With all the current talk about climate change, reducing green house gas emissions, National Emissions Trading Scheme and carbon trading it’s a lot of information and new ‘lingo’ to get your head around. One issue that has arisen is whether or not farmers may be able to store soil carbon and then sell it as a carbon credits.
The organic carbon found in soils is a mixture of plant residues that break down and decomposes at different rates. The total amount of carbon in the soil at anyone time is the difference between the inputs (plant residues comprising of roots, leaf litter etc) and the losses due to decomposition and mineralisation. The age of soil carbon can vary from the very recent to charcoal fragments that can survive in the soil for thousands of years.
Farmers can increase the amount of carbon in the soil through a variety of measures. Basically any farming system that will increase productivity and the return of plant material (be it leaves, stems or roots) to the soil will increase the soil store of carbon. For example, minimum tillage practices, use of fertilizers and lime to increase plant growth etc. Conversely any practices that increase decomposition (when plant materials or organic carbon is converted to carbon dioxide) will reduce the soil carbon store ie fallowing, burning stubbles and hard grazing.
There are also a range of factors outside a farmers control that will limit the potential amount of plant material that can be grown and hence the amount of carbon that can be stored. A key one here is the amount of rainfall received. Also important is soil clay content, soil depth and bulk density. All of these factors will determine the amount of soil carbon that can potentially be stored.
The only other option to further increase soil organic carbon is to add extra (over and above the plant residues). This could be done by adding compost etc on a regular basis.
Converting from annual to perennial pastures and changing from set stocking to rotational grazing may increase plant growth and productivity and increase soil carbon content. Clay spreading on non-wetting sands will obviously increase the clay content of the soil and the inherent ability of the soil to store carbon.
The time taken to increase soil carbon levels varies of course from site to site – depending on the soil, climate and farming systems. Using a soil carbon model (RothC) indicates for Roseworthy to double the soil carbon content from three to six percent would require a sustained production of wheat of eight tones per hectare per year for approximately 200 years! (Information taken from ‘Australian Grain’ May-June 2008).
At the moment the price offered per tonne of sequestered carbon (approx $20/t) would not be sufficient to justify a change in farming operations just for the carbon dollars alone. And there is still a lot of science to be undertaken in developing a simple carbon test that differentiates between the active and inactive carbon as well as accurately measuring changes over time.
The opportunity is there for farmers to increase the store of soil carbon. The challenge is, to be able to do this in a way that is economically viable. For most farmers such measures are simply just good farming techniques and provide additional benefits of improving the store of soil nutrients and improving soil structure whilst reducing the likely impacts of erosion and soil water loss.
Lyn Dohle, Snr Soil and Land Management Consultant Rural Solutions SA
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