Why Study the Soil?– The soil, as the major medium for plant growth, is the basic resource for all land use and development. Ecologically sustainable development is not feasible unless it includes, as a basic concept, the conservation and sustainable use of soils.
The National Soil Conservation Program (NSCP), which was established to ‘develop and implement national activities for the rehabilitation and sustainable use of the nation’s soil and land resources’, is broadly speaking physically and chemically based. Some of the major study areas include erosion, soil structural decline, acidification and salinisation, invasion of productive land by native woody plants, mass movement and water repellency. Originally the NSCP did not have biodiversity objectives in mind.
In 1993 the CSIRO started a multi-divisional program (MDP) ‘to produce a national framework for conserving biological diversity and maximising its economic benefits, through a national collaborative venture involving all appropriate agencies’. The group of which I am a part is particularly interested in two of the ten projects which make up this research program. These are ‘methods for characterising, sampling and quantifying biodiversity in soil and litter and on the soil surface’ and ‘effect of land-use practices on the biodiversity of functional groups of soil organisms’.
Soils are products of interactions between abiotic processes, including physical and chemical weathering, temperature regimes, hydrology etc., and biotic processes, including the production of organic matter (plants) by photosynthesis, uptake of water and nutrients by plants and the eventual return of this organic matter to the soil through decomposition.
The soil biota are extremely important in the regulation of plant nutrient uptake and release and organic matter decomposition. They are also important contributors to the formation of stable soil aggregates, and larger ones (especially earthworms, termites and ants) to soil porosity and the infiltration of water. These biotic processes, together with the abiotic physical and chemical processes, are prime regulators of soil fertility. If we are to protect our soil, arguably our most important resource, we must know more about the biota it contains.
The Magnitude of the Problem– Algae, bacteria, fungi, protozoa, viruses and invertebrates make up the soil biota that I am concerned with here. The total biomass of this biota in a fertile soil may exceed 20 tonnes per hectare. They range in size from the microscopic to earthworms that are more than one metre in length and can weigh more than 500g. The numbers of organisms in the soil can be enormous – invertebrate numbers can easily exceed 100 000 per square metre given the right conditions. However, what they are and exactly what they do is to a large extent a mystery.
It is well known that earthworms play an important role in promoting soil fertility, plant production and the rehabilitation of degraded soils. Introduction of selected European lumbricid species to pasture soils in Tasmania where those species were not formerly present has resulted in remarkable increases in pasture growth. We know little however, about our native species. Many plants are known to have associated mycorrhizal fungi that promote the uptake of nutrients from the soil, but little is known of their possible usefulness for plant establishment on degraded lands or for promoting the growth of plantations of native trees. The symbiotic nitrogen-fixing rhizobium bacteria have free-living stages that depend for their long-term survival in the soil on periodic encounters with their hosts. Agricultural practices favour the presence in the soil of strains that nodulate Trifolium species (grain legumes), while those associated with wild legume species become rare or disappear.
Less than 50 per cent of the estimated 200 000 Australian invertebrate species have been described. However, perhaps even more important, only about one per cent of these have been studied in detail, and these are often the pest ones which have been studied because of their economic and/or agricultural significance. The case with the micro fauna and flora is much worse. From these figures it is obvious that a reduction in biodiversity probably means that we are losing valuable resources that we do not even know about.
It is not feasible to describe all the species of invertebrates and micro- organisms; there are simply too many. So it is not constructive to argue that the conservation of biodiversity must wait until all species have been described. We must find ways to study biodiversity so that management decisions that have to be made now can be based on sound taxonomic and systematic knowledge.
Indicator Species and Functional Groups–Particular species have been used to indicate the biological effects of environmental disturbance. Dragonflies and aquatic beetles have been used to monitor the water quality in wetlands near Perth. A reduction in the diversity of these groups has then been attributed to several factors including the presence of pollutants such as pesticides and excessive nutrients. This reduction in diversity is then assumed to have occurred in other species occupying the same environment. Some groups of collembola and mites could be used as indicator species when comparing the biodiversity of soil in a natural ecosystem and a managed one.
Sometimes it might be more profitable to study a function in an ecosystem rather than a particular organism or group of organisms. For example it is obvious that cellulose decomposing organisms are important in a soil ecosystem (fungi, termites etc). We do not want to be up to our necks in undecomposed plant litter, but does it really matter which organisms or species do the decomposing in a particular ecosystem? If we wipe out one organism but another can take its place such that the function can continue at the required rate, is this good enough? I am not advocating that species recognised as redundant to a particular function should be allowed to become extinct of course, but asking what limits we really have. Many groups of organisms, especially invertebrates and the micro flora and fauna, are not sufficiently well known for redundancy to be recognised. They may contribute to ecosystem function in ways other than those of immediate interest, or their roles may become more significant under changing climatic or other environmental conditions. Again we just do not know enough.
Protecting Biodiversity–The level of biodiversity protection that should be deemed adequate is a difficult scientific question. All development is likely to cause some loss in biodiversity and disturbance to ecosystem processes. Protecting biodiversity means ensuring the conservation of species, ecosystems and gene pools such that essential ecological processes and life-support systems are maintained for future generations, namely, ecologically sustainable development is achieved. Over the next several years we shall be studying sections of the soil biota both in the pristine state (for which well-managed reserves set aside for the protection of biodiversity are crucial) and under different management regimes, with the specific intention of answering, or at the very least more clearly understanding, some of these questions.
CSIRO Division of Soils