Jones, C. E. (2006) Australian Farm Journal, February 2006, pp. 55-58

‘Potential for high returns from more soil carbon’

Australia has the highest per capita rate of greenhouse gas emissions in the world.

Appropriately managed farmlands could effectively ‘mop up’ most of the excess carbon being emitted to the atmosphere, converting a potential hazard into an extremely productive opportunity.

The risks and problems associated with low soil carbon and the benefits associated with high soil carbon should theoretically provide sufficient impetus for change. To date that has been slow to happen. In the context of ‘conventional’ agriculture, the greenhouse equation remains way out of balance.

Carbon credits for soil

Enter the era of carbon farming and carbon credits for soils. Carbon trading operates on many levels from simple bilateral agreements to fully government legislated and regulated arrangements. Emission reduction certificates for carbon sequestration (storage) are currently worth AUS$10–15/t of CO2 equivalent. Projections suggest values could even soar to $170/t CO2 equivalent as the carbon market ‘hots up’.

Measuring carbon you can see is a lot easier than measuring carbon you can’t. Hence carbon accounting and carbon credits schemes began with what was most easily quantifiable, that is, the carbon in trees. Not all areas of Australia are suited to forestry, and even if they were, we need to feed the nation. When cropping and grazing lands are nurtured in ways that improve levels of biological activity, not only will our food be healthier, but we will be sequestering atmospheric carbon in soil as a bonus.

Calculating soil carbon

 Healthy soil has both organic and inorganic components. The inorganic portion is derived from weathered rock particles and minerals, while the organic portion is derived from a wide range of living things including plants, animals, insects and microbes.

Soil carbon content is usually expressed as either a concentration (%) or a stock (t/ha). Unless the depth of measurement and soil bulk density parameters are known, it is not possible to accurately convert from one unit of measurement to the other.

For the sake of illustration however, some simple assumptions can be made. Changes in the stock of soil carbon (t/ha) for each 1% change in measured organic carbon (OC) status for a range of soil bulk densities and measurement depths are shown in Table 1. Numbers in brackets represent tCO 2 equivalent. An explanation of these terms follows.

 Soil bulk density (g/cm 3) is the dry weight (g) of one cubic centimetre(cm 3)of soil. The higher the bulk density the more compact the soil. Generally, soils of low bulk density are well structured and have ‘more space than stuff’. The lower the bulk density the more room for air and water and the better the conditions for soil life and nutrient cycling. Bulk density usually increases with soil depth. To simplify the table it was assumed that soil bulk density did not change with depth

CO 2 equivalent. Every tonne of carbon lost from soil adds 3.67 tonnes of carbon dioxide (CO 2) gas to the atmosphere. Conversely, every 1 t/ha increase in soil organic carbon represents 3.67 tonnes of CO 2 sequestered from the atmosphere and removed from the greenhouse gas equation.

For example, from TABLE 1 we can see that a 1% increase in organic carbon in the top 20 cm of soil with a bulk density of 1.2 g/cm 3 represents a 24 t/ha increase in soil OC which equates to 88 t/ha of CO 2 sequestered.

TABLE 1.Changes in the stock of soil carbon (tC/ha) for each 1% change in measured organic carbon (OC) status for a range of soil bulk densities and measurement depths. Numbers in brackets represent tCO 2 equivalent.

Soil bulk density (g/cm 3)

	1.0	1.2	1.4	1.6	1.8
Soil depth
0 - 10 cm	10 (37)	12 (44)	14 (51)	16 (59)	18 (66)
0 - 20 cm	20 (74)	24 (88)	28 (103)	32 (117)	36 (132)
0 - 30 cm	30 (110)	36 (132)	42 (154)	48 (176)	54 (198)

 

Value of soil carbon. Sequestered carbon is a tradeable commodity. It has different values in different markets and the price is subject to market fluctuation. If the CO 2 equivalent in the above example was worth $15/t, the value of sequestered soil carbon in ‘carbon credits’ would be $1,320/ha. If the soil carbon concentration was increased by 1% to a depth of 30cm rather than to 20 cm, this would represent 132 t/ha sequestered CO 2 at a value of $1,980/ha.

If organic carbon concentrations were increased by 2% to a depth of 30 cm in the same example, this would represent $3,960/ha, that is, almost $400,000 in ‘carbon credits’ per 100 ha of regenerated land. These levels of increase in soil carbon are achievable, and have already been achieved, by landholders practicing regenerative cropping and grazing practices.

Even if organic carbon levels were only increased by 0.5% in the top 10 cm of soil this would represent 22 t/ha sequestered CO 2 valued at $33,000 per 100 ha regenerated land (assuming a soil bulk density of 1.2 g/cm 3 and a price of $15/t CO2 equivalent).

Carbon credits for sequestered carbon are not an annual payment. In order to receive further credits, the level of soil carbon would need to be further increased. It is also important that the OC level for which payment was received is maintained.

This is not difficult with regenerative regimes in which new topsoil is being formed. Biological activity is concentrated in the top 10cm of most agricultural soils, but regenerative practices rapidly expand this activity zone to 30 cm and deeper. Many benefits in addition to potential carbon credits accrue to increased root biomass and increased levels of biological activity in soil.

Carbon accounting

Organic matter is not easily extracted from soil and is almost impossible to measure. Because all matter derived from living things contains carbon, it is the carbon that is usually measured in Australian laboratory tests. This may then be converted to an estimate of organic matter for the soil test report. Hence the terms ‘organic matter’ and ‘organic carbon’ tend to be used interchangeably. Technically speaking, they represent different things. A conversion factor needs to be applied in order to compare one with the other.

Soil organic matter is approximately 58% carbon and the conversion factor commonly used to convert organic matter (OM) to organic carbon (OC) is to multiply by 0.58. To convert OC to OM, multiply by 1.72. For example, 5% OM on a soil test equates to 2.9% OC. However, a landholder receiving a soil test result with 2.9% OC may believe they have less soil carbon than a landholder receiving a soil test result with 5% OM. This creates a great deal of confusion.

To further complicate the issue, the level of carbon in soil organic matter can vary and is not always 58%. There would be less inaccuracy if soil tests reported the measured levels of OC rather than converting these to a derived level of OM.

Simplifying soil carbon

The carbon in soils moves between various ‘pools’, some of which are short-lived while others remain for thousands of years. For scientific and research purposes it is important to know how much carbon is in each of these fractions. For practical purposes however, soil carbon accounting does not need to be difficult. As soil carbon levels increase, more of the carbon will evolve into stable forms.

Standardisation with respect to which organic fractions are measured and how they are measured is required. Some methods include all carbon-containing materials, whereas others may exclude various components of the soil carbon pool, such as undecayed plant or animal remains, charcoal, microbial biomass or important glycoproteins such as glomalin.

What is undisputed is that the majority of soils in Australia have lost enormous quantities of organic carbon and this process needs to be reversed. What has gone up must come down. Soils, plants, animals and people will benefit when we take ‘recycle and re-use’ to the next logical step and recycle the excess carbon currently in the atmosphere.

A new era

Turning things around will require working on the PLUS side of the Greenhouse Equation, using management practices that increase soil carbon levels and build new topsoil. The more regenerative the management regime, the more atmospheric carbon will be channelled into soils. The more carbon in soils, the more life. The more life the better the soil structure resulting in more air, more water, higher nutrient availability and potentially more green leaves. More green leaves means more capture of solar energy and higher productivity. This positive feedback loop begins with improvements to groundcover management resulting from fundamental redesign of land management techniques.

The current situation is this:

 

Greenhouses gas emissions from agricultural soils currently represent huge losses of valuable farm resources. A carbon credits trading scheme for soil carbon would create the economic incentive for rapid development of market-based innovative technologies to reverse this trend.

When soil carbon sequestration becomes the core business of the farm business, we will witness a new era in and on the land. Carbon compounds provide the fuel for the soil engine. The sooner landholders are rewarded for improved levels of soil carbon the better.

It can be done
If we can measure it
It can be rewarded

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Further information:

‘Carbon storage in soils – myth or reality?’ Australian Farm Journal, August 2005, pp. 18-21

‘Dollars for soil carbon’ Australian Farm Journal, September 2005. p. 58.

‘Managing the carbon cycle’ Australian Landcare, September 2005, p. 41

‘Smarter farming cuts global warming’ Australian Farm Journal, October 2005, pp. 54-56.

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