Soil Organic Matter and The Carbon Storehouse

The Carbon Storehouse

Soil carbon is implicated in almost all indicators of soil health, being beneficial for nutrient retention and availability, soil structure improvement, erosion resistance and water infiltration, retention and drainage characteristics.

Red Chromosol showing roots at surface and deeper lucerne roots.  Photo David McKenzie

Red Chromosol showing roots at surface and deeper lucerne roots. Photo: David McKenzie

With its key roles in both carbon sequestration and in improving soil condition, increasing the organic content of soils has a dual effect, in terms of both climate change mitigation and adaptation.  The potential store of carbon in soils is very large relative to anthropogenic greenhouse gas emissions, and represents an effective long term store of carbon drawn down from the atmosphere via photosynthesis. Therefore soils and soil management will undoubtedly play a significant role in any future carbon offset or credit mechanism.  Natural resource management policies need to recognise these coupled benefits of soil carbon and invest strategically to maximise the overall benefit.

As with other natural resources, the impacts and preferred management responses to the effects of climate change will vary in different regions.  However, improved understanding of the magnitude of the risks associated with climate change in different regions is needed in order to prioritise natural resource investment by governments and land managers.  In particular, the effects of changing climate on the various processes that degrade soils will be different across regions.

photo: Brian Murphy

Paddock sown using zero till, stubble retained and minimum disturbance. A practice that generally increases soil organic carbon Photo: Brian Murphy

There is some confusion in the meaning of the various definitions of soil organic matter and soil carbon.  The terms requiring definition include soil organic matter; soil organic carbon; soil inorganic carbon; soil organic carbon concentration; soil organic carbon stocks and bulk density.

Key management actions for improving and/or maintaining soil organic matter include maintaining and maximising biomass input by ensuring adequate yields and groundcover are maintained; minimising soil disturbance and soil organic matter loss; maintaining a nutrient balance in the soil and a favourable chemical environment.

POSITION STATEMENT

Soil Organic Matter (Soil Carbon)

  • Soil organic matter is critical for soil function through its influence on water holding capacity, structural stability, nutrient cycling, cation exchange capacity and soil buffering capacity.  Improved management to increase soil organic matter will improve the long term resilience and buffering capacity of the soil in the face of issues such as climate change and increased pressure from the production needs of agricultural soils.
  • There is confusion around the meaning of soil organic matter and soil organic carbon.
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    Highly organic soil at Guyra, NSW Photo: Sally McInnes-Clarke

    • Soil organic carbon is defined as organic carbon associated with the less than 2.0 mm mineral fraction of the soil.  This component of the total soil organic matter is intimately associated with the functioning of soils.  This means that organic materials in the soil, such as large pieces of root, leaves and stubble, are NOT included in the measurement of soil organic carbon.  In fact, these need to be removed prior to measurement of soil organic carbon and should be accounted separately.
    • Most measurements relating to soil organic matter actually give a value for soil organic carbon, because it is possible to estimate organic carbon in soils relatively easily and effectively.  Soil organic matter is about 58% carbon, and it is possible to estimate soil organic matter from soil organic carbon simply by multiplying by 1.72.  It is essential to be clear about which value, whether soil organic carbon or soil organic matter, is being presented in any report or discussion.
  • A clear distinction needs to be made between values of soil organic carbon concentrations or percentages in laboratory results and the values of soil carbon stocks, or soil carbon stores used for assessing carbon credits.
    photo: brian murphy

    Soil core being cut into depth intervals to determine bulk density and soil carbon stocks. Photo: Brian Murphy

    • The soil organic carbon concentration is given in grams of organic carbon per 100 grams of oven dry soil.
    • Soil carbon stocks are expressed as tonnes of organic carbon per hectare.  To calculate the soil organic carbon concentration multiply the soil bulk density and an area factor to obtain this value to a specified depth, typically 0-30cm.
  • Increasing soil organic matter has the benefit of improving soil productivity, land degradation and economic returns.  This is in addition to potential benefits that may be derived from carbon trading.
  • It is easier to lose organic matter than it is to build it.  There may be physical or economic limitations which restrict potential carbon sequestration rates.
  • Organic matter must be added continually in order to maintain or increase soil carbon over the long term. This is best achieved with photosynthesis of actively growing plants.

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    A saline site where vegetation has been severely restricted. Large losses of soil organic carbon occur by decomposition and erosion. Photo: Brian Murphy

  • Projects that aim to measure the effects of land management practices on soil organic carbon must allow for differences in climate, soil type, and landform that can influence the amount of soil organic carbon in soils.
  • Soils might vary in proportions of C, N, P and K, but without the right balance of these nutrients, soil organic matter cannot form in soils.
  • Methods for measuring soil organic matter depend on the purpose.  For example, measuring soil organic matter to obtain payment for any carbon credits requires a much more rigorous, appropriate and expensive process, than measuring organic matter to assess soil condition or benchmarking. These methods must also be economic and practical, to encourage adoption by land managers.