Afforestations are acknowledged as C sinks under the Kyoto protocol article 3.3. However, young afforestations may be considerable C sources. Losses of soil C may offset the C sink of the tree biomass. The aim of this thesis was to i) investigate the factors that affect the establishment success of the new forests, ii) quantify the impact of site preparation and management changes along with the afforestation on the C balance of the system, iii) understand how soil C dynamics are influenced by historical land use changes and activity of the soil fauna (earthworms), and iv) to explore soil C variability to set up an optimized sampling scheme for future soil C studies at the two afforestation sites.
 
The essence of this research is presented in the form of six manuscripts. This thesis sets the basis for the long-term experiment BIOTREE which was started at three sites in Thuringia with a total of 70 ha. The future aim of this experiment is to investigate the influence of tree diversity on ecosystem processes. Therefore, 300 000 seedlings from 19 different tree species were planted. The design of the experiment is outlined in manuscript 1 together with a description of the three study sites.
 
Manuscript 2 explores the differences between the establishment success of the tree species and the influencing factors. Establishment failure of the species up to 79% extends the time before afforestations become net C sinks. Experimental plots with higher tree diversity were found to be more resistant against damages by voles and rabbits than plots with less tree species.
 
Parts of the sites Kaltenborn and Mehrstedt were converted from cropland to grassland, 23 and 29 years ago, respectively. The impact of this historical land use change on soil C stocks and C fractions was investigated (manuscript 3). Surprisingly, there was no significant difference in soil C stocks between both land use types but a different vertical C distribution was observed. High C stocks at the clay rich Mehrstedt site were found below the ploughing horizon. The swelling and shrinking dynamic of the clayey soil was expected to enhance the C transport into the subsoil. Measurements of the ¹⁴C age of this subsoil C confirmed this hypothesis. In the uppermost horizon of the sandy soil in the Kaltenborn grassland mineral surfaces were found to be C-saturated, thus, this horizon cannot physically stabilise additional C. The large area of unsaturated mineral surfaces in the subsoil provides an unused capacity to stabilise and store additional C at of both sites.
 
Net C exchange fluxes between land surface and atmosphere were measured with two eddy covariance towers at the afforestation site Mehrstedt and an adjacent grassland site (manuscript 4). Gross primary productivity of the afforestation was reduced by 41% (first two years) to 14% (third year) compared to the grassland. Site preparation of the afforestation with deep ploughing damaged parts of the herbaceous vegetation that dominated the C fluxes. Enhanced C mineralisation was detected at the afforestation only during the first year, causing a net C loss of 1.2 t ha^-1. Seasonal C dynamics were determined by climatic factors (mainly precipitation during summer) and disturbances by site management (grazing on grassland site, mowing on the afforestation site).
 
The probability to detect expected soil C stock changes depends on the vertical and spatial heterogeneity of the C stocks. The variability of the soil C concentration was found to be one to two magnitudes higher than the variability of the bulk density. Both parameters directly affect the calculated soil C stocks. A simulation model revealed the possibility to improve the sampling design for soil C stocks with sample numbers reduced by 12-19% but unchanged statistical power. This is of major importance because high sample numbers are usually needed to make soil C stock changes detectable.
 
The effect of earthworms on soil C translocation and stabilisation was investigated to understand how afforestations may influence the C cycling indirectly by reducing the earthworm abundance (manuscript 6). Deep burrowing earthworms were found to be effective in translocating recently assimilated C into the subsoil by depositing it along the burrow walls. Contrary to the original hypothesis of C stabilisation due to earthworm gut passage, organic C in earthworm burrows was lost rapidly with half life times of only 3-5 years. Nuclear magnetic resonance (NMR) relaxation experiments and enzyme activity measurements showed no enhanced C stabilisation by earthworms. The C dynamics of the investigated afforestation sites were found to be influenced by different factors. Some of them, such as earthworm abundance and seasonal soil moisture pattern, change along with the forest development feeding back on the C cycle and the C sequestration.