-- Campaign For Old Growth | Reference Article --

The soil ecosystem is comprised of abiotic and biotic fraction whose interaction determines soil stability. The biotic fraction of soil can be managed to provide greater stability and carbon sequestering potential, with significant benefits for the terrestrial environment and the atmosphere.

The soil ecosystem has been described as a principal component of all agricultural ecosystems, the stability of which is essential to the development of sustainable agriculture (Senanayake 1990, 1991). The move to develop management strategies that achieve sustainable land use requires a good working knowledge of the ecosystem. Many functional components of the ecosystem have been researched in Australia (e.g. CSIRO, 1983), providing a good base for soil ecosystem management research.

As the current level of management skills has not generally led to sustainable methods of land use, there is a need to develop a higher degree of skill in soil management. The official tally in 1978 (CSCS, 1979), suggested that about 51 per cent of agricultural land in Australia was being subjected to some degree of degradation. Today, many new sources of land degradation have been identified and the percentage of land affected may be much higher. This state of the land has been one of the driving forces that established the current effort in Landcare (Roberts, 1990). However, agriculture is not the only reason to develop better skills in soil management. Studies on the nature of soil as an ecosystem demonstrate that it may play a significant role in the greater global cycling of carbon dioxide, the soil acts as a dynamic buffer.

The soil ecosystem is comprised of two distinct fractions called the "organic fraction" and the "inorganic fraction" (Fig. 1 [included in Journal]). There is a slow interchange between them in the form of mineralisation and decomposition. These fractions act as reservoirs that can be identified by their history. The organic fraction is composed largely of the breakdown products of photosynthetic compounds and their derivatives. The inorganic fraction is composed largely of the breakdown products of rocks. Most soils are comprised of a mix of these two fractions in various proportions.

The organic fraction of soil has been calculated to contain in total between 1200 and 1800 gigatonnes (Gt) of carbon (Kohlmaier et al., 1983). Globally, this reservoir maintains a flux with an atmosphere reservoir of 725 Gt of carbon in the form of carbon dioxide (Pearman, 1989). Most of this organic fraction is comprised of matter which originated in the process of photosynthesis with a propensity to loose energy over time. One model proposed to describe this flux is a steady state model (Houghton et al., 1985), here the net annual primary production of carbon (NPP) from the world⁴s terrestrial ecosystems is about 45-62 Gt and the loss of carbon dioxide to the atmosphere by decomposition will balance the input of plant debris to the soil. This suggests that an amount of organic carbon equivalent to that generated by NPP per cent each year. However, studies on the amount of carbon dioxide released from soils suggest a higher figure of about 75 Gt per year (Schlesinger, 1977). The implication of these figures is that not all soils are currently in a steady state with respect to soil organic matter. While accumulation of carbon occurs in certain soils (Armentano et al., 1984; Lemon, 1977), the major trends in land use such as logging and cultivation lead to increase in decomposition because of increased soil temperature, aeration and moisture (Witkamp, 1971; Marks and Borman, 1972).

Although Pearman (1987) suggests that soil organic matter is being maintained in a dynamic equilibrium and that the soil ecosystem is very effective at conserving fixed carbon, present trends in agriculture and land use, especially techniques that reduce the organic matter content of soil, may destabilize this equilibrium. The impact of modern agriculture on the organic matter content of soil has been demonstrated to be severe in many instances and has begun to affect many regions of the world (Lal, 1987). When displaced from its original matrix and exposed to changes in climate, soil organic matter will release carbon dioxide ad volatilise in a relatively short time. The mechanism of repeated drying and wetting has been demonstrated to increase carbon dioxide production 16 to 121 per cent (Sorensen, 1974). Similar rends have been observed as a result of chemical input such as increased salt in the soil environment (Theng et al, 1968) or biological processes such as a the catabolic stimulation of native soil organic matter y exogenous organic matter (Broadbent andNakashima, 1974). An end result of the process of soil degradation is thus an increase in the volume of carbon dioxide entering the atmospheric pool.

Another fact to be considered when addressing the cycling of organic matter in soil, is that soil organic matter does not degrade evenly. It is comprised of many different classes of compounds, with different classes of compounds, with different rates of cycling. This organic matter can be subdivided based on different rates of cycling. This organic matter can be subdivided based on different variables. Van Rees et al, (1985) suggest three fractions based upon size and time of residence. In this scheme, one fraction is very short lived (1-2 years) and comprises about 1-2 per cent of the total organic matter, another fraction has a moderate age (3-100 years) and comprises about 10-20 per cent of the total organic matter, and the last fraction is very old (100-4000 years) and comprises about 78-89 per cent of the total organic matter. This relationship is illustrated in Figure 2, and suggests three distinct cycles operating in a temporal sense as well as a biochemical sense.

The distribution of the various fractions in the soil suggests a process of biochemical distillation that leads to the formation of complex humic compounds. Photosynthetic products accumulate on the surface of the soil where they enter the respiration process and pass through various organic systems until the hmic compounds are formed. These large complex molecules are distributed through the soil and often serve to delineate the part of the profile termed the "solum", or the region of the soil profile that contains a visible concentration of organic matter. While many of the humic compounds have ages measured in thousands of years the "younger" molecules are concentrated at the surface of the soil and decrease exponentially with depth (O⁴Brien and Stout, 1978), suggesting that the surface soil is the site of primary synthesis. Thus management of the surface layer will have potential to affect the rate of organic matter cycling.

The process of biochemical distillation of photosynthetic products can keep atmospheric carbon dioxide tied to or sequestered by the biological system for periods exceeding 4000 years (Beckman and Hubble, 1974). While about 16 per cent of the long-lived fraction identified as "old carbon" can have lifetimes from 5700-15,000 years (O⁴Brien and Stout, 1978) the role of soil in sequestering, or tying up, atmospheric carbon dioxide has to be recognized. An evaluation of the sequestering potential of various soil ecosystems may identify terrestrial ecosystems, other than forests, that have the potential to be used in the management of atmospheric carbon dioxide.

The rate of sequestering will depend on the quality of the photosynthetic material and the condition of the soil ecosystem. Some compounds such as glucose are more readily metabolised than lignin. In reality, a gradient of substrates from "readily metabolised to "resistant" exists in the organic matter pool. The products of readily metabolised organic compounds are predominantly carbon dioxide and soil microbial biomass (Tate, 1989). This fraction releases about 80-90 per cent as carbon dioxide within 12 weeks (Kassim et al.,1981) while 3-8 per cent is incorporated into microbial biomass. Similar studies on the "resistant" fraction have demonstrated a carbon dioxide output of only 5-13 per cent and microbial biomass increase from 0-0.7 per cent within one year (Stoutet al., 1983). However, these rates vary considerably with plant species (Shanks and Olson, 1961), suggesting a range of sequestering values for different vegetation formations in similar environments. The increment of photosynthetic material into the pool of long-lived compounds can be estimated by examining the pool of humic compounds and the nature of the ecosystem that it resides in.

At the level of practical management this pool is easily detected by the dark color that it imparts to most soils. It helps delineate the A horizon of the soil, also sometimes know as the organic layer or top soil. Studies on agricultural techniques suggest a strong correlation between management and solum depth or quality. For instance, the differential effect of various cultivation techniques on the status of the solum, as measured by its carbon, nitrogen and phosphorous content, is now becoming evident (Russell and Williams, 1986). Most of the studies to date identify solum state in terms of organic carbon or nitrogen. Other indicators such as microbial population ecology have been suggested (Senanayake, 1990). Still others, like the ratio of short to long-lived compounds in sequestered carbon pools, can be described and developed.

These studies suggest that certain types of land management provide a better carbon sequestrating potential, biodiversity conservation value and other characters that accompany an increase in solum status. However, these trends in solum status may differ depending on the end-use of the ecosystem in question. For instance, an increase organic matter or a build-up of earthworms as dominant soil macro-organisms are seen to be indicators of good management in terms of agriculture, but the environmental conditions that favor such a process are also hostile to much of the native biota (Matthews, 1976; Douglas, 1987). Therefore, management goals need to be clearly stated in terms of the end-use of each ecosystem. If agricultural production is the goal, management to avoid such change will become the management response. An understanding of the differential nature of these ecosystems is central to the development of sustainable land use, and is a consideration that must be attended to when designingfor carbon sequestering by the ecosystem.

Australia provides an example of this process. Most of the crop or pasture plants used in Australian agriculture have been developed under European or Mediterranean conditions. Here, a friable soil high in organic matter has been seen as the ideal. In fact his aspect of soil has been seen as an ideal through the 800 or so years of man⁴s agricultural history (Allison, 1973). However, the clearing and preparation of forest land for agriculture initiates a process of organic matter loss that brigalow (Aacia harpophylla) scrub had a substantially higher organic carbon content than adjacent grassland communities on similar soils (Isbell, 1966). The clearing of this shrub resulted in a loss of organic matter from 14-41 per cent below the virgin level (Graham, 1976). However, impoverished soil can often regain organic matter depending on management practices. For instance, Jackman (1960) reported an increase in soil carbon from 4.6 to 20 per cent in 22 years following pasture in Taupo pumice soils of New Zealand. This represents an accumulation rat of approximately 1100 kg carbon/ha/yr. In a similar study Barrow (1969) reported a gain of 440 kg/ha/yr during a period of 30-40 years in Western Australia. In long term experiments at Rothamsted, soil that received no farmyard manure has maintained a constant amount of organic matter of about 25 t/ha, while soil that was given annual dressings of farmyard manure increased in organic matter from 25 t/ha in 1852 to 50 t/ha in 1871 (Attiwill and Leeper, 1987). Once land has been cleared and demarcated for agriculture, management to build up the organic matter content becomes an important goal.

The condition of the soil ecosystem, while being crucial to sustaining agricultural productivity, is also important in terms of providing a long term sink for atmospheric carbon. This ecosystem has the potential to sequester carbon for very long time periods. As one current global response to control carbon emissions is the proposal of a carbon tax (Krause et al., 1989) the logical corollary, that of extending a carbon credit that draws on the tax, has also been put forward (Anon, 1991a). Examples of scientifically directed action to provide sequestering have also been developed (Faure et al., 1990). The current estimates of the revenue which might be generated by a carbon tax suggest a figure of US 139 billion for America alone (Brown 1991). This figure was obtained from estimates made by the US congress at a tax rate of $100 per ton (Anon, 1990). However, in 1991 the OECD has proposed a rate of $200 per ton (Anon, 1991b). This economic scenario suggests that soil management has tremendous potential for development. The performance of Australian pasture which acts as a sink for atmospheric carbon at a rate of 3.7 million t/yr (Russell, 1986), is an indication of the need for economic re-assessment of the value of rural production.