This is the third of a series of articles that will investigate the scientific basis of biodynamic viticulture and the harmful effects of chemical agriculture.
Depending on their management, vineyards are systems based on perennial plants (grapevines) intercropped with different soil cover strategies that might go from bare soils to annual legume-grass covers to perennial mixed species swards. The biology of vineyards soils plays a key role in the final quality of wine. Let’s see in depth how it works.
Ingham and Rollins (2006) report that according to their research, soils normally contain “enormous amounts of total nutrients, but are not available to plants in their current form”. According to Brady et Weil (2014), 95% to 99% of soil nitrogen is retained as organic. It is microbial action that transfers organic carbon and nutrients within the soil. A complex soil food chain constituted by protozoa, nematodes, micro-arthropods and earthworms then processes the bacteria and fungi and releases the excess nutrients in their faeces in soluble forms making it available to plants. There is increasing evidence that nutrient cycling, soil hydrology, and carbon storage are jointly mediated by plants and soil organisms to regulate productivity of the ecosystem (Drinkwater and Snapp in Cardon & Whitbeck 2011).
Fungi are better equipped than bacteria for carrying out the decaying of insoluble plant material on soil surfaces as they produce a wide range of enzymes, can penetrate new substrates and transport nutrients, therefore being able to proliferate even when nutrients are limited, which makes them better prepared to colonize bulk soil. Fungi can also remain active at very low water potential and are better suited to exist in inter-pore spaces, (Bardgett 2005). They are made of more recalcitrant compounds (e.g. chitin), further stabilizing soil organic matter. Their soil aggregation effects tend to bind organic matter, protecting it from rapid decomposition.
As a consequence of these characteristics, fungal dominated agro-ecosystems seem to require fewer inputs to sustain organic matter decomposition and nutrient cycling, and seem to be associated to more stable and efficient ecosystems, richer in organic carbon (Six et al. 2006). However, by their very nature, these systems are less tolerant of disturbance.
Bacteria are present as individual cells and therefore can better tolerate disturbance. As they have a more rapid turnover (weeks vs. 4 to 5 months in natural soils, as cited in Strickland and Rousk 2010), they can efficiently colonize and take advantage of newly available organic matter and more labile organic components, as in the environment provided by roots in the rhizosphere.
Fungi, bacteria and wine quality
A result of the characteristics of soil biology and its interaction is that, according to Ingham (2005), ecosystems evolve along successional stages from bacterially dominated microbial communities to fungi dominated communities. Basing on her observations of many natural ecosystems, early plant colonizers of soils are typically annual or bi-annual plants that have high levels of inorganic nitrogen requirements to maintain dominance. These plants favor bacteria dominated systems for rapid nutrient cycling and N fixation, as these organisms have shorter generation times, smaller body size, rapid dispersal and generalist feeding (Neher 1999).
Later successional plant species (perennial plants) require more fungi in the balance, as they need to explore larger soil areas for permanent nutrition and to withstand environmental challenges and disease pressures, for which fungi might be a better partner (as it is evident in the case of mycorrhizae). Also, as disturbance decreases and more cellulose and lignin based residue remains on soil surface, more complex decomposition pathways are required: initiated by fungi, bacteria can only intervene later in the process (Neher 1999). At the same time soil physical and chemical characteristics (aggregation, pH) and nutritional dynamics are modified as a consequence of the new ecosystem functioning.
Similar microbial community and soil characteristics changes might be expected when transitioning from conventional to organic systems, as has been demonstrated by De Vries et al. (2006) in the case of grasslands fertilized by manure as compared to inorganic fertilizers, where organic amendments resulted in higher Fungi/Bacteria ratios, lower ph and less nutrient leaching, compatible with a more efficient system.
In order to express “terroir” identity as described in the previous article of this series, it is reasonable to conclude that more sustainable systems that are able to maximize wine quality should be based on perennial organic schemes that correspond to higher F/B ratios.
The relationship between soil biology, plants and soil structure
P. Lavelle (2013) proposes that soils can be considered as self organizing systems, as structures and processes mutually reinforce each other for the construction of a habitat optimized to host all the participating living organisms maintaining order and function.
As a consequence of this perspective, soil structure cannot be considered separately from soil biology, as they are effectively interdependent. As Bronick and Lal (2005) review, there are many biological and chemical mechanisms of aggregation that create soil structure and stability. Bacteria operate mainly at the micro-aggregate scale (below 250 micron) by bonding clay particles with their exuded polysaccharides. But macro-aggregates (larger than 250 micron) are mostly the result of roots and fungal hyphae (mostly mycorrhizal fungi). As it is the smaller sized macro-aggregates (1-0.105 mm) that seem to improve soil hydrological properties like water retention capacity, infiltration and aeration (Boix-Fayos et al., 2001), therefore facilitating plant establishment and growth, it is possible to infer that the succession path from bacterially dominated to fungi dominated systems, also implies the development of better soil structural conditions that can support a more complex ecosystem. As indicated in Seiter et al. (2004) the improvement in soil quality also facilitates nutrient availability and uptake, thus contributing to the efficiency of the system.
On the contrary, poor or disturbed soil structure can impair both soil biological function and plant growth. Whalley et al. (1995) review the effect of soil compaction on soil biology. They conclude that compaction reduces biological activity in the case of roots and earthworms and tends to create anaerobic conditions for microbes, which constrains the activity of beneficial fungi and bacteria and promotes the development of pathogens. This explains how in lower quality soils compaction drives a vicious cycle of soil biology deterioration that derives in further loss of structure and restrained plant performance.
Passioura (2002) reviews the effects of compaction on plant growth. As bulk density increases, plants trigger protective mechanisms that decrease root growth and rate of leaf production and size as a feed-forward mechanism anticipating conditions of decreased humidity or mechanical root growth restriction.
The potential of Mendoza region
Interestingly, a special case might occur in arid climates like Mendoza where in absence of organic matter, carbonates might play a significant role in soil aggregation. Under conditions of decreased moisture, dissolved CO2 precipitates carbonates and bicarbonates originated from rock material with Calcium (or Magnesium) cations to form calcium carbonate coatings on soil particles, which then conducts to aggregation (Bronick and Lal, 2005). This might be a relevant process in some of Mendoza’s soils. It is important to note that Vitis vinifera is calcicole (calcium loving), as opposed to other members of the Vitis genera that are normally used as rootstocks, as Vitis riparia or Vitis labrusca, which originated in North America (Bourgignon, 1991). This preference might reflect its adaptations to Mediterranean soils that although poor in organic matter could offer sufficient structural conditions because of the abundance of calcium carbonates.
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