| Yhteenveto: | Knowledge of the importance of decomposer food-web structure to the ecosystem functioning is largely based on laboratory experiments. My aim was to develop theoretical models for enclosed model ecosystems, and to use the models to clarify the role of microbial-feeding nematodes in the nutrient cycling in boreal coniferous forest soil. Experiments focusing on heterotrophic decomposer populations and decomposition processes were conducted using food chains assembled from organisms grown in pure cultures. Experiments were made using both sterilized litter and humus and mineral soil supplied with glucose intended to mimic rhizosphere. The experimental systems were modelled as piecewise linear models, with the model phases determined by resources limiting the growth of decomposer populations. The contribution of microbial-feeding animals has been estimated to be over 30 % of total nitrogen mineralization. In contrast, models in which the population dynamics of a fungal-feeding nematode was taken into account explicitly, indicated that the maximal contribution of the nematode was less than 3.5 %. In nutrient-limited conditions the fungal-feeding nematode increased fungal biomass towards the end of the experiment and this was eventually reflected in enhanced glucose decomposition. This was however not solely due to nutrient mobilization by the nematode, but the presence of the nematode modified the functional properties of the fungus. The most most surprising result was that an introduction of a bacterium, whether grazed by a bacterial-feeding nematode or not, induced similar changes in fungal biomass and functioning as the fungal-feeder. As for the nematode, the effect of the bacterium was larger than could be explained by nutrients cycled by it. The stability of the fungus-nematode food chain was tested by directly manipulating initial population size of the nematode and as resistance to and recovery after a freezing disturbance. There was no evidence of sensitivity to initial population size or to the freezing disturbance. Addition of the bacterial-based food chain did not increase system stability.
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