Further uncertainties in the Carbon cycle

The Carbon cycle is far from being fully understood or quantified. The absorption and release of carbon dioxide by the oceans and from biological plants and fungii – both on land and in the ocean – are a long way from being established. The amount of Carbon locked up in the earths crust is equally subject to great uncertainty.

A new paper shows that deep soils hold much higher levels of carbon than is usually assumed.

R. J. Harper, M. Tibbett, The hidden organic carbon in deep mineral soilsPlant and Soil, July 2013, Volume 368, Issue 1-2, pp 641-648

Abstract: Current estimates of soil organic carbon (SOC) are based largely on surficial measurements to depths of 0.3 to 1 m. Many of the world’s soils greatly exceed 1 m depth and there are numerous reports of biological activity to depths of many metres. Although SOC storage to depths of up to 8 m has been previously reported, the extent to which SOC is stored at deeper depths in soil profiles is currently unknown. This paper aims to provide the first detailed analysis of these previously unreported stores of SOC. ….. Mean SOC mass densities for each of the five locations varied from 21.8–37.5 kg C m−2, and were in toto two to five times greater than would be reported with sampling to a depth of 0.5 m.

PhysOrg reportsCurrent estimates of soil organic carbon are based largely on measurements to depths of 30 cm. This approach has evolved in North America and Europe, where soil is generally more shallow. 

However, many plant species have roots extending many metres deep, suggesting there is also carbon stored at such depth and inspiring researchers to explore the storage potential of deeper soils in older landscapes such as the Amazon or Australia. Researchers in the Amazon had previously sampled soils to 8 m. 

The researchers took soil measurements from samples taken to almost 40 metres deep at a range of sites in south-western Australia. They found that small amounts of carbon were present throughout the soils all the way to the bedrock, and that deep soils store up to five times more carbon than is normally reported.

Lead researcher Professor Richard Harper, an expert in water management and sustainability at Murdoch University said the findings extend our concept of the amounts and potential of carbon stored in soils.

“This carbon has been previously overlooked, and this opens up several lines of inquiry – for example, what happens to this carbon with land use change such as deforestation and reforestation?” Professor Harper said.

“There is likely more carbon stored in the world’s soils than previously considered. What will happen to this carbon – that is, will it be released as a result of either land-use change or climate change – is unknown. This is what we are working on now,” he said.


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