The hidden organic carbon in deep mineral soils

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.

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Acknowledgments

We thank David Chittleborough, Jock Churchman, Guy Kirk, Stan Sochacki, Bernard Dell, Chris Mitchell, Rod Keenan, Keith Smettem and two anonymous reviewers for useful comments, Ian Truckell (Cranfield University) for preparing the Figures, Alex Winter (Forest Products Commission, Western Australia), Marianne Harkins (CSBP), Michael Smirk and Evonne Walker (UWA) for assistance in the field and laboratory and all landholders for access to sites. Drilling was undertaken by the Forest Products Commission (Western Australia) as part of the Australian National Action Plan for Salinity and Water Quality project “Strategic Tree Farming”.

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Correspondence to R. J. Harper.

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Responsible Editor: Kees Jan van Groenigen.

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Harper, R.J., Tibbett, M. The hidden organic carbon in deep mineral soils. Plant Soil 368, 641–648 (2013). https://doi.org/10.1007/s11104-013-1600-9

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Keywords

  • Soil carbon
  • Regolith
  • Global change