Measuring organic carbon in Calcarosols: understanding the pitfalls and complications

Abstract

The measurement of soil organic carbon (OC) is important for assessing soil condition and improving land management systems, as OC has an important role in the physical, chemical, and biological fertility of soil. The OC contents of Calcarosols often appear high compared with other Australian soil types with similar fertility. This may indicate either systematic overestimation of OC in Calcarosols or the existence of a mechanism of OC stabilisation specific to carbonate-rich soils. This study compares three dry combustion techniques (dry combustion with correction for carbonate-C determined separately, dry combustion following sulfurous acid treatment, and dry combustion following treatment with hydrofluoric acid) and two wet oxidation techniques (Walkley–Black and Heanes) for the measurement of soil OC content, to determine which method is best for Calcarosols. Nine calcareous and nine non-calcareous soils were analysed. Of the methods, dry combustion with carbonate-C correction and dry combustion following sulfurous acid pre-treatment were found to be unsuitable for highly calcareous soils. Dry combustion with carbonate-C correction was unsuccessful primarily due to incomplete conversion of carbonate to CO2 under the combustion conditions used. However, even if this problem could be overcome, the method would still not be suitable for highly calcareous soils since it would involve the measurement of a relatively small value (OC) as the difference of two much larger values (total C and carbonate-C). Sulfurous acid pre-treatment was unsuitable because it did not remove 100% of carbonate present. Although the remaining dry combustion technique (i.e. following hydrofluoric acid treatment) did not have such problems, it did give very different (and much lower) OC estimations than the two wet oxidation techniques for the highly calcareous soils. These results are consistent with carbonate minerals interacting with and stabilising a substantial quantity of soluble OC. This has implications for the way OC levels should be measured and interpreted in Calcarosols, in terms of both fertility and C stabilisation and sequestration.