Deep soil carbon 5 times more than before

A joint Australian-UK study has found that deep soils store up to five times more carbon than previously thought. (ABC RURAL)

The research, published in the journal Plant and Soil, shows carbon is stored at depths of up to 40 metres.
Many will be astounded at the findings and the challenge they represent to the conventional view of soil carbon. Professor Snow Barlow, from the National Climate Change Adaptation Facility, says the findings mean planting trees to sequester carbon may become a more profitable option for farmers. He implies that the soils in question had been recently under trees. But no, the abstract tells us the soils n question "had been either recently reforested with Pinus pinaster or were under agriculture." Could grasses have deposited this carbon? No, only trees get a guernsey: "There may be value in actually putting those back to some form of woody vegetation which could sequester carbon, at the same time giving you those shelter and biodiversity benefits," he said."In some places in the landscape, particularly Western Australia, that might give you some salinity benefits as well."
The authors of the study have called for a reassessment of the current measurements used to judge soil carbon stores. "The paper demonstrates the need for a reassessment of the current arbitrary shallow soil sampling depths for assessing carbon stocks, a revision of global SOC estimates and elucidation of the composition and fate of deep carbon in response to land use and climate change," the paper says.
SOC levels when measured to 35m were "two to five times greater than would be reported with sampling to a depth of 0.5 m."     

Bob Wilson gets down to where deep soils in WA contain up to
 5 times as much carbon as previously thought. Tim Wiley was behind the camera.

Plant & Soil

Plant and Soil An International Journal on Plant-Soil Relationships© 

The hidden organic carbon in deep mineral soils

R. J. Harper¹   and M. Tibbett^{2, 3}

School of Environmental Science, Murdoch University, South Street, Murdoch, WA, Australia, 6150

National Soil ResourceInstitute, Department of Environmental Science and Technology, Cranfield University, Cranfield, Bedfordshire, MK43 0AL, UK

School of Earth and Environment, University of Western Australia, Crawley, WA, 6009, Australia

Abstract

Aims

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.

Methods

Soils from five sites in the deeply weathered regolith in the Yilgarn Craton of south-western Australia were sampled and analysed for total organic carbon by combustion chromatography. These soils ranged between 5 and 38 m (mean 21 m) depth to bedrock and had been either recently reforested with Pinus pinaster or were under agriculture. Sites had a mean annual rainfall of between 399 and 583 mm yr⁻¹.

Results

The mean SOC concentration across all sites was 2.30 ± 0.26 % (s.e.), 0.41 ± 0.05 % and 0.23 ± 0.04 % in the surface 0.1, 0.1–0.5 and 0.5 to 1.0 m increments, respectively. The mean value between 1 and 5 m was 0.12 ± 0.01 %, whereas between 5 and 35 m the values decreased from 0.04 ± 0.002 % to 0.03 ± 0.003 %. 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.

Conclusions

This finding may have major implications for estimates of global carbon storage and modelling of the potential global impacts of climate change and land-use change on carbon cycles. The paper demonstrates the need for a reassessment of the current arbitrary shallow soil sampling depths for assessing carbon stocks, a revision of global SOC estimates and elucidation of the composition and fate of deep carbon in response to land use and climate change.