Variability of soil carbon stocks in a mixed deciduous forest on hydromorphic soils

•High soil carbon stocks (> 30 kg C m− 2) in a deciduous forest on hydromorphic soils•High spatial variability in carbon stocks and concentrations•High carbon stocks caused by management history, stand age, and soil properties•Temporal variation likely artifacts of spatial variability and low sample numbers

Forests play an important role for carbon (C) storage and a large part of C in forest ecosystems is stored in the soil. Although large-scale inventories of soil C stocks have been published recently, detailed site-specific studies are still needed because C storage depends on various site specific soil, ecological and management factors. Quantification of soil C stocks is hampered by spatial and temporal variability. Here, we assess C stocks in an ancient forest site at a stand which has not been managed for > 60 years and analyse the spatial variability of C concentrations and stocks to evaluate inter-annual variability and elucidate possible causes. Samples were taken down to 1 m depth by auger cores in May of three consecutive years (2013, 2014, 2015) on regular hexagonal grids covering an area of 0.25 ha and 19 sampling spots. In addition, to address small scale variability, 36 auger cores were taken in November 2014 using a nested sampling design.C stocks were on average 34 kg C m− 2 (0-1 m depth). This is distinctly higher than soil C stocks reported for most other forest sites in similar climatic conditions, and is likely caused by the interplay of several parameters, i.e., hydromorphic soil conditions, high biomass production due to high soil fertility, stand structure (tree age, species and density), and the management history of the site. The coefficients of variation for C concentrations (> 50%) and for C stocks (30-50%) show that spatial variability is high. Geostatistical analysis revealed a larger scale component with ranges of 5-20 m and a small scale component represented by high nugget effects in the semivariograms (> 50%).As hydromorphic soil types are widespread in NE Germany, large amounts of organic C could be sequestered into soil under unmanaged old forests, thus contributing to alleviating the rise in anthropogenic CO2 concentrations in the atmosphere.