Physical fractionation of soil organic matter: Destabilization of deep soil carbon following harvesting of a temperate coniferous forest

Developing a better understanding of the processes involved in controlling soil carbon (C) storage and turnover in native forest soils is critical if we are to fully understand the role land management activities play in the global C cycle. Separating soil organic matter (SOM) into discrete fractions has been successfully used to isolate changes in the structure and function of the SOM pool in response to land management activities but investigations in native forest systems are rare. Using a density fractionation procedure, we isolated and characterized three distinct SOM fractions (free, intra‐aggregate, and organo‐mineral) across a postharvest forest age sequence. We describe age related variations in each of these fractions with respect to their contribution to soil mass, C storage, C concentrations, C‐to‐N ratios, and δ 13 C ratios. In conceptual models of SOM pool structure, the organo‐mineral fraction is assumed to be largely stable. We show that harvesting may increase the potential for loss of soil C (i.e., destabilize the soil C pool) and that a significant portion of the soil C pool may be cycling on decadal timescales. Isotopic evidence is consistent with a period of C loss attributable to increased rates of decomposition, with losses below 20 cm driving the trend. We encourage investigators studying the effects of forest harvesting on SOM storage to consider the deeper mineral soil (20+ cm) and how we may increase SOM turnover time and stabilization capacity in a native forest system.