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6.1 Indirect ‘manipulation’ of soil organisms through land management practices

A variety of land management practices are used around the world and each is based on a required output in relation to a chosen (or available) input. There are many factors involved in deciding which practice to apply, but currently economic issues predominate during the selection of the majority of land management practices. However, societal values, land tenure issues, and environmental considerations are all relevant when choosing a land management system.

Although much has been written about the sustainability of managed environments, it has been difficult to define the central concepts of sustainability. Regardless of how sustainable a management practice is, any practice that manages the land, alters the habitat of soil organisms. Therefore, all land management practices indirectly, and possibly unintentionally, ‘manipulate’ soil communities.

In this section, the management of land for agriculture, horticulture, forestry and livestock grazing of natural rangelands and the restoration practices applied to severely disturbed sites are viewed in terms of how they influence soil biological fertility.

Can agricultural and horticultural practices be used to manage soil organisms?

Agriculture has a very long history and a great deal of practical research has addressed agricultural land management issues. However, practices since the 1920’s have not generally focused ways of developing and maintaining soil biological activity. A major reason for this is that the cost of fertilisers and pesticides is relatively low compared with the value of the food or flower crops produced. Without an incentive to make use of biological soil processes, practices have arisen that override and underestimate the biological fertility of the soil. Another reason that agricultural and horticultural practices commonly fail to capture the benefits of soil biological processes is because of their highly intensive practices, including soil disturbance and addition of synthetic chemicals. Furthermore, there is a disincentive to make effective use of organic matter from previous crops because of the potential for carry-over of plant pathogens.

Nevertheless, primary producers understand the need for integrated land management practices. The decisions that they make day-to-day requires consideration of many factors simultaneously. To what extent can land management practices be selected that develop and maintain the soil biological resource so that it becomes part of a normal, integrated practice?

The selection of management practices that enhance biological processes in soil cannot be identified simply on the basis of plant productivity because this is obtained under conditions of high chemical fertility. Practices that maximise the contributions of soil organisms will vary between soil type, environment and availability of resources and these factors need to be considered during the selection process. In addition, the impacts of soil organisms and improved soil biological fertility on productivity are only likely to be seen over the long-term and if only measured over short periods (several years) there may be no discernible improvement. Nor will simple measures of the quantity of organic carbon or nitrogen in soil give much indication of the extent of biological activity.

When selecting systems to increase the biological component of the soil, minimising the impact of diseases also needs to be considered. Knowledge about the life-cycles of plant pathogens can be used to select practices that avoid the widespread development of plant disease. Similarly, the use of pesticides needs to be based on knowledge of their effectiveness, toxicity to non-target organisms and influence on important soil biological processes. Information is available to understanding biological processes in sustainable farming systems (Brussaard and Ferrera-Cerrato 1997; Pankhurst 1994).

If a soil is not already physically or chemically degraded, cropping systems that balance inputs with cycling of nutrients from organic matter need to be identified. For degraded soils, the most important factor is to increase the input of plant organic matter to the soil and to encourage a dense root mass because this will stimulate growth of soil organisms.

‘Modern’ agricultural and horticultural technology

The improvement in technological knowledge during the twentieth century changed modern agricultural and horticultural practices considerably. This has resulted in a high level of soil manipulation, precision sowing and harvesting of crops, and dependence on synthetic chemicals to control pests and diseases and ameliorate nutrient disorders in plants. Considerable off-site problems have arisen from such high input agriculture, including eutrophication of waterways with excessive inputs of phosphorus and nitrogen, and contamination of groundwater by pesticides.

The adoption of reduced tillage methods and integrated pest management has improved opportunities for managing soil organisms. However, an undesirable outcome of the use of high levels of fertilisers is that plants are selected which grow best in soils with high chemical fertility but not necessarily those with high levels of biological fertility. The consequence of this is that some of the commercial varieties of plants used are less able to benefit from microbial associations. Furthermore, the increasing manipulation of plant genetic material (through conventional breeding and genetic engineering) is continuing to alter the potential for soil organisms to contribute to better land management.

Low input sustainable agricultural (LISA) systems

Low input sustainable agriculture (LISA) seeks to minimise inputs and reduce the risk of environmental pollution. This approach has the potential to encourage and sustain the activities of some important soil organisms. The level of agricultural production achieved depends on quantity and form of inputs, the original fertility of the soil, as well as the management practices used before the introduction of LISA systems.

Practices that specifically rely on soil biological fertility

A variety of approaches to agricultural and horticultural production have been implemented that rely heavily on soil biological activity. Many of these practices are not new and they are often based on procedures that were more common before the advent of modern technological advances in agriculture, including the widespread use of synthetic agricultural chemicals. They seek a holistic approach to agricultural and horticultural production and pay particular attention to the impact of land management on the environment and quality of food for human consumption. Examples of these practices are:

Use of manure

Agriculture systems relied on farmyard manure as a major source of nutrients before the widespread use of chemical fertilisers. Use of animal waste continues as a nutrient source but management issues also now also focus on the effective use of this nutrient source without off-farm environmental impacts.

Organic agricultural systems

Organic agriculture prohibits the use of synthetic chemicals and depends on biological processes for nutrient cycling and pest management. Productivity varies depending on the type and level of allowable inputs, and the original physical, chemical and biological fertility of the soil. Organic agriculture excludes the use of genetically engineered organisms.

Biodynamic Agriculture

Biodynamic agriculture, based on the principles of Rudolf Steiner, specifically seeks to maximise soil biological activity (Koepf et al. 1976). The approach is based on a holistic view of agricultural production and includes the application of special preparations according to lunar cycles that are aimed at stimulating soil organisms. The biodynamic program also includes practices such as rotational grazing to enhance soil conditions that encourage soil biological activity. Restrictions on the application of soil amendments are similar to those of organic agriculture. Biodynamics is practiced with specific instruction from experienced practitioners.

Permaculture

Permaculture has been developed in recent years to address environmental issues during production. Permaculture is a holistic approach that aims to reduce the consumption of non-renewable resources and make effective use of all resources, and therefore, includes enhancing soil biological fertility. Permaculture integrates the design of production systems with ecological processes in the environment. It uses knowledge of natural systems and focuses on both local and global issues related to food production (Mollison 1991).

Is microbial activity different in different management systems?

Many comparisons have been made between the levels of microbial activity in soil from 'conventional' or 'modern' agriculture and those from organic agriculture. Commonly, microbial activity is greater in farming systems that include practices that stimulate growth and activity of soil organisms (Carpenter-Boggs et al. 2000). Many factors may be responsible for differences in activity of soil organisms with various types of management. This includes differences in the amounts and form of carbon added to the soil, frequency of cropping, and frequency and severity of soil disturbance.

Agricultural systems, such as organic and Biodynamic, that are designed to increase microbial contributions to soil physical and chemical fertility need to maintain different types of biological activity (not just those associated with nutrient cycling) at certain levels. This is to keep the soil in a state to satisfy certification standards for food products.

The dominance of various biological processes depends on the agricultural system as well as the soil type and environment. The criteria for benchmarking soil characteristics need to take into account the purpose of the agricultural system. This is discussed further in relation to identification of indicators of the biological state of soil.

How does rangeland management influence soil organisms?

Rangelands are low input and low management systems. The link between vegetation in these systems and microbial processes is strong because of the dependence of many microorganisms on plant organic matter for their carbon, nutrient and energy requirements and the dependence of plants on nutrients recycled by soil organisms. The productivity of rangelands is often low because of low rainfall, but the rate of recycling also depends on the natural level of chemical fertility of the soil.

In rangeland management, maintenance of the soil in a stable state in order to retain the microbial crusts and prevent erosion is essential. Biological processes in rangeland soils are similar to those elsewhere and control of grazing pressure is needed if soil biological resources in these fragile environments are to be retained.

How does the management of natural ecosystems influence soil organisms?

Because soils supporting natural vegetation have all the requirements for nutrient cycling and maintenance of soil biological activity, natural ecosystems require little management to maintain soil biological activity. However, knowledge of the potential for spread of plant pathogens is required before areas are disturbed. Sustainable management practices in forests are applied in a mosaic and therefore only areas directly disturbed will have altered soil conditions. Exceptions to this are if the water table is altered or serious erosion occurs.

The cycling of nutrients from leaf or other debris in managed natural ecosystems occurs at a rate that is adequate for naturally occurring plant species. Fertiliser addition stimulates plant growth and may either increase or decrease the break down of organic matter. Similarly, pesticides affect some organisms but not others.

What happens to soil organisms in severely disturbed soil?

Severe land disturbance, for example mining, disrupts soil biological processes through the removal of soil organisms and formation of soil environments that limit their growth and reproduction. Retention of topsoil for re-application after mining preserves some organisms and organic matter, but not all organisms survive. The chances of survival depend on the storage method and climatic conditions.

Rehabilitation of minesites

Restoration of land severely disturbed by mining usually requires improvement in the physical, chemical and biological components of soil fertility. Improvement of physical conditions is usually necessary before biological fertility can be re-established. Commonly, the low level of soil organic matter in mine waste restricts the formation of effective communities of soil organisms. Furthermore, toxic substances or other adverse chemical conditions such as high levels of salinity, acidity, alkalinity or sodicity may require amelioration before soil organisms can be re-established at levels sufficient to restore nutrient cycling and other biological processes.

Re-establishment of plant communities on waste dumps or newly exposed land surfaces is facilitated by the return of soil organisms in the topsoil. Enhancement of nutrient cycling processes carried out by heterotrophic soil organisms will occur with the spread of plants and gradual accumulation of plant organic matter. AM fungi are not dependent on organic matter and their return in the topsoil will contribute in the early stages of revegetation by assisting phosphorus uptake into seedlings. The formation of mycorrhizas has the potential to increase plant diversity by allowing those species that are more dependent on mycorrhizas to become established early in the revegetation. Symbiotic nitrogen-fixing bacteria present in topsoil are generally able to survive in numbers that allow nitrogen-fixing symbioses to form with re-introduced legumes.

Bioremediation in surface and subsurface environments

Several bacterial and fungal species are able to degrade recalcitrant toxic compounds in soil or ground water. A fungus widely studied is the white rot fungus, Phanerochaete chrysosporium. This fungus occurs naturally in decaying wood and produces enzymes that facilitate lignin breakdown. However, knowledge about the functioning of these types of organisms in degrading complex molecules in the soil and the conditions that maximise this function remain limited.

Soil organisms that degrade toxic substances can be manipulated by the addition of nutrients to increase their growth and capacity for bioremediation. For example, the addition of phosphorus or nitrogen can overcome deficiencies in these nutrients for bacteria that are able to degrade complex carbon-based molecules. These nutrient sources are necessary because of deficiencies in these elements either in the contaminants or in the soil environment.

Although bioremediation seems relatively simple, in reality contaminated ground-water and soil are extremely difficult to manage. This is partly because the indirect manipulation of naturally occurring organisms is difficult. Furthermore, organisms that have the potential to degrade toxic contaminants may do so by co-metabolism without deriving carbon and/or energy for their growth. Hence the process may be slow or it may require addition of other carbon compounds such as phenol or toluene to provide a readily available source of energy and carbon. This is not a very practical. Another difficulty is that it is not easy to ensure that the added carbon sources are placed where they will be accessible to the biodegrading bacteria. This is a particular problem where contaminants are dispersed throughout a large volume of soil.

Summary Points (6.1)

• Because soil organisms are sensitive to changes to their environment, all are affected to some degree by land management practices.

• Modern agricultural practices do not maximise soil biological fertility.

• Many alternative approaches to modern agriculture seek to enhance soil biological fertility.

• Management of natural ecosystems can also be concerned with maximising soil biological fertility.

• Principles that support sustainable land management are the same in any environment.

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