Fine root traits, belowground interactions and competition effects on the rhizosphere of <i>Fagus sylvatica</i> and <i>Fraxinus excelsior</i> saplings

European beech (Fagus sylvatica L.) and European ash (Fraxinus excelsior L.) are common tree species in Central European forests and of high ecological as well as economic value. However, knowledge about the structure and function of the ecologically important fine root system of beech and ash and its impact on rhizosphere processes is scarce. Moreover, little is known about the direct intra- and interspecific belowground competition effects of these two species. This thesis presents results from different greenhouse experiments on the species-specific effects of beech and ash saplings on key belowground dynamics as well as their root competition effects. The main objective was to disentangle species-specific effects from competition/biodiversity effects on rhizosphere and fine root properties. In a competition experiment with saplings of beech and ash grown in different rhizobox-treatments (monoculture, mixture or single plant) we investigated morphological, C/N and δ13C responses in the fine root system employing a root order-related analysis. We observed large differences in various root traits between the root order classes 1 to 4, which underscores the ecological significance of the position of roots in the root system, e.g. 1st order roots, i.e. root tips, had significantly higher specific root areas and contributed to 65-70% to the total length of the analysed root segments. While the species-specific fine root characteristics of beech and ash were obvious, no major root morphological or chemical (nitrogen concentration, C/N ratio) alterations in response to competition were found. This partly contradicts observations in mature stands, where fine roots of beech were shown to act very plastic in changing their specific root length in a competitive environment. Thus, adaptive root responses to competition may not be a universal phenomenon and are likely to vary with site conditions, species and plant age. In contrast to the fairly unaffected root morphological and chemical traits, fine root survival, which was analysed by sequential digital imaging of root growth through a root window, showed significant differences between competition treatments and species. Competition with conspecific or allospecific roots altered root longevity in both directions, either toward a shorter lifespan or higher longevity. Mean root lifespan differed significantly among species with higher fine root longevity in ash and also depended on competition treatment. Fine root mortality increased in beech roots grown in mixture with ash and in beech monoculture compared to beech plants grown in isolation. Ash fine roots apparently profited from the presence of beech roots, while beech root growth and survival were negatively affected by ash. These results indicate size-asymmetric belowground competition. Thus, competition represents an important force influencing the fine root lifespan of beech and ash saplings. In a rhizotron experiment with beech and ash saplings we investigated root-induced trace gas fluxes, microorganisms and root exudations. The results showed species-specific as well as root biomass effects on greenhouse gas fluxes. The CH4 uptake of the soil planted with ash was higher and the N2O emissions were lower than from soil under beech. In contrast, the CO2 efflux was much higher in beech than in ash, although root biomass was smaller than that of ash. Thus, soil biological activity is not only quantitatively affected by root biomass, but also qualitatively related to the species identity of the tree. This qualitative effect is also supported by the findings of the species differences in the composition and concentration of organic acids measured in the closest proximity of fine roots. We additionally observed species-specific effects on soil microorganisms and total soil carbon content. In particular, the fine roots of beech altered carbon dynamics in the soil by reducing soil pH and thus decreasing the carbon use efficiency of bacteria, while more leaf litter-derived carbon was channeled into higher trophic levels in the presence of ash. The last experiment dealt with the incorporation of plant carbon and microbial nitrogen into the rhizosphere food web of beech and ash. We conducted a 5-month 13CO2 greenhouse labeling experiment to follow the flux of carbon from plant shoots to the rhizosphere and into the soil animal food web. In parallel, we used 15N labeled mineral nitrogen to trace the flux of nitrogen via saprotrophic microorganisms and mycorrhiza into the soil animal food web. The litter and soil were minimally enriched in 15N and 13C whereas fine roots of beech and ash were highly enriched. Maximum values of 13C were observed in the ectomycorrhiza of beech. The isotopic signature of soil animals was low, suggesting that the studied animal species did not exclusively feed on mycorrhizal fungi. Furthermore, the isotopic signature of the soil animals did not significantly vary between beech and ash. Overall, the comparison of the roots and rhizosphere of beech and ash and their interactions indicate that tree species identity needs to be considered in competition and biodiversity studies in the field. Further research on specific fine root traits and rhizosphere dynamics is needed for more tree species to predict realistic carbon models