The aims of this study were i) to investigate the influence of forest development on the habitat and structure of soil decomposer assemblage, and ii) to assess the consequences on ecosystem performances and nutrient cycling, especially on carbon trajectory. Monitoring of soil ecosystem parameters (i.e. abiotic parameters, habitat structure, soil fauna and microbial communities) within two chronosequences and laboratory manipulations were carried out to test and answer the following hypotheses:
- Soil decomposer assemblage and humus structure are strongly affected by successional changes of above ground system during forest rotation.
- The nature of the tree species (coniferous vs. deciduous) may strongly control or shape the effects observed on soil decomposer assemblage during forest monoculture rotation.
- Changes in decomposer assemblage may have a functional implication at the ecosystem-level and might subsequently affect ecosystem performances to a certain extent.
Two forest chronosequences in Germany were selected as study sites: a spruce forest on acid soil at Tharandt and a beech forest on base-rich soil in Leinefelde. At each chronosequence, the structure of the soil faunal and microfloral communities, as well as environmental parameters were assessed by common methods. Furthermore in the spruce chronosequence, description of the humus micromorphology was performed in order to evaluate the habitat and resources modifications during spruce forest growth. Results clearly demonstrated the influence of forestry cycles on soil biota communities and topsoil structure. A direct comparison between both investigated forests is a far more complicated task considering the numbers of parameters (climatic, geographic, history, etc ) differing between the forests. Nevertheless, two major phases during forest rotation can be identified. Firstly, the transition from a mature to regeneration stand, which implies a sudden and strong perturbation, i.e. a clear-cut with removal of trunks followed by plantation of new trees. Secondly, the growth of the forest from the regeneration stand to the mature stand that, compared to clear-cut, operates as a long-term process (in our case, over a century). In general, and regardless from the chronosequence investigated, both phases showed specific effects on soil biota and soil structure. However, all changes in the decomposer food web did not lead to modifications of the ecosystem performance. In the beech chronosequence on calcareous soil, biomass stability of ecosystem engineer as earthworms are suggested to be responsible for stability of meso- and microbiota leading subsequently to constancy of ecosystem processes. A major finding is the indication of a certain degree of ecosystem integrity maintained during the whole cycle. This is important for maintaining site productivity for sustainable timber production, as with a more stable soil biota community, plant available nutrients (e.g. N, P, K) released would become more stabilized, and the physical conditions of the soil would become more suitable for root growth and plant water uptake. However, the recovery of the soil fauna community from disturbances caused by clear-cutting and tree removal takes a rather long time. The large panel of reactions of soil fauna to ecosystem changes monitored in the beech forest chronosequence raises the question of the value of soil fauna as bioindicator of ecosystem perturbations or of environmental conditions. To conclude, this work provides ample evidence (in the field or in a microcosm experiment) for substantial ecosystem-level implications of changes in the soil food web. Forest management aiming at conserving soil organic matter pools must then adapt to regional differences in soil and substrate conditions. It is also of a particular importance when monitoring or modelling potential soil sinks or sources of carbon.
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