Integrating physical and chemical methods for isolating stable soil organic carbon

A better understanding of the effects of climate and management on long-term soil organic carbon (SOC) sequestration necessitates separation of the stable from the labile SOC fraction. An array of physical and chemical methods is currently used for the separation of SOC fractions, but these methods have not been subjected to a comparative evaluation to understand their effectiveness. Therefore, the present study was conducted with the objective to evaluate the pool size and 13C isotopic signatures of stable SOC fractions derived from physical and chemical approaches applied separately and in combination on a Typic Fragiudalf of Ohio, USA. Five methods were evaluated: recovery of silt-associated SOC by physical method (method 1a), clay-associated SOC by physical method (method 1b), bulk soil-associated SOC by chemical method (method 2), chemically stable silt-associated SOC by combined physical and chemical methods (method 3a), and chemically stable clay-associated SOC by combined physical and chemical methods (method 3b). The corn-derived fraction of the stable SOC pool was calculated using 13C natural abundance technique because there was a clear vegetation shift from C3 (forest) to C4 species (corn, Zea mays L.) at the study site. Across land use and soil depth increments, the pool sizes of stable SOC isolated by chemical treatment, individually and in combination with physical method (methods 2, 3a, and 3b), were substantially lower (1.5% to 29.4% of total SOC) than that of standalone physical method (methods 1a and 1b) (17.3% to 64.8% of total SOC). However, among the chemical methods (method 2, 3a and 3b), the pool size of chemically resistant SOC was generally comparable. In the surface layer, the proportion of young, corn-derived SOC was lower after chemical treatment than after individual physical method. But, the chemically stabilized SOC in subsoil showed an enrichment of 13C signatures and increased proportion of C4–C than that of surface soil, and it is mostly similar to that derived from physical methods. The data obtained from this study indicate that integrating physical and chemical methods is effective in isolating the SOC fraction with greatest stability in the surface soil layer, but its effectiveness to separating the stable SOC fractions in the subsoil was comparable to individual physical and chemical methods.