The soil carbon story

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Soil carbon
Why is soil carbon so topical?
What is soil carbon sequestration?
Management practices that might increase soil carbon on dairy farms
Management practices that reduce soil carbon on dairy farms?
What about Bio-Char?
What are the opportunities for dairy farmers in soil carbon trading schemes?
References and further resources

Please note - before reviewing soil carbon, it's important to sound a note of caution. All the scientific evidence suggests that pasture-based dairy farmers should approach the ‘opportunity’ to sequester soil carbon and seek payment though the Carbon Farming Initiative, with considerable caution. There are significant technical and logistical barriers, and significant risks and liabilities to consider.

Soil carbon

Up to a third of the carbon in soils is not from organic origins – it comes from the breakdown of calcium containing rocks or is sometimes added to soil because carbon is a component of lime and gypsum. This inorganic carbon is quite stable, has little interaction with the biological carbon cycle and is usually ignored in discussions of soil carbon and carbon sequestration - as it is from here on in this document! The focus in discussions of soil carbon is on organic carbon, that carbon that is added to the soil through organic processes as outlined in the soil carbon cycle shown in Figure 1 or that can be viewed as an animation on the DPI Victoria website.

Figure 1. A simple diagram of the soil organic carbon cycle showing additions from plant and animal litter, and the reductions from respiration and erosion. An indication of the proportion of the soil organic carbon pool that is in the fast, slow and passive pools is shown in red, and the contributions to respiration are shown in blue. Source Bureau of Rural Sciences report March 2009 – Science for decision makers series - Soil Carbon Management and Carbon Trading

Why is soil carbon so topical?

Increasing the carbon stored in soil can potentially off-set emissions from things like electricity generation and transport. It has been proposed that carbon sequestration on farm might become a profitable ‘commodity’ for farmers.

The interest is so strong for at least four reasons:

Most projected impacts on agriculture from any schemes to put a price on carbon the CPRS or other emissions reduction schemes are negative for farmer incomes, so the prospect of being paid for increasing the soil carbon store is a very welcome change
The existing soil store of carbon is huge - about 2-3 times the size of the atmospheric store – so that modest increases in the soil carbon store could make a significantly reduce atmospheric carbon
Every farmer knows that soils with more organic matter (ie a larger soil carbon store) are more fertile, thus providing a win/win both farmers and the environment
There are strong advocates for the ‘soil carbon case’ and this has sometimes resulted in an ‘overstating’ of the case and a focus on anecdotes rather than proven results.

NB - the Kyoto rules call for any sequestration to be permanent, additional, verifiable, and enforceable. To which the Carbon Farming Initiative has added:
Avoidance of leakage – the project must not cause material increases in emissions elsewhere, which nullify or replace the abatement that would otherwise result from the project.
Conservative – conservative assumptions, numerical values and procedures should be used to ensure that abatement and other claims are not over-estimated.
Supported by peer-reviewed science – scientific evidence must have been subject to independent review and critique by scientific peers prior to publication in scientific journals.

What is soil carbon sequestration?

Soil carbon sequestration is the process of transferring carbon from atmospheric carbon dioxide into plant material, some of which is added to the soil carbon store as dead plant material or animal waste. Soil is a complex mixture of organic compounds at different stages of decomposition. Soil organic carbon is divided into different ‘pools’ that are classified according to their rate of decomposition – as shown in Figure 1.

Fast pool - freshly added plant, animal and micro-organism residues that decompose easily
Slow pool - well decomposed organic material, the humus. This pool is relatively stable unless physically disturbed, ie by activities such as ploughing
Passive pool - the fraction that is old, resistant to further breakdown and represents products in their last stage of decomposition, e.g. charcoal

The fast pool is the easiest to increase though addition of organic matter but it tends to decompose (ie return carbon to the atmosphere) quickly. The slower pools are more important for long term sequestering carbon into the soil so as to reduce atmospheric CO2 levels but are harder to add to.

The amount of carbon in the soil depends on:

The climate and soil fertility: fertile soils in high rainfall zones (or with irrigation) can support high levels of plant growth and therefore have the potential to return large amounts of organic matter to the soil. The proportion of organic matter returned to the soil that is used for respiration by soil organisms depends on the soil temperature (higher temperatures, more respiration) and soil water content. The climate and soil therefore set the upper limit for soil carbon sequestration.
The agricultural production system: more carbon tends to build up under pastures than under crops – more details below.
Management: When soils are ploughed or otherwise disturbed, soil carbon previously protected from microbial action is decomposed rapidly. Systems that encourage the addition of plant litter to the soil (eg stubble retention or lax grazing) have some potential increase the soil organic matter pool and eventually the soil carbon content but the rates of change are slow (see below).

Dairy farmers have no control over their climate, little effective control over their soil fertility (most dairy soils are already highly fertile) and have a production system based on grazed pastures. Management is therefore the only significant option if dairy farmers wish to increase soil carbon.

Management practices that might increase soil carbon on dairy farms

In theory any management practice that increases pasture production should lead to increased soil carbon because of the associated increase in plant material (roots and litter) and animal dung. Practices such as fertiliser application, improved rotational grazing, irrigation, and improved pasture species all have the potential to increase pasture production and thus soil carbon – though the impacts can be small and slow (see text box about the long term P experiment). Application of dairy effluent and sludge to pasture will also provide additional carbon inputs to the system.

These activities are already ‘best practice’ on most Australian dairy farms because of the impact that increasing soil fertility and pasture production has on farm profit. Therefore while some farmers may have the option of implementing these management practices, for most the opportunities to significantly boost soil carbon will be limited and if they are already considered good or best practice, such sequestration does not meet the requirement for ‘additionality’.

For those dairy farmers who grow crops and make silage, minimum tillage systems will reduce the rate of soil carbon decline in cropping paddocks - again minimum tillage is already best practice for most soil types.

The Hamilton Long-Term Grazing Experiment

This experiment was set up in the 1980’s to compare a range of superphosphate application rates (0 to 33 kg P/ha annually) on pasture performance (resulting in current pasture production from 4t/ha to 16 t/ha), carrying capacity (9 to 28.5 dry sheep equivalents/ha) and wool production (from 55 to 133 kg wool/ha) in a 700mm/yr rainfall environment.

These are extreme management differences that have been applied on the same soil type and in the same climate for 25 years and provide perhaps the best opportunity to explore the impacts that management practices can have on soil carbon in a relatively high and reliable rainfall environment.

There has been a trend of slowly increasing carbon sequestration with increasing rates of fertiliser application and the associated increasing rates of pasture production. However, at this stage the measured differences are not the level accepted for scientific publication.

Projecting into the future using soil carbon models indicates that low pasture production associated with zero fertiliser application will lead to a slow loss of soil carbon, particularly in soils with an initially high content – such changes though would be difficult to measure or prove over periods of less than 50 years. The high fertiliser treatment, with associated high levels of pasture production, on the other hand, would lead to long-term soil carbon gains that should be statistically significant after perhaps 30 years.

Conclusion - increased plant and animal production (ie increased fertiliser use) is likely to cause a very gradual increase in soil carbon levels.

On the positive side, farmers can have confidence that improved pasture systems are not likely to be detrimental to soil carbon sequestration and that any improvements in soil carbon will provide direct benefits in terms of soil structure and fertility.

Graham, J. Robertson, F. and Skjemstad, J. (2008) Greenhouse Emissions in the Broad Scale Grazing Industries – effect of different Pasture systems on Soil Carbon Sequestration. MLA report.

Management practices that reduce soil carbon on dairy farms?

These practices can reduce soil carbon levels:

Overgrazing
Soil disturbance (eg renovating pasture)
Changing from perennial pasture to annual pastures
Forage conservation or cut and carry systems that reduce litter and prevent the return of grazed material as dung

These practices either reduce the plant and manure inputs into soil or increase the decomposition soil organic material by enhancing its exposure to soil animals and microbes.

Some practices such as increased use of annual pastures and fodder crops are being implemented as an adaptation strategy to increased climate variability and reduced irrigation water allocations.

In practice, increasing soil carbon beyond the levels currently measured is difficult and slow within a productive dairy system, while reducing soil carbon can be relatively fast!

What about Bio-Char?

Bio-char is a charcoal like material produced by the pyrolysis (heating to between 350-600°C under limited oxygen) of organic matter. This converts easily-decomposable organic matter into a highly stable (i.e. biologically and chemically stable) form of carbon that potentially has both soil improvement and carbon sequestration benefits.

There are many issues and challenges (CSIRO have extensively reviewed the possibilities – see References and resources below) to overcome before the production of bio-char becomes a practical carbon sequestration option.

Those agricultural industries that produce large amounts of waste organic matter (eg crop and mill residues from sugar cane) may have an opportunity in the future to convert this waste into bio-char and claim a carbon credit. Bio-char is not explored further here as dairy farms do not have a significant source of organic matter for conversion to bio-char, however for those who are interested, follow the link to the CSIRO report.

What are the opportunities for dairy farmers in soil carbon trading schemes?

To assist dairy farmers and the industry understand the soil carbon debate and the potential for dairy farmers to profit from soil carbon sequestration schemes, Dairy Australia commissioned a major review – “Soil Carbon Sequestration under Pastures in Australian Dairy Regions”. The report is available on the Dairy Australia web site www.dairyaustralia.com.au

In summary, the review concludes:

Well managed dairy pastures are generally regarded to be close to their physical storage capacity - so significant permanent addition is unlikely.
Australian soils are relatively dry and warm – this significantly limits the ability to build carbon content in the soil.
Soil carbon can be increased by growing additional dry matter - or for already highly producing pastures by allowing more pasture to decompose. Adding carbon (eg biochar) is also possible but that would be a cost to dairy farmers (not a source of income).
Raising soil carbon in the top 10cm of soil by 1% over 5 years would require adding to the soil more than 10 t DM/Ha above current levels – this is clearly impossible even for dairy pastures.
The potential price of carbon would need to be very high (over $200/t) to deliver a better return as soil carbon compared to using it for feed in milk production.
Building soil carbon requires significant nutrient inputs (especially N, P, S). If these have to be applied to raise soil carbon the fertiliser cost must be taken into account in any analysis.
Under certain climate conditions soil carbon increases could lead to higher emissions of nitrous oxide (another powerful greenhouse gas). This could see greenhouse emissions from participating farms increase.
It is expensive to accurately measure soil carbon with current technology and if the farmer has to pay for this ‘verification’ then cheaper methods would need to be developed.
Soil carbon can change significantly with changes in weather, soil moisture, land use etc. This raises the question of what is the risk for farmers claiming credits at one point in time if they are audited later under different climate/land use and have to repay.
The requirement to retain claimed carbon in soil for at least 100 years has implications for long term land use options, the value of land, and the passing of obligations across generations. For example, a shift from perennial pasture to annual cropping in response to other factors such as water availability, temperature, markets etc can reduce soil carbon and hence may lead to an obligation on farmers to re-purchase carbon permits for “claimed carbon credits” that are subsequently “lost”.

Please note - all the scientific evidence suggests that pasture-based dairy farmers should approach the issue of soil carbon sequestration with considerable caution.

More work is required in the following areas:

Soil carbon measurement.
Impact on soil carbon of changing weather conditions across different soil types.
Determination of the long term liabilities associated with farmers selling soil carbon credits.

The basic science however is not going to change and the science strongly indicates:

It is very unlikely that dairy farmers would be able to benefit significantly from soil carbon sequestration even if carbon prices operate at or above the highest current estimates.
There are significant long term (including intergenerational) risks associated with selling soil carbon sequestration when the future of that soil carbon is unknowable.

References and further resources

The DPI Victoria website has an animation of the carbon cycle that demonstrates as simply as possible some of the stocks and flows of carbon in soils.

Bruce, S. et al (2009) "Science for decision makers" – soil carbon management and carbon trading. Bureau of Rural Sciences, Department of Agriculture, Fisheries & Forestry, Canberra.

Chan, Y (2008), Increasing soil organic carbon of agricultural land, PrimeFact 735; NSW Department of Primary Industries.