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

Abstract

[1] 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 d 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.

Comparison of forest soil carbon dynamics at five sites along a latitudinal gradient

“…The temperature dependence of MOM-C decomposition was in agreement with studies indicating a potential for significant short-term turnover of MOM-C (Diochon and Kellman, 2009;Dumale et al, 2009). The annual fractional loss rate of C from U (k 1 , yr −1 ) was calculated as:…”

“…A physical separation of whole soil by wet sieving was used here to distinguish two broad categories of soil organic C that appear to have distinctly different turnover rates. Many investigators have used density-based separations of soil organic matter to quantify "light" and "heavy" fractions (Boone, 1994;Cadisch et al, 1996;Diochon and Kellman, 2009;Sollins et al, 1984;Spycher et al, 1983), but the characteristics of light fraction organic matter (LF-OM) and POM are somewhat similar (Six et al, 2002). Both POM and LF-OM are relatively labile, and consist of partially decomposed plant residues with high C:N ratios and low potential net Table 3 Summary statistics (mean ± SE) for mean soil density, cumulative soil C stocks (0-20 cm soil depth), and mean C:N ratios in two successive years at the five forest sites.…”

Depth Distribution and Predictors of Soil Organic Carbon in Podzols of a Forested Watershed in Southwestern Canada

“…This points to the need for field studies, taking into account subsoil as well as topsoil C, especially because controls on C dynamics may be different (Salomé et al, 2010). Although SOC turnover generally decreases with depth, both fast cycling and more persistent SOC forms are present in significant amounts in the lower part of the soil profile (Trumbore, 2000;Diochon and Kellman, 2009). …”

Effects of Thinning on Soil Organic Carbon Fractions and Soil Properties in Cunninghamia lanceolata Stands in Eastern China

“…Recently, SOC fractions, which were separated by physical methods, were widely used to study the changes in soil carbon pools in forest management [12,13]. The light fraction of SOC is comprised of slightly decomposed plant residues and animal tissues, and is readily affected by changes in management activities [14], and is commonly characterized by a rapid turnover.…”