In search of stable soil organic carbon fractions: a comparison of methods applied to soils labelled with 14 C for 40 days or 40 years

Summary A reliable method for the isolation of a stable fraction of soil organic carbon (SOC) would be very helpful for improving our understanding of the mechanisms responsible for stabilization of SOC and the dynamics of SOC turnover. We tested acid hydrolysis, physical fractionation (particle density/size), photo‐oxidation, treatment with chemical oxidants (NaOCl and NaS 2 O 8 ) and thermal treatment on two soils incubated with 14 C‐labelled barley straw for either 40 days or 40 years. Different intensities of the treatments were included. Acid hydrolysis, photo‐oxidation and treatment with a chemical oxidant consistently removed more 40‐year‐old C than 40‐day‐old C, which suggests that the isolated fractions contained a large proportion of material with a relatively rapid turnover. The clay + silt associated SOC fraction contained a small proportion of 40‐day‐old C and a large proportion of 40‐year‐old C. This is consistent with a SOC fraction with medium turnover. The thermal treatment removed more 40‐year‐old C than 40‐day‐old C. At 400°C there was still a small proportion of the 40‐year‐old C remaining, whereas almost all the 40‐day‐old C was removed. This is consistent with a stable SOC fraction. However, because only 2–3% of the C remained after this treatment, the isolated SOC fraction may be of little quantitative importance. Furthermore, the thermally resistant fraction is likely to be heavily altered by the treatment, and therefore unsuitable for further studies of the chemical nature of stable SOC.