Indirect biotic interactions in the rhizosphere

University dissertation from Media-Tryck, Lund

Abstract: The rhizosphere (the soil environment around plant roots) is characterized by complex direct and indirect interactions among its organisms. Studies of interactions among organisms in the rhizosphere are difficult to carry out due to a high diversity of inconspicuous organisms on a micro scale and the highly heterogeneous nature of the soil environment. In this thesis, grassland plant species and soil were used as rhizosphere environments to study interactions among plants and soil invertebrates, such as collembolans and nematodes, as well as microorganisms. Stable isotope analyses of C and N, and molecular 16S rRNA gene analysis,were applied to determine direct (trophic) and indirect interactions among the soil organisms. Biomass and structure of microbial communities were determined with phospholipid fatty acids analysis (PLFA). We showed that indirect interactions among invertebrates in the food web are present and changes in food web components induced dietary switching of collembolans in the food webs. This was detected by analyses of natural abundances of ? 15N values of the collembolans and was a general trait among different plant species. By using a 13C pulselabeling technique we detected indirect effects on microorganisms through plant C allocation into biomass of different microbial groups. The grass Holcus lanatus had the largest influence on 13C allocation into microorganisms such as bacteria and arbuscular mycorrhizal fungi. The presence of the grass species explained the carbon allocation in species mixtures. Among functionally dissimilar soil organisms arbuscular mycorrhizal fungi increased plant community productivity and nutrient uptake, while the effects of root feeding herbivores, such as nematodes and wireworms, were less pronounced. Bacterial diversity (16S rRNA genes) differed among nematodes of various feeding groups and among bacterial feeding nematodes. The nematodes did not feed on the most abundant bacteria in soil. Furthermore, several nematodes contained bacterial symbionts and parasites.

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