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 below.
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.
-
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).

Figure: A simplified illustration of the carbon cycle in soil. Source: McKenzie 2010.
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.
In conclusion:
-
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 (e.g. 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”.
Management practices that might increase soil carbon on dairy farms
The magnitude and rate of soil organic carbon decomposition and sequestration depends on a range of soil and environmental factors.
To boost organic carbon concentrations in soil, two main options are available: reduce the decomposition and/or improve the rate of addition of organic materials.
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.
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.
Biochar is the solid by-product resulting from bioenergy production (Figure below). The pyrolysis conditions can be optimised for bioenergy or biochar production.

Figure: Biochar, especially when combined with bioenergy production, can result in net removal of carbon from the atmosphere. Source: CSIRO 2009.
Biomass (‘feedstock’) for biochar production can comprise most urban, agricultural and forestry biomass such as wood chips, saw dust, tree bark, corn stover, rice or peanut hulls, paper mill sludge, animal manure and biosolids.
There are many issues and challenges to overcome before the production of bio-char becomes a practical carbon sequestration option for dairy farmers. However, some large dairy farms and feedlots may produce sufficient manure from dairy effluent to make biogas generation from methane an option. For more information on biogas in dairy systems download the Fact sheet: Biogas feasibility.
See also Emissions Reductions Fund method - Destruction of methane generated from dairy manure in covered anaerobic ponds.