Fleece Weight and its Component Traits€¦ · Web view6 - 14 – RSNR403/503 Sustainable Land Management ©2009 The Australian Wool Education Trust licensee for educational activities

6. Soil Sustainability

Kathleen King

Learning objectivesOn completion of this topic you should be able to:

Describe the main groups of beneficial decomposer organisms that live in soil Explain the roles of soil biota in the sustainability of Australian temperate agricultural

systems by improving chemical and physical fertility of soil, with a focus on sheep production

Understand adverse or stimulatory impacts of management practices on soil biota

Key terms and concepts Economic, ecological or environmental sustainability Soil biota, microfauna, mesofauna, macrofauna, soil microbes, bacteria, and fungi Chemical and physical fertility of soil Decomposer biota, decomposition of organic matter, nutrient cycling Soil animals fragment organic detritus, soil microbes digest the latter with 50-60 enzymes Soil structural stability, soil pores, water-stable soil aggregates Biodiversity, species richness

Introduction to the topicIn agricultural systems, an adequate level of economic return from animal production is the final objective but economic sustainability will not be attained if ecological (or environmental) sustainability declines. This topic will emphasise the ecological sustainability of sheep production enterprises from the point of view of soil health. Soil health, or quality, has been defined as ‘the capacity of a soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality and promote plant and animal health’ (Doran & Parkin 1994). Soil health has three main aspects, all of which interact with each other. The three aspects are chemical, physical and biological. Of these, it is the soil biota (organisms, other than plants, living in the soil) which perform vital functions that help to sustain both the chemical and physical fertility of soils. Without soil organisms, organic matter would not decay and release plant nutrients and chemical fertility would decline. Without soil biota, soils would also have poor physical structure and water and air transmission into and through soil would be impeded. Hence the emphasis of this topic will deal with roles of soil biota in pasture ecosystems and how management can affect their function, and thus sustainability.

RSNR403/503 Sustainable Land Management - 6 - 1©2009 The Australian Wool Education Trust licensee for educational activities University of New England

6.1 Types of soil organisms When you scoop up a handful of soil, it just looks like a heap of 'brown stuff'. But there are myriads of beneficial organisms, mostly microscopic, living there that are vital to the functioning of that soil. Anywhere in the world where there is soil, there will be organisms living in it. Such diverse ecosystems as tropical rainforests in South America, the McMurdo Dry Valleys in Antarctica, hot deserts in Namibia, coral cays of the Great Barrier Reef in Australia and pastures in the New England Tablelands, all have their own communities of soil organisms which consist basically of two main groups: soil microbes and invertebrate animals.

Main groups of soil biotaAside from plant roots, the two main groups of biota living in soil are:

soil microbes soil invertebrate animals.

Of course, plant roots are living things too, but in this topic we are only concerned with the way roots interact with soil animals and microbes.

For images of these soil biota, see the UNE Living Soils website: http://sciences.une.edu.au/livingsoils. This website contains much of the content of this module but the emphasis of this topic is on pasture soils.

Soil microbesThese are mainly microscopic bacteria and fungi. Although individual cells may be small, the overall biomass of microbes in soil is very large. In Armidale pastures, for every sheep grazing above ground there is an equivalent weight of 4 sheep 'living below ground' in improved pastures as microbial biomass.

Bacteria are single-celled biota Fungi form networks of filaments (hyphae), a single cell thick, which grow through soil and

the overlying organic residues (litter, dung). At times, fungi make themselves obvious as the microscopic filaments coalesce and form the visible fruiting bodies of mushrooms and toadstools

Most bacteria and fungi are 'decomposer' organisms because they use dead organic matter as a food source, decomposing it and releasing nutrients back into the soil solution

There are other soil microbes which form relationships with plant roots. These are mutually beneficial (symbiotic) to both organisms (e.g. mycorrhizal fungi in roots, nitrogen-fixing bacteria in legume roots)

Mycorrhizal fungi infect most plant roots. The fungus forms an extension of the root by growing out into the soil from the infected root, and helps the plant absorb nutrients such as phosphorus and zinc. In return, the fungus receives food (sugars) from the plant

The Rhizobium bacterium contributes to another symbiotic relationship with legumes by forming a nodule on the roots where it lives. It fixes atmospheric nitrogen thus aiding the plant's nutrition.

Soil invertebrate animalsThere are three groups of soil animals: micro-, meso- and macrofauna (Figure 6.1) classified depending on their size.

Microfauna are the single-celled protozoa which feed on bacteria and fungi Mesofauna are small multicellular animals which are several mm long. These are nematode

worms, springtails and mites. Most soil nematodes are free-living (not the parasitic ones of animals and

plants) and are 'decomposers' with a major role to play in recycling of nutrients. Mesofauna have diverse feeding habits consuming bacteria, fungi and organic detritus. They can also be predatory (i.e. feeding on each other). As an example, springtails are mainly fungal and detritus feeding but there are a few predators amongst them which feed on other springtails

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Macrofauna include the animals you can see easily with the naked eye. Decomposer macrofauna feeding on organic matter include earthworms, millipedes and woodlice. Predatory macrofauna include spiders and centipedes. Some groups of animals are omnivorous (e.g. some ants) while others are herbivorous, feeding on live plant roots (e.g. some beetle larvae).

Figure 6.1 Mesofauna and macrofauna in soil. (1) A springtail. (2) Mites. (3) A nematode worm and a protozoan (on right). (4) Macrofauna: beetles, earthworms, millepedes,

cockroach, woodlouse. Photos 1-3 were taken under the microscope. Source: Photos taken by Kathleen King UNE.

6.2 Roles of soil organismsWhat do soil organisms do? How can they affect the chemical and physical fertility of soil and hence the sustainability of the temperate pasture system? The main roles of soil biota include:

sustaining soil chemical fertility by decomposing organic matter (dead plants, animals, dung) and releasing nutrients back into the soil solution for plants to re-use (nutrient cycling)

enhancing soil structural stability (soil physical fertility).

Organic matter decomposition and nutrient cycling (chemical fertility)Decomposition of dead plants and animals, serves to eliminate the build-up of organic detritus in ecosystems which can restrict the germination and establishment of plants. Decomposition is also important to the release and recycling of nutrient elements from dead plants and other organisms and also excreta. This sustains the chemical fertility of soils. Decomposition is fundamentally a biologically-mediated process carried out by soil biota. Decomposer invertebrate animals and microbes have quite distinct roles in breaking down organic detritus. Soil microbes, the tiniest of soil biota, are the main agents for decomposition of organic matter and the breakdown of organic molecules into inorganic ones that plants can absorb through their roots (for reasons, see below). Invertebrates play only a small part in this chemical degradation of dead organic residues (detritus) due to the small number of digestive enzymes they possess. However, soil animals do help microbes in many other ways to do their job of releasing nutrients from organic detritus.

Role of invertebrate animals in decomposition and nutrient cyclingSoil animals and soil microbes have different ways of decomposing organic detritus. Soil animals with hard mouthparts bite off bits of organic matter and fragment it into small pieces. On passage through the invertebrate gut, some further physical disintegration of the organic matter occurs. Take the case of the woodlouse.


The dead plant leaf is fragmented as the woodlouse munches and this helps the function of soil microbes in decomposition in several ways. Firstly, the leaf is reduced to tiny fragments, increasing the surface area of the original piece of organic matter and hence the number of sites for colonisation by microbes. This physical fragmentation of organic matter is one of the main ways invertebrates help decomposition because, although soil animals digest a small proportion of this organic matter internally, the variety of digestive enzymes is far fewer in number than those produced by soil microbes. By increasing the access of microbes to well-fragmented organic matter, decomposition rates are enhanced.

Secondly, microbes can hitch a ride either on the outside of the animals’ bodies or in their guts. So microbes are picked up by the woodlouse in one spot and transported to and deposited in, another point of colonisation for the microbes, therefore helping the microbes’ mobility and access to organic matter, throughout the soil.

Thirdly, invertebrate animals graze senescent microbial colonies and stimulate them into renewed growth. Even the tiny springtails and mites can increase nutrient transfer from plant litter to soil by up to 50%, so their contribution to decomposition of organic residues and nutrient release can be substantial.

Role of microbes in decomposition and nutrient cyclingMicrobes digest organic material externally by exuding a huge variety of digestive enzymes (> 50 different ones) on to their organic matter food. These enzymes help chemically degrade organic detritus, breaking up large organic molecules (e.g. proteins) in which nutrient elements are bonded. Eventually, simpler molecules (e.g. amino acids) are split off and passed back through the microbial cell walls to be absorbed into their tissues and used for growth and maintenance. Simple organic molecules are eventually reduced to an inorganic form which passes into the soil solution in forms which plant roots can absorb.

Figure 6.2 Cotton cloth strips buried in soil for 17 and 44 days. Source: Photo taken by Kathleen King UNE.

Figure 6.2 shows cotton cloth strips (a source of cellulose which is a common constituent of plant detritus) which have been buried for 17 and 44 days and a fresh, unburied strip for comparison. The microbes have been using the cellulose fibres in the cloth as a source of energy, just as they would if growing on a dead leaf, and by 44 days in the soil, the microbes have started to rot the strip completely away in spots. You can also see how the colonies of bacteria and fungi colonising the strips have progressively stained and degraded the strips over time.


Enhancement of soil structure (physical fertility)Good soil structure helps the physical fertility of soil. Soil structure refers to the arrangement of soil particles, and to the connected network of pores in between the particles. Good soil structure is beneficial in many ways as it facilitates the movement of water and gas in soil, it lowers soil bulk density making it easier for root penetration, and lowers the potential for erosion. The maintenance of the porous network is essential for the infiltration, transmission and storage of water into and within the soil. Pores of different sizes and the interconnections between them control the rates of water release and the circulation of air through the soil. Soil pores can be formed through the burrowing activities of large soil animals. Also when soil particles clump together they form larger granules (macroaggregates) with larger volumes of space (macropores) between them. These large soil aggregates are generally stable to disruptive forces of water and help keep good structure in soil. The soil term 'well-granulated', expressing looseness, denotes better percolation of water and air.

Soil structural stability refers to the ability of the soil to withstand disruption of the structural arrangement of pores and particles, when exposed to different stresses such as cultivation, trampling by domestic livestock, trafficking by farm machinery, rainfall and irrigation.

The destruction of soil aggregates results in slaking and dispersion with breakdown in soil structural stability where the connected network of soil pores collapses and clay particles block the pores preventing movement and storage of water and gas. Soils with a high content of water-stable macroaggregates are resistant to the stresses that cause structural decline, and can maintain the pore network. Addition of organic matter can increase the number of water-stable aggregates in soil, thus sustaining soil physical fertility.

Macroaggregates are vulnerable to disruption due to agricultural practice (e.g. trampling by livestock). Soil structural stability is enhanced by the activities of soil biota. Both larger soil animals and tiny microbes have roles in improving soil structure. See the summaries below which show the roles of soil biota in enhancing soil structural stability.

Macropores Large soil animals (e.g. earthworms, dung beetles) make tunnels (macropores) through the

soil Such macropores help in vertical and lateral transmission of water through soil Some earthworms, dung beetles, spiders, ants and cicadas (as they emerge from the soil as

adults), make vertical tunnels that open to the soil surface and down which water can infiltrate easily during rainfall. This also reduces run-off of water and the risks of soil erosion

Other tunnellers (e.g. earthworms, termites) form macropores that do not open to the soil surface but these are still used for lateral and vertical water transmission.

Soil aggregates Bacteria and fungi help in the formation of water-stable soil aggregates Fungal hyphae grow around and between soil mineral and organic particles and physically

bind them together. Both mycorrhizal fungi and free-living decomposer soil fungi perform this function

Both bacteria and fungi secrete polysaccharide mucilages which are sticky and glue the soil particles together into aggregates

See Figure 6.3 for a diagram of a water-stable soil macroaggregate These aggregates can be stable to the action of water for several months and help prevent

slaking and dispersion of the soil.


Figure 6.3 Water-stable soil aggregate. Source: King (2006).

Mixing soil layersThrough their tunnelling and nest-building activities, soil animals mix soil layers together and also mix organic matter with mineral soil layers. Ants, earthworms, termites and other soil animals bring organic matter into the soil (as a food supply, or deposit it as dung) from the surface and deposit it deeper in the soil, thus increasing its organic matter content. This helps water retention in soil with the organic matter acting like a sponge. The incorporation of surface organic detritus such as leaf litter and dung helps in the formation of water-stable aggregates in surface soil.

Soil structural stability and soil erosionMacroaggregate formation lowers the potential of soil loss by water and wind erosion. Larger aggregates, being heavier than individual particles, are more resistant to wind erosion. If surface soil lacks a cover of organic matter in the form of plant litter, the surface is exposed to the impact of raindrops. Soil aggregates can be disrupted and pores collapse, with the sealing of the soil surface preventing infiltration of rain into the soil increasing the risk of subsequent run-off and erosion.

6.3 Zones of biological activityNinety percent of soil organisms live in the top 10 cm of soil and in the layers of organic matter that overlie the soil such as leaf litter, dung, carrion and logs (Figure 6.4). This 10-cm topsoil zone is where most roots, soil nutrients and soil organic matter (the food or energy source for decomposer soil organisms) occur. Below this 10-cm depth, biological activity rapidly declines. Hence, any disturbance to this top 10-cm soil layer can affect the sustainability of the pastoral enterprise.

'Hotspots' of intense biological activity occur where there is a food supply – in the case of decomposers, around any bits of organic detritus such as litter, dung, invertebrate faeces including earthworm casts, dead roots, dead animals, the mucous lining of earthworm burrows and any other bits of organic matter in soil. In addition, the rhizosphere (the zone extending a few millimetres out from living roots) is a hotbed of biological activity as root exudates leak from them, providing soil biota, particularly soil microbes, with a rich source of readily available energy and nutrients.


Predators of soil microbes (protozoa, nematodes) aggregate here along with other predators (mites, springtails, beetles, centipedes), creating a ferment of biological activity along this complex food chain.

Figure 6.4 Zones of high biological activity in soil. Source: King (2006).

6.4 Species richness of soilPractices used in management of grazing lands for sheep and wool production can affect the number of species (species richness) and types of species (assemblages or composition) present. Topic 8 (plant sustainability) will cover the effects of management on plant species diversity so here we are only considering the species diversity of the soil biota.

We have little idea of the diversity of soil microbial species and how management practices might affect it, as only a very small fraction of soil microbes have been described. However there is evidence that species of soil animals are affected by management of pastures.

In Armidale pastures:

grazing and pasture improvement changed the species composition of springtails and nematodes

a decline in number of species occurred in springtails with heavy grazing and pasture improvement, native species being particularly affected.

Does this diversity of species in soil matter? This depends on whether we are concerned about species richness from the viewpoint of the conservation of species or species presence from a soil functional viewpoint. From a conservation perspective, species diversity of some groups of soil biota (e.g. native species of springtails) can decline with overgrazing and pasture improvement. However, there is considerable controversy as to whether lowered diversity in soil affects its functioning.

Some people subscribe to the ‘rivet’ hypothesis and others to the ‘redundancy’ hypothesis. In the rivet hypothesis, if enough key species are lost (like rivets from a plane), then ecosystem function (such as nutrient cycling) may be adversely affected. An example of this could be nitrogen-fixing microbes, such as Rhizobium.

But in the complex environment of the soil, there are, globally, about 8000 species of soil arthropods, 1.5 million species of soil fungi and 1 million species of nematode, and 1 g of soil can contain 1 million species of bacterial species. Hence, there is a wealth of diverse and well-adapted species and there is a high probability that species are capable of substituting for each other in


functions that are important for the long-term stability of the ecosystem. For example, if several native species of springtails are replaced in improved pastures by large numbers of one cosmopolitan species of springtail, is decomposition of organic materials adversely affected? Our data show this is probably not the case since litter decay still proceeds rapidly in improved pastures dominated by the cosmopolitan species. However, since research on diversity and ecosystem function has largely ignored soil biota and their food webs, then the possibility remains that there may be particular cases where declining soil biodiversity could lead to functional decline. Thus, the jury is still out on the importance of diversity of species among soil organisms in ecosystem function. Wardle (2002), a prominent soil ecologist, when referring to species diversity among soil biota, states that 'there is no undisputed evidence……..which suggests……. that species richness is a key driver of ecosystem stability in nature'.

6.5 Effects of management practices on pasture soil organisms

What happens to the soil biota when we disturb soil during pastoral operations? We clear the land of trees, we physically disturb the soil through ploughing to sow pastures, replacing native plants with introduced productive grasses and legumes, and we apply inorganic fertilisers such as superphosphate. Large ungulate herbivores graze the land and we use agrochemicals for suppressing weeds and the parasites of livestock. How do the activities listed below affect soil biota?

tree clearing grazing pasture improvement with fertiliser and introduced pasture plants cultivation pesticides pasture ley phase in a cropping system.

Tree clearingMost information on the effects of tree clearing on soil biota is from studies of earthworms. When trees are felled and replaced by pastures, some effects include:

after clearing deciduous European forests, an increase in earthworm numbers in temperate grassland occurred

European species of worms tend to dominate Australian agricultural soils in the temperate zone when compared with forest soils.

GrazingHigher grazing levels affect the soil biota in following ways:

numbers and biomass of soil mesofauna and macrofauna are reduced soil microbial activity is higher in the stock camps than over the general paddock diversity of some species of soil biota declines (e.g. springtails).

Effects of grazing on the habitat, microclimate and food supply of soil biota:

Heavy grazing results in low litter levels. Litter provides both a living-space for soil animals as well as forming organic residue food for soil and litter biota

Litter protects the soil environment against climatic extremes. In pastures at Tamworth, NSW, summer temperatures at 5-cm depth in overgrazed, bare pastures can reach 50°C. However, the soil temperature can be < 25°C at the same depth in lightly grazed pastures with a good litter layer cover

Litter layers slow the evaporation of moisture from soil from 10 mm/day in overgrazed pastures to < 2 mm/day in summer in less intensively utilised pastures of the same type


Trampling by high numbers of sheep, particularly in wet weather, compacts the soil, squeezing the pores tight, making them smaller. It is then difficult for mesofauna to access the soil as they are too small to move soil particles aside and create their own tunnels. They have to use existing soil pores. In addition, compacted soil makes it difficult for larger animals which can build their own burrows (e.g. earthworms) to move through soil. Root growth is also slowed in compacted soil

Soil nutrients and organic matter (in the form of livestock dung) are concentrated in stock camps on higher ground or in corners of paddocks, where sheep (particularly Merinos) congregate to rest. This concentration occurs at the expense of nutrient and organic matter status over the rest of the paddock.

Pasture improvementPastures, sown to productive introduced plant species and well-fertilised, can triple the annual plant and domestic animal production of unfertilised native pastures. These two contrasting pasture types are the two ends of a 'pasture improvement' spectrum – most pastures fall somewhere in between these two extremes and are sometimes called 'semi-improved'. The most commonly applied inorganic fertilisers used to improve the chemical fertility of pasture soils are:

superphosphate – a source of phosphorus (P, 9%) and sulphur (S, 10.5%) lime is used mainly as a soil ameliorant to reduce soil acidity.

In Australian pastures, nitrogen (N) is generally supplied by the legume component (e.g. white clover Trifolium repens). Mostly, inorganic fertilisers increase soil biological activity because they increase plant production providing large amounts of organic residues of high quality (high nutrient content). There may be some short-term adverse effects, possibly due to pH change (e.g. some types of N fertilisers) but in the long-term, effects of inorganic fertilisers on overall biological activity are mostly stimulatory. However, species types and diversity of soil biota are likely to change with fertiliser use.

Superphosphate is not considered to be harmful to soil biota in the long-term. The increased secondary production that follows from their use, along with the increased quality of organic residues that form on those farming systems, increases biological activity.

A general 'rule-of-thumb' is that with pasture improvement, plant productivity, domestic animal productivity and soil biological activity increases by three-fold.Effects of pasture improvement on soil biota include:

increase in earthworm numbers by between 2-5 times increase in protozoa, springtails, mite and nematode numbers by up to 3-4 times increase of microbial abundance in soil by 2 times species diversity of some groups decline (e.g. springtails).

Soil acidity and use of limePastoralists use lime to improve the structure of clay soils and to counteract the effects of soil acidity. Some soils are naturally acidic but concerns are being raised with the development of acid soils under temperate Australian pastures which are associated with the use of legumes and phosphatic fertilisers. The main cause of increasing acidity in Australian temperate pastures is through the leaching of NO3

2- from legume-based pastures, leaving behind H+ which lowers the pH of topsoils. Long-term acidification of soils can lead to a decline of pH of about 1 unit in 50 years.

Invertebrates are affected by soil pH both in terms of abundance and species present. Some invertebrates are more tolerant to acidic conditions and can be abundant in acid pastures (e.g. some earthworms).


Effects of liming on soil biota include:

changes in earthworm species from acid to alkaline tolerant. Overall numbers may remain the same

increase in bacterial to fungal ratio. Fungi are more acid tolerant than bacteria.

CultivationPastures are cultivated when new pastures are sown, or during pasture renovation. Direct effects of cultivation on soil organisms include:

physical injury to animals (e.g. abrasion even to mesofauna) inversion of soil brings animals to the soil surface where they desiccate or are eaten by birds the inversion of soil traps the smaller soil biota in deeper levels in soil numbers of large animals (earthworms, millipedes) are lowered. They can take up to 2 years

to recover to pre-cultivation levels because they have long life cycles springtails and mites can also decline by 50% after cultivation. They can recover their original

numbers within 6 months as they have short life cycles soil microbial biomass declines after cultivation probably as a result of the reduced soil

organic matter levels, their energy source. Fungal biomass is reduced by the physical disruption of extensive hyphal networks which extend throughout the soil.

Indirect effects of cultivation on the habitat and food supply of soil biota include:

burial of the protective layer of dead plants on the soil surface – this affects the living space for litter-dwellers and the soil microclimate (increased soil moisture loss and extremes of temperature)

destruction of earthworm burrows physical disruption of soil habitat (e.g. pore channel continuity, pore size) – this limits the

mobility of animals which are too small to move soil particles about and which can't form tunnels and must make use of existing soil pores (e.g. mesofauna)

reduction of organic matter levels in soil.

Direct drilling of seed will help reduce adverse effects of cultivation during pasture renovation.

PesticidesIn pastures used for sheep production, the main pesticides applied are herbicides for spray topping and winter cleaning of pastures and antiparasitic drugs. Chemicals used to control weeds, pests and diseases in pastures and domestic livestock can have toxic side-effects on the non-target soil biota (ecotoxicity). However, the extent of effects of pesticides depends on the nature of the chemical, its dose, the method of application, temperature and moisture conditions in soil and the rate of decay of the chemical. For example, some herbicides are more persistent if sprayed onto a mulch rather than sprayed onto bare soil as they may be quickly inactivated by adsorption on to soil particles.

A general 'rule-of-thumb' is that if the ecotoxic effect of a pesticide on a non-target organism lasts longer than 60 days, then the chemical can be regarded as 'persistent' and its toxic effects only slowly reversible.

The following points can be made about the ecotoxic effects of herbicides and antiparasitic drugs.


HerbicidesCommonly used herbicides on Australian pastures are simazine and the newer chemicals, glyphosate and the sulphonyl ureas. Effects, both adverse and stimulatory, on soil biota are:

newer herbicides tend to be less ecotoxic as they are designed to affect enzyme pathways specific to plants

older herbicides (e.g. simazine) can have some toxic effects and reduce abundance of soil biota (e.g. springtails, mites)

there may also be some reduction in species diversity (e.g. free-living decomposer nematodes)

many fungi are tolerant to herbicides earthworms can increase in numbers following herbicide application as their food supply, in

the form of dead plants, is increased.

Antiparasitic drugsSome antiparasitic drugs commonly used to protect domestic livestock have been shown to be toxic to soil fauna. Levamisole would seem to be relatively harmless to dung fauna but ivermectin and some of the synthetic pyrethroids have been shown to have adverse effects on dung fauna. Most ecotoxicological studies on antiparasitic drugs have been done on the widely-used avermectins. Ivermectin is one example of this family of drugs. Avermectins have a broad spectrum of action, affecting helminth and arthropod parasites, both internally and externally. Avermectins are eliminated from livestock mainly in the dung.

Some effects which have been documented are:

dung fauna (dung beetles and fly larvae) are adversely affected by ivermectin few effects are seen on earthworms but ivermectin-containing dung pats contain fewer

arthropods (e.g. insects, millipedes) ecosystem function can be affected by ivermectin. Decay of dung pats containing ivermectin

is slower with delayed release of P and N from dung by soil biota microbial biomass is not affected by the presence of ivermectin in sheep dung moxydectin (in the same chemical family as the avermectins) has less adverse effects on

dung beetles.

Pasture ley phase in cropping systemsPasture leys are often incorporated into cropping rotations in the sheep/cereal zone in order to increase soil organic matter, to provide a disease break, to improve soil structure and, if a legume is sown, to increase soil fertility. These conditions enhance the habitat of invertebrates and mostly have stimulatory effects on many soil invertebrates. An additional advantage of the pasture phase is that it remains relatively undisturbed for several years which favours invertebrate animals.

Sheep/cropping systemsIf sheep production is combined with cropping systems, some further considerations of management effects on soil biota and their functioning are:

More frequent cultivation of the soil for cropping will have greater adverse consequences for soil biota than an enterprise based largely on permanent pastures. Once grasslands are cultivated, soil organic matter starts to decline rapidly and this affects soil biological activity. In addition, cultivation affects soil structure and heavy machinery can compact the soil, affecting the habitat of soil biota

Crop residue treatment impacts on soil biota. Compared with conventional tillage systems where the stubble residues are ploughed back into the soil or are grazed by sheep, minimum tillage systems have higher biological activity in the soil as it is less disturbed, the soil has higher organic matter content and the crop stubble on the soil surface has an insulating or moderating effect on the soil microclimate


Different crop rotations affect soil biota. Legume crops or green manures will increase the soil fertility through quality of incorporated organic residues (higher N content) leading to an increase in the abundance of soil biota

In Australia, N fertilisers are used in cropping rather than in pasture systems. Some N fertilisers can have an initial negative impact on soil biota through drop in pH but this is generally short-lived and long-term effects of N fertilisation can be stimulatory to soil biota with increased plant production providing better quality residues. Urea would have less impact on soil biota than anhydrous ammonia as the pH drop in the soil after application is minimal

Long periods of fallow, where the soil is bare of vegetation for months, can affect soil biota (e.g. mycorrhizal fungi decline in abundance causing a problem called 'long-fallow disorder')

More pesticides are used in cropping systems than in pastures. More herbicides are used and insecticides and fungicides are applied to deter insect and fungal pests and disease. Insecticides and fungicides are targeted at organisms which are closely related to beneficial soil biota (invertebrate animals and microbes). A general 'rule-of-thumb' is that toxicity of pesticides, from the least to the most toxic to soil biota, follows the order: herbicides < insecticides < fungicides

Repeated burning of stubble residues depletes soil organic matter and biological activity declines. There are few direct effects of stubble fires on biota if they are protected within the soil but litter dwelling biota are destroyed.

6.6 Principles of sustainable soil management in temperate pastures

Permanent pastures are more sustainable agricultural systems than cropping systems from the point of view of soil health. The habitat for soil biota is generally undisturbed by cultivation and plants and their residues are commonly present year-round. Rhizosphere populations and mycorrhizal and free-living fungal networks occur throughout the year. Perennial pastures can also provide year-round vegetative cover for soil, which helps to prevent soil erosion. Soil structure is better in pasture grassland as the number of water-stable soil aggregates is high. This is partly related to the greater length of roots and fungal hyphae in pasture soil when compared with cropping soil. In pastures, fewer pesticides are used than in cropping soil.

However, some areas of concern include the following:

Overgrazing and management of soil organic matter. Soil organic matter will accumulate under pastures since the annual addition of dead plant mass to the soil is regular and generally greater than under crops. However, if the pastures are overgrazed, plant and litter cover are at low levels and the litter and dung fragments on the soil surface become vulnerable to erosion. In heavy rain, this fragmented organic matter is washed over the surface and lost off-farm (e.g. into streams). These organic particles are richer in nutrients than the mineral soil particles in eroded sediments. Organic residues can have between 3 to 5 times the content of P, S and N than topsoil. Hence, you not only lose organic matter off the farm but also substantial amounts of nutrients. Pasture improvement produces organic matter that is higher in nutrient content than native pastures. Litter and dung formed on improved pastures can have between 2 to 3 times the P, S and N content of that formed on native pastures. Hence, loss of nutrients in organic detritus that is washed off improved pastures is higher than similar losses from native pastures Overgrazing and removal of ground cover. This includes the cover provided both by green and dead standing vegetation and litter lying on the soil surface. Once ground cover is increasingly reduced below 75% the potential for rainfall run-off and subsequent soil erosion is increased markedly. This is another reason for not overgrazing pastures Overgrazing and soil compaction. Even moderate stocking levels lead to soil compaction over time. Animal treading, especially during wet conditions, can cause soil compaction and reduction in soil pore space in the top 10 cm of pasture soil. Restoration of compacted soil takes several years


Pasture improvement. Improving the chemical fertility of, and plant and animal production from, pastures by the application of inorganic fertilisers and the use of legumes increases the biological activity in soil by around three fold. In addition, unless nutrients that are exported from the system in products (wool, meat, hay) or erosion are replaced, chemical fertility will decline even in native pastures. However, some would argue that the continuing application of fertilisers is not an ecologically sustainable practice even though the farm can be more productive. For profits to remain sustainable, wool and sheep prices would have to be able to offset the cost of fertiliser application. In addition, the long-term effects of fertiliser and legume use leading to soil acidification is not an ecologically or economically sustainable practice unless acidity can be profitably counteracted with lime Use of pesticides. Although some herbicides and antiparasitic drugs affect soil biota adversely, there are some positives. Newer herbicides are better at targeting only plants with fewer side-effects on non-target soil biota. At Armidale, we have calculated that in a worse-case-scenario for the ecotoxic effects of ivermectin, where sheep were pulse-dosed several times per year and when the decay of dung containing ivermectin was delayed for a full year, that nutrient cycling would decline, at the most, by 5%. Such an ecological cost is small in relation to the benefit received from the drug. However, the use of slow-release formulations of the drug where the device delivers the drug over much longer time intervals (e.g. 100 days) has been shown to affect dung fauna on pastures for much longer than single pulse doses and may be less compatible with ecological sustainability of pastures. Timing of dosing in a worm control program could possibly be altered so that the relatively harmless antiparasitic drugs could be used during the insect breeding season.

Readings The following readings are available on CD:1. Chan, Y. 2004, 'Soil structure and soil biota: their interactions and implications on soil health', in Proceedings of a Workshop on Current Research into Soil Biology in Agriculture, Tamworth Sustainable Farming Training Centre, August 2004, 'Soil Biology in Agriculture' (ed. R. Lines-Kelly), pp. 39-45. NSW Department of Primary Industries, Orange.2. King, K.L. 1994, 'Pasture management effects on soil biota', in Proceedings of the 9th

Conference of the Grassland Society of NSW Inc. July 1994, pp. 8-15.3. Schwenke, G. 2004, 'Soil organic matter, biological activity and productivity: myths and realities', in Proceedings of a Workshop on Current Research into Soil Biology in Agriculture, Tamworth Sustainable Farming Training Centre August 2004, 'Soil Biology in Agriculture' (ed. R. Lines-Kelly), pp. 25-32. NSW Department of Primary Industries, Orange.

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SummarySummary slides are available on CD.Economic and ecological sustainability of pastoral enterprises is dependent on the chemical, physical and biological fertility of the soil. The two main types of soil biota are invertebrate animals and microbes. The major roles of soil biota in sustaining the chemical and physical fertility are the decomposition of organic detritus and the recycling of nutrients for re-use by plants and in enhancing the structural stability of soil. Many management practices in a sheep production enterprise affect the abundance, species diversity and functioning of the soil biota (e.g. overgrazing, pasture improvement and pesticide use). Some principles for the sustainable management of soil in sheep production are outlined: prevention of overgrazing, importance of preservation of adequate vegetative and litter ground cover, organic matter management and prevention of erosion.

ReferencesDoran, J.W. & Parkin, T.B. 1994, ‘Defining and assessing soil quality’, in Defining Soil Quality for a

Sustainable Environment. (Eds. D.W. Doran, D.F. Bezdicek & D.C. Coleman), Soil Science Society of America Special Publication No.35, pp. 3-21.

Wardle, D.A. 2002, Communities and Ecosystems. Linking the Aboveground and Belowground Components, Princeton University Press, Princeton.

Glossary of termsArthropod A hard-bodied invertebrate animal with jointed legs (e.g. insects,

millipedes)

Biodiversity The variety of genes, species and ecosystems

Chemical fertility Nutrient status of soil

Dispersion The breakdown of soil aggregates

Earthworm cast Earthworm faeces - a mixture of organic matter and mineral soil

Ecotoxicity Toxic effect of a chemical on non-target organisms in the environment

Free-living Organisms that are not parasitic or in a symbiotic relationship with other organisms

Hypha Fungal filament

Macropores Large spaces in soil made by large soil animals or roots which have died and rotted

Pasture ley A pasture phase in a cropping rotation, often lasting more than one year

Physical fertility Physical state of the soil

Polysaccharides Sticky secretions from soil microbes

Rhizosphere The zone a few millimetres wide around roots – a hotbed of biological activity

Species assemblage The community of different species of a particular type within an ecosystem

Species richness The number of species within a defined community of organisms

Slaking The breakdown of soil aggregates

Superphosphate A phosphatic fertiliser which also contains sulphur (9% P, 10.5% S)

Soil biota Non-plant organisms that live in soil


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