Online Tutoring on Soil Carbon Management and Carbon Trading

Key Points

Soils play an important role in the global carboncycle, both as sources and sinks of carbon. In soils, carbon exists in two forms— organic and inorganic. Between the two, the organic form (referred to as organic soil carbon) is most likely to be included in carbon trading. Most organic soil carbon comes from the decay of organic matter such as plants, animals and microbes.
There is potential to increase stores of organicsoil carbon. Relatively small increases in the proportion of organic soil carbon could make a significant contribution to reducing atmospheric carbon. In addition, increasing organic soil carbon can improve productivity and provide other beneficial ecosystem services such as erosion control.
The ability of any soilto absorb additional carbon depends on many factors, including existing levels of carbon, soil type, temperature, rainfall, and how the land is managed.
Accurately measuring changes in organicsoil carbon for the purposes of carbon trading can be difficult and expensive.
There are risks of potential leakage of organiccarbon from the soil pool and some undesirable environmental consequences associated with methods for increasing organic soil carbon. For example, increasing fertiliser use to improve plant productivity may adversely affect the environment and generate greenhouse gas emissions.
There may be a role for organicsoil carbon in carbon trading. Important issues include the economic potential of landholders to participate in carbon trading and the ability of soil carbon projects to meet certain technical requirements.
More work is required to develop consistent methods for measurement and to understand the risks from climatevariability and climate change on organic soil carbon as well as the effects of different farming systems and land-use practices on stores of organic soil carbon.

Introduction

Soil is both a source of greenhouse gases and a sink for carbon. In total, soils contain about 3 times more carbon than the atmosphere and 4.5 times more carbon than all living things. Hence, relatively small increases in the proportion of soil carbon could make a significant contribution to reducing atmospheric carbon. Agriculture occupies about 60 per cent of Australia’s land surface and, if there is a role for soil carbon in reducing carbon in the atmosphere, much will depend upon agricultural managers.

The purpose of this Science for Decision Makers brief is to investigate the role of soil in capturing and storing (or sequestering) carbon emissions.

What is soil carbon?

Soil carbon exists in various forms with differing longevity. Inorganic carbon, such as calcite and dolomite, makes up to a third of total soil carbon 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 carbon on agricultural production and carbon sequestration.

Organic soil carbon is the organic form of carbon found in soil organic matter. It is more manageable than inorganic soil carbon, especially as a carbon store.

Soil organic matter comprises leaf litter, plant roots, branches, 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 carbon content of this non-living matter that is relevant to carbon sequestration. ‘Living organic matter’—living plants, animals and microbes—typically accounts for a small and variable proportion of soil organic matter.

Figure 1: Composition of Soil Organic Matter[1]

There is a continuum of forms of organic carbon in most soils. For convenience, organic soil carbon may be grouped into three conceptual pools—fast, slow and passive—with different times to break down (known as residence or turnover times). The passive pool is the most stable, followed by the slow pool. The proportion of organic soil carbon in these pools varies.

Typically, organic soil carbon accounts for less than 5 per cent of soil mass and diminishes with depth. The store of organic carbon in the top 30 centimetres of Australian soils commonly ranges from 5 to 250 tonnes carbon per hectare.

What benefits are provided by organic soil carbon?

Increasing organic soil carbon enhances the level of services provided by soils to humans, including:

Carbon storage
Food and habitat for biodiversity
Nutrient storage and supply
Erosioncontrol
Buffering capacity (to moderate changes in pH and perhaps adsorb pesticides)
Soil moisture retention.

Increases in organic soil carbon 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. This ‘win-win’ situation is shown in the below figure.

Figure 2: Win-Win Situation[2]

The impacts of increasing organic soil carbon 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 carbon change?

At any one time, the organic carbon content of the soil is a balance between carbon inputs and losses. The major inputs are dead and decaying plants, animals and microbes. Organic carbon is lost from the soil through leaching, erosion, and conversion to carbon 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 carbon for carbon sequestration purposes. Losses of organic soil carbon in any one year occur most rapidly in the fast carbon pool, less in the slow carbon pool and are usually negligible in the passive pool.

Organic soil carbon is influenced by soil type, position in the landscape, climate, management and soil biota. Typically, organic soil carbon content is greater at the surface and diminishes with depth. However, in some soils, high concentrations of organic soil carbon can be found at depths greater than 50 centimetres. The variability of organic soil carbon across fields can be substantial and can show different patterns at different depths in the profile.

Climate can influence the amount of organic carbon 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 carbon 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 carbon inputs into the soil are a function of the vegetation present. Increasing plant biomass production would be likely to increase organic soil carbon. Animals such as earthworms, ants and termites may also influence the amount of stable organic soil carbon at lower depths in some soils. There is limited information on how vegetation and organisms affect the organic carbon levels in the stable organic carbon pools at different levels down a soil profile. However, decomposition of organic soil carbon is normally slower with increasing depth in a soil.

How can the organic carbon in soils be increased?

There are two major strategies for managers to reduce soil carbon losses or potentially increase the amount of carbon 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 carbon 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 carbon measured and estimated?

The density of organic soil carbon (mass of organic soil carbon per unit area) is used for calculating changes in the amount of organic soil carbon in the soil profile and would be important if organic soil carbon were included in a carbon trading scheme. Measurements of the density of organic soil carbon are determined from the concentration of organic soil carbon at various soil depths and bulk density (weight per unit volume of soil). Organic soil carbon 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 carbon is likely to be mixed and change with time. Increased atmospheric carbon dioxide may increase inputs of carbon to soil (through greater inputs of plant biomass). However, the increased soil temperature is likely to hasten the breakdown of organic matter and the emission of carbon dioxide from the soil.

Changes in rainfall, nitrogen availability and fire regimes are also likely to influence the level of organic soil carbon. A reduction in rainfall is likely to decrease inputs of carbon (such as plant biomass) to the soil, and decrease rates of organic soil carbon breakdown as a result of limited water availability. An increase in rainfall may lead to an increase in the breakdown of organic soil carbon, provided there are sufficient oxygen and soil nitrogen. Burning can adversely impact organic soil carbon levels and may also be a significant emitter of greenhouse gases.

Is there a role for organic soil carbon in carbon trading?

There may be a role for organic soil carbon in a voluntary carbon market or carbon trading scheme. Important issues include the economic potential for landholders and the ability of soil carbon projects to meet the technical requirements for carbon trading.

Farmers are likely to be interested in sequestering carbon in their soils if the price per tonne of carbon received is greater than the opportunity cost of capturing and storing the carbon. The opportunity cost of carbon-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 carbon has the potential to reduce atmospheric carbon and offers benefits for farmers through improved ecosystem services, such as better soil condition and increased productivity.

Inclusion of organic soil carbon in a carbon trading scheme or voluntary offset market will require improved methods for measuring and monitoring and a better understanding of soil carbon in the Australian context, such as:

The limits to storing carbonfor long times in Australian soils under changing climates
Management practices that can demonstrably increase sequestration of organiccarbonin soils
Effects these practices have on the delivery of other ecosystem services, including food production
Practices that increase the level of stable carbonpools deeper down a soilprofile.

Acknowledgements

Bureau of Rural Sciences, Canberra.

[1] Department of Primary Industries and Regional Development. (2019). What is Soil Organic Carbon? https://www.agric.wa.gov.au/measuring-and-assessing-soils/what-soil-organic-carbon

[2] Vermeulen, S., Bossio, D., Lehmann, J., Luu, P., Paustian, K., Webb, C., Augé, F., Bacudo, I, Baedeker, T., Havemann, T., Jones, C., King, R., Reddy, M., Sunga, I., Unger, M.V. & Warnken, M. (2019). A Global Agenda for Collective Action on Soil Carbon. Nature Sustainability. 2:2-4

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