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on.jpg
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Report Text.docx
Soil Ca
on Management and Ca
on Trading
Bureau of Rural Sciences, Department of Agriculture, Fisheries and Forestry
By Jeff Bradshaw, Peter Sainsbury and Suzanne Wicks
Key Points
1. Soils play an important role in the global ca
on cycle, both as sources and sinks of ca
on. In soils, ca
on exists in two forms— organic and inorganic. It is the organic form (refe
ed to as organic soil ca
on) that is most likely to be included in ca
on trading. Most organic soil ca
on comes from the decay of organic matter such as plants, animals and microbes.
2. There is potential to increase stores of organic soil ca
on. Relatively small increases in the proportion of organic soil ca
on could make a significant contribution to reducing atmospheric ca
on. In addition, increasing organic soil ca
on can improve productivity and provide other beneficial ecosystem services such as erosion control.
3. The ability of any soil to abso
additional ca
on depends on many factors, including existing levels of ca
on, soil type, temperature, rainfall, and how the land is managed.
4. Accurately measuring changes in organic soil ca
on for the purposes of ca
on trading can be difficult and expensive.
5. There are risks of potential leakage of organic ca
on from the soil pool and some undesirable consequences associated with methods for increasing organic soil ca
on. For example, increasing fertiliser use to improve plant productivity may adversely affect the environment and generate greenhouse gas emissions.
6. There may be a role for organic soil ca
on in ca
on trading. Important issues include the economic potential of landholders to participate in ca
on trading and the ability of soil ca
on projects to meet certain technical requirements.
7. More work is required to develop consistent methods for measurement and to understand the risks from climate variability and climate change on organic soil ca
on as well as the effects of different farming systems and land-use practices on stores of organic soil ca
on.
8. Click here to refer to our conclusions.
Introduction
Soil is both a source of greenhouse gases and a sink for ca
on. In total, soils contain about 3 times more ca
on than the atmosphere and 4.5 times more ca
on than all living things. Hence, relatively small increases in the proportion of soil ca
on could make a significant contribution to reducing atmospheric ca
on. Agriculture occupies about 60 per cent of Australia’s land surface and, if there is a role for soil ca
on in reducing ca
on in the atmosphere, much will depend upon agricultural managers.
The purpose of this Science for Decision Makers
ief is to investigate the role of soil in capturing and storing (or sequestering) ca
on emissions.
What is soil ca
on?
Soil ca
on exists in various forms with differing longevity. Inorganic ca
on, such as calcite and dolomite, makes up to a third of total soil ca
on but is relatively stable and, except for lime applications, is not strongly influenced by land management. Therefore, it is usually ignored when considering the effects of soil ca
on on agricultural production and ca
on sequestration.
Organic soil ca
on is the organic form of ca
on found in soil organic matter. It is more manageable than inorganic soil ca
on, especially as a ca
on store.
Soil organic matter comprises leaf litter, plant roots,
anches, soil organisms and manure. Its chemical, physical and biological properties influence soil quality and function. Soil organic matter decays at varying rates, depending on chemical composition, into various components of ‘non-living organic matter’ such as particulate organic matter, dissolved organic matter, humus and inert organic matter. While these components are important for production, it is the actual organic ca
on content of this non-living matter that is relevant to ca
on sequestration.
‘Living organic matter’—living plants, animals and microbes—typically accounts for a small and variable proportion of soil organic matter.
There is a continuum of forms of organic ca
on in most soils. For convenience, organic soil ca
on may be grouped into three conceptual pools—fast, slow and passive—with different times to
eak down (known as residence or turnover times). The passive pool is the most stable, followed by the slow pool. The proportion of organic soil ca
on in these pools varies.
Typically, organic soil ca
on accounts for less than 5 per cent of soil mass and diminishes with depth. The store of organic ca
on in the top 30 centimetres of Australian soils commonly ranges from 5 to 250 tonnes ca
on per hectare.
What benefits are provided by organic soil ca
on?
Increasing organic soil ca
on enhances the level of services provided by soils to humans, including:
• ca
on storage
• food and habitat for biodiversity
• nutrient storage and supply
• erosion control
• buffering capacity (to moderate changes in pH and perhaps adso
pesticides)
• soil moisture retention.
Increases in organic soil ca
on could be a ‘win–win’ situation, helping to reduce greenhouse gas levels in the atmosphere and improving soil quality with flow-on ecosystem service benefits for agricultural and forestry industries.
The impacts of increasing organic soil ca
on should be considered on a case-by-case basis, noting that some strategies adopted by land managers might have adverse impacts. For example, increasing fertiliser and pesticide use to improve plant productivity may increase nitrous oxide emissions (another greenhouse gas) or adversely affect the local environment.
How does organic soil ca
on change?
At any one time, the organic ca
on content of soil is a balance between ca
on inputs and losses. The major inputs are dead and decaying plants, animals and microbes. Organic ca
on is lost from the soil through leaching, erosion, and conversion to ca
on dioxide through mineralisation or respiration. A key loss is through mineralisation, primarily by microbial activity in the upper layers of the soil. Because of its highly transient nature, respiration from live plant roots is not calculated in losses of organic soil ca
on for ca
on sequestration purposes. Losses of organic soil ca
on in any one year occur most rapidly in the fast ca
on pool, less in the slow ca
on pool and are usually negligible in the passive pool.
Organic soil ca
on is influenced by soil type, position in the landscape, climate, management and soil biota. Typically, organic soil ca
on content is greater at the surface and diminishes with depth. However, in some soils, high concentrations of organic soil ca
on can be found at depths greater than 50 centimetres. The variability of organic soil ca
on across fields can be substantial and can show different patterns at different depths in the profile.
Climate can influence the amount of organic ca
on in soil because biological processes such as decay are affected by soil temperature, oxygen levels and soil moisture. As long as soil moisture is sufficient, higher temperatures lead to a faster rate of decomposition and respiration. Soils in humid regions generally have higher organic soil ca
on contents because of increased plant growth and biomass production. However, wetter soils lead to faster rates of decomposition provided there is sufficient oxygen.
The amount and quality of organic ca
on inputs into the soil are a function of the vegetation present. Increasing plant biomass production would be likely to increase organic soil ca
on. Animals such as earthworms, ants and termites may also influence the amount of stable organic soil ca
on at lower depths in some soils. There is limited information on how vegetation and organisms affect the organic ca
on levels in the stable organic ca
on pools at different levels down a soil profile. However, decomposition of organic soil ca
on is normally slower with increasing depth in a soil.
How can the organic ca
on in soils be increased?
There are two major strategies for managers to reduce soil ca
on losses or potentially increase the amount of ca
on sequestered in soils: changing land management practices to achieve ‘attainable’ levels or enhancing sequestration to achieve ‘potential’ levels.
The ‘attainable’ level is determined by soil type and climatic factors, through effects on plant growth and rates of mineralisation. Available estimates show that the attainable level of organic ca
on in many Australian soils is generally limited by rainfall, although it may be limited by temperature or nutrition in some areas and seasons.
How is organic soil ca
on measured and estimated?
The density of organic soil ca
on (mass of organic soil ca
on per unit area) is used for calculating changes in the amount of organic soil ca
on in the soil profile and would be important if organic soil ca
on were included in a ca
on trading scheme. Measurements of the density of organic soil ca
on are determined from the concentration of organic soil ca
on at various soil depths and bulk density (weight per unit volume of soil). Organic soil ca
on concentration can be directly measured through the use of wet or dry combustion techniques.
What will be the impact of a changing climate?
The effect of a changing climate on organic soil ca
on is likely to be mixed and change with time. Increased atmospheric ca
on dioxide may increase inputs of ca
on to soil (through greater inputs of plant biomass). However, increased soil temperature is likely to hasten the
eakdown of organic matter and the emission of ca
on dioxide from the soil.
Changes in rainfall, nitrogen availability and fire regimes are also likely to influence the level of organic soil ca
on. A reduction in rainfall is likely to decrease inputs of ca
on (such as plant biomass) to the soil, and decrease rates of organic soil ca
on
eakdown as a result of limited water availability. An increase in rainfall may lead to an increase in the
eakdown of organic soil ca
on, provided there is sufficient oxygen and soil nitrogen. Burning can adversely impact organic soil ca
on levels and may also be a significant emitter of greenhouse gases.
Is there a role for organic soil ca
on in ca
on trading?
There may be a role for organic soil ca
on in a voluntary ca
on market or ca
on trading scheme. Important issues include the economic potential for landholders and the ability of soil ca
on projects to meet the technical requirements for ca
on trading.
Farmers are likely to be interested in sequestering ca
on in their soils if the price per tonne of ca
on received is greater than the opportunity cost of capturing and storing the ca
on. The opportunity cost of ca
on sequestering practices includes any loss in revenue from changed agricultural production, increased fixed costs because of new machinery and additional transaction costs.
Conclusion
Increasing organic soil ca
on has the potential to reduce atmospheric ca
on and offers benefits for farmers through improved ecosystem services, such as better soil condition and increased productivity.
Inclusion of organic soil ca
on in a ca
on trading scheme or voluntary offset market will require improved methods for measuring and monitoring and a better understanding of soil ca
on in the Australian context, such as:
• the limits to storing ca
on for long times in Australian soils under changing climates
• management practices that can demonstrably increase sequestration of organic ca
on in soils
• effects these practices have on the delivery of other ecosystem services, including