We tested (1) how the presence of a diverse soil faunal community affects ecosystem carbon balance and (2) whether habitat patchiness modifies the influence of soil fauna on plant growth and carbon dynamics. We constructed cylindrical microcosms that contained coniferous forest humus and different litter materials either mixed or in separate patches, and in the presence or absence of diverse soil mesofauna. A birch seedling was planted in the centre of each microcosm. The experiment continued for two growing periods during which net carbon assimilation was measured continuously. At the end of the experiment, the microcosms were destructively sampled for plant biomass, soil fauna, and soil physical and chemical properties. All systems, independently of treatment, were net CO2 producers in the beginning. In the presence of a diverse fauna, the plant growth was drastically increased, and the mixed-litter systems respired more than the patchy ones. During the second season, the patch effect disappeared, while the birch seedlings and mosses continued to grow better in the microcosms with diverse fauna. In the long term, patchiness did not modify the effect of fauna on plant growth or carbon balance. By the end of the experiment, the carbon balance approached zero in the refaunated microcosms, while it remained negative in the "simple" systems. The weak impact of patchiness in comparison to the faunal effect may be due to a homogenising role of plant roots and progressive decay of the substrates.
Keywords: Decomposition; Ecosystem; Food web; Forest floor; Mineralisation; Nitrogen.