Causes of variation in mineral soil C content and turnover in differently managed beech dominated forests

Forest soils are important carbon stores and considered as net CO2 sinks over decadal to centennial time scales. Intensive forest management is thought to reduce the carbon sequestration potential of forest soils. Here we study the effects of decades of forest management (as unmanaged forest, forest under selection cutting, forest under age class management) on the turnover of mineral associated soil organic matter (MOM) in German beech (Fagus sylvatica L.) dominated forests. Radiocarbon contents were determined by accelerator mass spectrometry (AMS) in 79 Ah horizon MOM fractions of Cambisols (n = 13), Luvisols (n = 51) and Stagnosols (n = 15). Mean residence times (MRTs) for soil organic carbon (SOC) were estimated with a 2-pool model using the litter input derived from a forest inventory. MOM fractions from Ah horizons contained 64 +/- 8.8 % of the bulk SOC. The radiocarbon content of MOM fractions in Ah horizons, expressed as Delta C-14, ranged between -2.8 aEuro degrees and 114 aEuro degrees for the three soil groups. Almost all samples contained a detectable proportion of 'bomb' carbon fixed from the atmosphere since 1963. Under the assumption that depending on the soil texture between 19 % and 24 % of the SOC from the labile pool is transferred to the stable SOC pool, the corresponding MRTs ranged between 72 and 723 years, with a median of 164 years. Our results indicate that the MOM fraction of Ah horizons from beech forests contained a high proportion of young carbon, but we did not find a significant decadal effect of forest management on the radiocarbon signature and related turnover times. Instead, both variables were controlled by clay contents and associated SOC concentrations (p < 0.01). This underlines the importance of pedogenic properties for SOC turnover in the MOM fraction.