April - Soils and biodiversity

What types of species do you have in your mind’s eye when you consider biodiversity?
Biodiversity often conjures thoughts of mammals, plants, birds, insects, spiders and molluscs. Organisms which are not easily distinguished by eye are probably less likely to come to mind. Visible organisms are far more likely to be considered when it comes to biodiversity than the seething masses of microscopic invertebrates, fungi and bacteria which inhabit soils.
There is a whole world down there below our feet!

Topsoil is the most prolific part of healthy and productive terres trial ecosystems. The animals, plants and micro-organisms living in topsoils are both highly numer ous and highly diverse. For in- stance, in a cubic centimetre of soil there can easily be as many as 6 billion microorganisms – about as many people as there are alive on the planet! Figures a thousand-fold higher are not uncommon. Some estimates are that 25% of the living matter on the planet is contained within the soil. It is clear that soil biodiversity is a major and essential component of terrestrial biodiversity.

Soil conservationists examining severe soil erosion in central western NSW.
Photo : DLWC

It is the soil-dwelling species that we cannot see which are arguably the most important for maintaining healthy ecosystems. Soil micro-organisms can be divided into functional groups, depending on the vital roles they perform in breaking down, absorbing, recycling and making available plant and animal nutrients – nitrogen, phosphorus and other elements which are essential to plant and animal life (such as Attiwill and Leeper, 1990).

Soil organic matter is the main product of soil biology. Soil organic matter is a key component of terrestrial ecosystems and variation in its ecology has profound effects on many processes (Spain et. al, 1983). The amount of photosynthetic energy an ecosystem requires to accumulate soil organic matter is astounding. It has for instance been estimated that the amount of energy required for an increase in soil organic matter by one percent of soil weight would require plants to produce the equivalent of 10,000 litres of petrol over 50 years. Plants are known to exude large portions of photosynthetic product from the roots into the soil, such as 30% for wheat (Martin, 1977). Soil organic matter can be considered the bank account for terrestrial biodiversity. With soil-formation rates being very slow, and most Australian soils being old and infertile, the management of the capital (organic matter) becomes an important consideration. This applies not only to biodiversity values but also to sustainable food and fibre production.

Many soil-inhabiting species remain undescribed. For instance, around half of the estimated 108,000 species of Australian insects have not been named. Knowledge of the taxonomy of soil- dwelling insects is below average for all Australian insects (Greenslade and Greenslade, 1983). The ecological roles and lifecycles of many soil-living species are poorly known.

The distribution of soil organisms is complex and obviously dependent on soil type. For instance, waterlogged soils are often devoid of oxygen (oxygen diffuses 10,000 times more slowly through water than through the atmosphere). Wet soils run out of oxygen due to biological demands and tend to have a very different group/ community of micro-organisms, which creates entirely different soil conditions and hence plant species and related organisms.

To assist our understanding of soils and biodiversity, among other things, the Department of Land and Water Conservation has a program which characterises the distribution of soil and land types across eastern and central NSW (Soil Landscape Mapping). One use of this information will be to use soil-type distribution as a sample site selection guide for those who are interested in investigating soil biodiversity and the role of soil organisms on ecosystem functions.

The effect of human impacts on soil biodiversity is difficult to estimate and directly measure. However, we can all readily recognise that severely degraded land has been stripped of most of its biodiversity. Severely degraded land may be difficult to rehabilitate. It is certainly cheaper to manage and maintain healthy soils than it is to repair degraded landscapes.

Soil conservationists examining severe soil erosion in central western NSW
Photo : DLWC.

Soil management is fundamental to ecosystem management. Where vegetation has been cleared but the soil resource remains complete, it is much easier to rehabilitate the ecosystem. This is because the foundations remain: soil organisms, seed stores and nutrients are able to be readily used by recolonising species.

Once the soil has been significantly degraded through erosion, compaction, salinisation or acidification, and soil organic matter is destroyed, then the job of recovery is much harder.

Consider the biodiversity values of the degraded land in the photos with this article. The energy, nutrients and difficulty of re-establishing a soil biological culture on these sites in order to re-establish a functioning ecosystem can be very challenging. Care and prevention is better than cure.

In NSW the land degradation survey of 1988 found that over 70% of points surveyed on a 5 km by 5km grid in the east and a 5km by 10km grid in the west of the State was affected by at least one form of land degradation (Graham, 1988). Approximately 29% of the points surveyed were severely affected by one or more forms of land degradation. The question of sustainable soil management is often overlooked when biodiversity is discussed.

References
Attiwill PM and Leeper GW (1990) Forest Soils and Nutrient Cycles.
Melbourne University Press.

Graham OP (1988) Land Degradation Survey NSW 1987-1988.
Soil Conservation Service of NSW.

Greenslade PJM and Greenslade, Penelope (1983) "Ecology of Soil Invertebrates".
In Soils – an Australian Viewpoint.
CSIRO, 645-671.

Martin JP (1977) "Factors influencing the loss of organic carbon from wheat roots".
Soil Biol Biochem, 9, 1-9.

Spain AV, Isbell RF and Probert ME (1983) "Soil Organic Matter".
In Soils – an Australian Viewpoint.
CSIRO, 551-555.

* Greg Chapman is Manager of the Soils Information and Planning section of the NSW Department of Land and Water Conservation.