Soil Enzyme Activities – Vital to Vineyard Soil Health

Healthy agricultural soils are characterised by:

• Adequate organic matter: organic matter serves as a reservoir of slow-release nutrients that can be made available to the soil over time
• Good soil structure: organic matter causes soil to clump and form soil aggregates, which improve soil structure
• Improved water permeability: better soil structure allows improved permeability (infiltration of water through the soil)
• Improved water-holding capacity: organic matter acts like a sponge with the ability to absorb water
• Rich soil biology: healthy soils contain large populations of beneficial organisms and small populations of plant pathogens and insect pests
• Balanced chemical composition: sufficient but not excessive nutrient supply with no chemicals or toxins that may harm the crop
• Sufficient depth: for healthy plant root development
• Good soil drainage: allows water to drain at a moderate rate, without water pooling and puddling, and that provides adequate soil aeration vital for maintaining healthy plant roots.

The careful management of soil – with soil health being key – is critical to ensure its sustainability.

It is well-known that mismanagement of soil can lead to widespread degradation.

Examples of mismanagement in vineyards:

• Over fertilisation, which can – over time – lead to the build-up of excessive levels of certain nutrients, cause the pH to become unfavourable for the vine growth, cause the release of greenhouse gases, and contaminate water sources through deep drainage and/or runoff
• Loss of organic matter limits the soil’s ability to provide the grapevine with nutrients in a more sustainable manner, and reduces the biodiversity of the soil
• Increased salinity, which causes plant toxicity (e.g. root injury); saline soils cause plants to become dehydrated and thus cause wilting or death of plants
• Over-irrigation which causes water logging and runoff
• Over-use of pesticides and herbicides can cause soil, surface and ground water contamination, may have effects on non-targeted organisms, and may alter soil fertility, nutrient cycling and metabolism
• Heavy use of farming equipment and excessive tillage practices cause compaction and erosion
• Long-term effect of copper sprays results in heavy metal contamination.

Importantly, as the quality of the soil declines, it diminishes its capacity to support plant growth. Such soils are typified as suffering from “poor” soil health. In order to address this, suitable indicators of soil health are needed.

Indicators of soil health

Soil health is an all-inclusive concept that recognises the three main components of soil, i.e. chemical, physical and biological (figure 1). Within each category is a subset of measurable attributes (or indicators) that are used to consider the assessment of soil health. For example, indicators such as available bulk density, soil hardness and aggregate stability are considered physical; organic matter, microbial respiration, microbial enzyme activities and active carbon are considered to be biological; and pH, macronutrients and micronutrients, are considered chemical.

The scientific relevance of a suitable indicator is considered based on factors like its sensitivity to soil management, good correlation to beneficial soil functions, cost and ease to measure, and accuracy in measurement (Laishram et al., 2012).

Grape growers must weigh up the short-term input costs against the long-term economic benefifi ts of soil improvement measures.

Physical and chemical indicators – In the past, management-induced changes affecting soil health used to be almost entirely measured in terms of chemical and physical changes that soils undergo. Chemical changes like changes in the soil organic carbon content may nevertheless be slow and several years may pass before anything significant is detected. Physical and chemical methods are thus best used as indicators of long-term changes in soil health and are typically straightforward to measure.

Biological indicators – The biology of soil is very important to its overall health, productivity and sustainability. Biological indicators are a function of living organisms and these functions are very closely linked with both the chemical and physical properties of a soil. Importantly, they are dependent on and contribute to the fluctuations in soil parameters such as pH, nutrients, soil structure and aggregate stability. Biological indicators can effectively measure the impact of agricultural practices on soil health, at times preceding detectable changes in physical or chemical indicators. Therefore, they are best used as indicators of the short-term changes in soil health and measurements can be performed relatively easily on a routine basis.

Enzyme activities as biological indicators of soil health

Among the various biological indicators that have been proposed to monitor soil health, soil enzyme activities have great potential to provide a unique integrated biological assessment of the health of soils. Enzymes can be of plant, animal or microbial origin, but microorganisms are considered to be the main source.

Microorganisms mediate metabolic processes in soils mainly through the synthesis of enzymes that catalyses
biochemical reactions in soil. These processes are central to functions that soil perform and are at the basis of the cycling of key nutrients, e.g. carbon, nitrogen and phosphorus cycling in soil, respectively through the activities of the enzymes ß-glucosidase, urease and phosphatase.

Enzymatic reactions are what gives soil life. The changes that management practices may impose on the soil, for example variability in mineralisable substrates, fluctuations in soil moisture and temperature, not only affect microbial populations, but also the availability and activity of the enzymes that they produce.

These changes can be accurately detected in changes in the level of soil enzyme activities and within relatively short periods (from within weeks to months to 1 – 2 years).
Subtle improvements or decline in soil health can thus be anticipated long before they are detected by chemical or physical means. Therefore, soil enzyme activities could serve as early signs of soil improvement or early warnings of soil degradation.

Using an integrated approach

Measuring soil enzyme activities, in certain instances, present some limitations and must be considered in conjunction with other biological, physical and chemical measurements if we are to diagnose soil health correctly, i.e. data collected can be used in a comparative manner and the information on enzyme activities can be integrated or supplemented with other physical and chemical soil properties.

Because enzyme activities are closely linked to nutrient cycling in ecosystems, they typically correlate with soil organic matter content, with soil organic matter being the site of enzyme synthesis and enzyme stabilisation. In actual fact, enzymes are the direct expression of the soil microbial community to metabolic requirements and nutrient availability.

They are thus indicative of the soil’s potential to sustain overall microbiological activity, while also producing relevant information on the capacity of a soil to perform certain functions that help maintain overall soil fertility and productivity.

Enzyme activity indices

Individual enzymes can vary due to (amongst others) differences in substrate specificity, seasonal differences, differences in soil types or due to the kind of land use under consideration. Thus, individual enzyme activity data can sometimes be difficult to interpret. For this reason and for ease of interpretation, multiple enzyme activities can be simultaneously expressed by combining them into single numerical values or indexes.

Research on enzyme activity indices at ARC Infruitec-Nietvoorbij

Researchers from ARC Infruitec-Nietvoorbij have been testing an enzyme-based soil alteration index (AI3) that relates to three enzymes, namely: ß-glucosidase, phosphatase and urease.

Preliminary findings have shown that the AI3 index:

• is sensitive to management-induced changes in soil properties
• correlates with soil nutrient content (in particular organic matter) in vineyard soils (figure 2)
• is applicable to vineyard soils of dissimilar textures (figure 2)
• differentiates between municipal and winery wastewater irrigation treatments in vineyards (figure 3)
• differentiates between cover crop management practices in vineyards (figure 4)
• differentiates between the top-and subsoil layers in vineyards (figure 5)

Thus, the AI3 is potentially useful for the monitoring of soil health in local vineyards. The above findings also stressed the importance of nutrient content, texture, infiltration and water holding capacity, as well as depth, as these inherent soil properties are known to have an impact on grape chemical composition.

Conclusions

• Acknowledgement of the biological component of soil as being critical to the overall productive capacity of a soil is important if we are to diagnose soil health correctly
• Using enzyme activities as indicators of soil health has definite advantages over physical and chemical methods
• Enzyme activities must nevertheless be considered in conjunction with other biological, physical and
  chemical measurements, using an integrated approach
• Individual enzyme activities may potentially be combined and better expressed as indices
• Grape growers can consider using biological indicators, in conjunction with physical and chemical indicators, as tools to make the “health” of vineyard soils both “measurable” and “manageable”
• Grape growers must weigh up the short-term input costs against the long-term economic benefits of soil improvement measures.

References

• Doran, J.W., & M.R. Zeiss. 2000. Soil health and sustainability: managing the biotic component of soil quality. Applied Soil Ecology 15:3–11.
• Laishram, J., Saxena, K.G., Maikhuri, R.K., & Rao, K.S. (2012). Soil quality and soil health: A review. International Journal of Ecology and Environmental Sciences 38(1): 19–37.