. The primary focus on healthy soil management practices is usually on increasing soil organic matter content, because of its fundamental role in supporting many biological, physical and chemical dynamic aspects of soil health. Over the last 15 to 20 years, studies have examined the role of soil management practices in vineyards of Spain, France, South Africa, California and Australia. These practices include use of cover crops, reduced till, compost and other organic amendments. Recent studies in California by Dr. Lazcano and Dr. Decock are also evaluating the use of sheep grazing to manage vineyard cover crop (Figure 1).

Figure 1. Sheep grazing the cover crop at a field trial established at Tablas Creek Vineyard (Paso Robles, California, USA).

Even though these studies typically show improvements on soil organic matter, the observed benefits for soil health, crop yield and final grape quality seem to be highly variable between studies which prevents the establishment of guidelines and best management practices for wine grapes. This lack of clear and consistent outcomes in the literature likely stems from the wide variability in inherent soil health properties which affect more dynamic soil properties. For instance, soil texture influences carbon sequestration potential. In a recent project supported by the American Vineyard Foundation and in collaboration with the Napa Valley Grapegrower Association, Dr. Lazcano and Dr. Decock are studying variability in soil carbon and other soil health indicators among the different soils of Napa Valley in California. The final goal is the establishment of a baseline of soil health properties for different soil types that will help growers know what to expect when using sustainable management practices. 

In summary, vineyards have a great potential to sustain high levels of soil health and to sequester C, contributing to environmental quality locally and globally. Current and future research efforts should be addressed at defining the most important aspects of soils for winegrape production and helping growers fine tune their practices to balance agronomic and environmental outcomes.

Dr. Cristina Lazcano is an assistant professor at the University of California Davis. She is interested in finding the connections between soil biota, soil health, crop productivity and nutritional quality. She studies how soil organisms interact with each other and their abiotic environment to regulate ecosystem processes such as nutrient cycling which are essential to support crop productivity. Dr. Lazcano started working in vineyard soils four years ago, investigating how soil management affects soil biota, C and N cycling and grape quality. Dr. Lazcano completed her PhD at the Soil Ecology research group from the University of Vigo (Spain). Afterwards, she worked as a postdoctoral scholar at the Technical University of Denmark, the University of Calgary and UC Davis. Before joining the faculty of UC Davis in 2019, Dr. Lazcano was an assistant professor of soil ecology at the California Polytechnic State University, San Luis Obispo.

Charlotte Decock is an assistant professor of soil health and fertility at Cal Poly. Her teaching and research focus on sustainable plant nutrition and soil conservation management. Her work at Cal Poly aims to contribute to the development and assessment of soil and nutrient management practices that protect soil, water and air resources, while maintaining agronomic productivity and economic viability in California’s cropping systems. She is also interested in the evaluation of novel indicators for the assessments of soil health and nutrient availability in arable soil. She earned a Ph.D. in Soils and Biogeochemistry from UC Davis and a M.Sc. and B.Sc. in Environment Engineering from Ghent University (Belgium). During the last 10 years, her research has focused on nutrient cycles and greenhouse gas emissions in agroecosystems around the world; and how they are affected by management practices and climate change.

Stewart Wilson is an assistant professor of soil resources at California Polytechnic State University, San Luis Obispo. Dr Wilson’s research is broadly focused on how soils form and are distributed across the landscape, and how soil formation and the spatial distribution of soils influences biogeochemical cycling. Dr. Wilson conducts research in soil genesis and mineralogy, phosphorus biogeochemistry, predictive soil mapping and the sustainable management of vineyard soils. Dr. Wilson holds a B.S. in wildland soils from Humboldt State and dual masters degrees, one in soils and biogeochemistry and one in geography (GIS and modeling) from UC Davis. Dr. Wilson earned a Ph.D. from UC Davis in soils and biogeochemistry. Dr. Wilson teaches courses in introductory soil science, digital soil mapping and soil genesis and morphology.

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