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I am heading the Department of Terrestrial Ecology at the University of Cologne. My research covers diverse aspects of terrestial ecology, with long-standing research expertise on multitrophic interactions between belowground and aboveground systems. My main research activities are focussed on the analysis of invertebrate – microbial interactions in the rhizsophere of plants with particular emphasis on functional interactions of protists, bacteria and mycorrhiza in soil.

More recently, new research areas on natural abundance of stable isotopes and stoichiometry in terrestrial food webs, on island biogeography and community ecology of lizards in Vietnam were established.

My research is at the interface between community and ecosystem ecology. Most of my past research has centered on interactions of plants with soil fauna and microbes outside of the relatively well-defined symbioses, and aims to understand the functional significance of belowground biodiversity for aboveground systems. My research is addressing questions pertaining to the importance of ecosystem engineers and both fungal and bacterial grazers in shaping rhizosphere microbial diversity and functioning and their feedback on plant productivity, plant competition and on the performance of aboveground herbivores.

We have found that indirect effects of soil fauna, either on microbial diversity and functioning, or on defense mechanisms of plants, are generally much more important for plant performance, root pathogen infection, and on aboveground herbivores than their direct effects on nutrient cycling. Tiny shifts in the competitive ability of plants caused by soil organisms can multiply strongly at the plant community level. My special interest applies to rhizosphere microbial interactions. For example, by focusing on predator-prey interactions in the plant rhizosphere, we have shown that bacterivorous protists affect the turnover, community composition and function of rhizosphere bacteria, with significant feedback effects on the hormonal balance in plants, root architecture and the plant’s nutrient uptake efficiency. These effects have been shown to cascade over several trophic levels and affecting the performance of aboveground herbivores.

Using the common soil protist Acanthamoeba castellanii and the rhizobacterium Pseudomonas fluorescens CHA0 as a model system, we have recently begun to elucidate the mechanisms of a complex chemical warfare between amoebae and pseudomonads, which can be linked to the pseudomonas-induced systemic plant defence of fungal root pathogens.

We further investigate soil processes that affect nutrient cycling, priming by reactive and storage of recalcitrant carbon. We are studying how these cycles interact, how microbial stoichiometry feeds back on these cycles and how these cycles depend and respond to human-driven changes in climate, land use, and atmospheric chemistry, with particular emphasis on the response of ecosystems to atmospheric deposition of N from air pollution. Our studies have shown that plant release of reactive carbon appears crucial in these interactions, that the microbial N:P ratio is a critical predictor for accumulation or degradation of soil carbon stocks, and N enrichment modified plant–soil feedback relationships, resulting in changes to plant community composition.

More recently, mechanisms of community assembly became major a focus of our research. Working with a range of collaborators, we are investigating neutral processes, species assortment and habitat filtering in metacommunities on very different spatial and temporal scales, from microbes (micro-food webs of epiphytic bacteria and protists) to invertebrates (arthropod food webs on lake islands) to vertebrates (reptiles on limestone karst mountains in Vietnam and Laos).

This work is generally connected to uncovering energy flows and nutrient pathways through terrestrial above- and belowground food webs using natural abundances and labeling techniques with the stable isotopes 15N and 13C. For example on lake islands, we found a clear species-area relationship. While the 15N signatures of arthropod predators indicate their trophic positions, their 13C signatures show the degree of prey subsidy from aquatic systems, and variations of isotope signatures indicate niche breadth. Comparing isotope signatures across trophic levels thus gives detailed information on vertical diversity (i.e. food chain length) as well as on changes in horizontal niche breadth due to competitive processes with changing island size.

We recently started a collaboration with Cologne Zoo and the Institute of Ecology and Biological Resources (Hanoi, Vietnam) on community assembly of lizards in tropical rain forests in Vietnam and Laos. This research is focusing on patterns of geographic genetic differentiation and attributes of the ecology and life history of reptiles. We are especially interested in placing this work in the context of ecosystem-level consequences of biodiversity loss due to factors such as habitat degradation and destruction, and on shifts in tolerances to changing temperatures as expected by Global Change.