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Abstract

Background and aims 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.
Methods 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.
Results MOM fractions from Ah horizons contained
64±8.8 % of the bulk SOC. The radiocarbon content
of MOM fractions in Ah horizons, expressed as Δ14C,
ranged between −2.8‰ and 114‰ 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.
Conclusions 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.