Can secular stable soil organic carbon be isolated? An assessment of Zimmermann fractionation using a long-term bare fallow

Abstract. Useful soil organic matter fractionation techniques require fractions with accurate and distinct residence times and must be implementable within reasonable timeframes and with reasonable equipment. Among these techniques, the Zimmermann fractionation is a size- and density-fractionation process which aims at separating soil organic matter into five fractions known to be linked to kinetic compartments of carbon, including one fraction corresponding to the organic carbon in silt and clay fractions resistant to chemical oxidation (rSOC) considered to be stable at the scale of millenia, which is challenging to verify. In this paper, we used a 97-year-old long-term bare fallow experiment located at Versailles (France), to assess the stability of this fraction at the centennial scale. We applied the Zimmermann fractionation to archived soil samples collected in 1929, 1962 and 2021. We observed that the rSOC content was nearly constant over the lifetime of the trial, in contrast to total SOC and to other fractions. However, the rSOC only accounted for a small proportion of total SOC and did not contain all soil carbon persisting at the centennial time scale in this experiment. Our results show that rSOC represents an almost pure but not exhaustive pool of centennially stable SOC and that other methods are needed to obtain a complete estimate of centennially stable SOC.