Organic layer and clay content control soil organic carbon stocks in density fractions of differently managed German beech forests

Forest management and associated litter inputs and decomposition rates are thought to affect the carbon storage in mineral soils. Here, we studied the effects of forest management on soil organic carbon (OC) stocks in density fractions of Ah-horizons in soils that developed on loess. We used 82 beech (Fagus sylvatica L.) dominated forest plots in Thuringia, Germany that differed in their management (unmanaged forest, forests under age-class management and forests under selection cutting forest). After density fractionation of the mineral soil with a 1.6 g cm-3 polytungstate solution we determined OC concentrations and stocks as well as CN-ratios in the free (f-LF) light fraction, the occluded (o-LF) light fraction and in the mineral associated organic matter (MOM) fraction. In our study, Ah-horizons of beech forests stored on average 2.6 ± 0.2 kg m-2 (38.7 ± 1.3 kg m-3) OC. The results showed that 37% of the bulk soil OC was stored in the light fractions. We could show that OC stocks in the light fraction were significantly affected by the amount of C stored in organic layers (p = 0.011). The OC stocks in the organic layers, in turn, were higher in unmanaged forests and in forests under selection cutting. This suggests a sensitivity of unprotected OC in the f-LF of beech forests against forest management. In contrast to the f-LF, the OC stocks in the MOM fraction are mainly controlled by pedogenic properties such as clay and iron oxide content. Even after several decades of forest management and with large sample size, an effect of forest management on the stable MOM fraction could not be detected.