Australasian Science: Australia's authority on science since 1938

Myths about Carbon Storage in Soil

By Robert White and Brian Davidson

Goals of sequestering carbon in agricultural soil ignore the law of diminishing returns.

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The idea that an increase in the carbon content of the world’s soils could substantially offset greenhouse gas emissions has been enthusiastically promoted by politicians and environmental groups. It arises because the amount of organic carbon in the world’s soils is impressive – some 1500 billion tonnes to a depth of 1 metre, which is about twice the amount of carbon in atmospheric carbon dioxide.

This carbon is stored in soil primarily as organic matter, comprising the residues of plant material, animal excreta and dead organisms. Although it’s also present in insoluble carbonates, these are much more stable than soil organic matter and not very amenable to human manipulation.

A prominent example of this idea is the “4 per 1000 Initiative” launched by the French Ministry of Agriculture at the COP21 meeting in Paris last December (www.4p1000.org). The thinking is that even a small annual increase of 0.4% in soil carbon, averaged over all soils, would not only improve soil fertility and agricultural production but would also help to limit the global temperature increase to 1.5–2°C, as advocated by the Intergovernmental Panel on Climate Change.

While this may be a laudable aspiration, the concept is flawed. It implies that soil carbon will increase by a slightly bigger increment each year as...

The full text of this article can be purchased from Informit.