The soil food web of temperate deciduous forests: litter and root resources as driving factors, and soil fauna effects on ecosystem processes

Above- and belowground communities have been investigated independently for long, but there is increasing evidence that both are intimately linked and depend on each other. Plants provide energy and nutrients for the belowground consumer community, both via litter and root exudates, thereby affecting soil biota. Simultaneous investigations of both litter and root exudate based pathways are scarce and therefore, the relative importance of roots as compared to litter as food resource for soil organisms remains unknown. Soil organisms, in turn, contribute to ecosystem processes like litter decomposition and nitrogen cycling that are essential for nutrient supply of plants and primary production. Their individual effects may be facilitated or reduced by co-occurring species. The underlying mechanisms and the reasons for the variability of soil fauna interactions are little understood, but complementarity effects are likely to be related to dissimilarity of traits. The present thesis focuses on litter- and root-derived resources for soil communities and analyzes feedbacks of detritivores to ecosystem processes and hence to plant nutrition. In order to disentangle the relative importance of identity and diversity of root-derived as compared to litter-derived resources for soil microorganisms (Chapter 2) and mesofauna invertebrates (Chapter 3) a field experiment was performed in which aboveground litter and root species were manipulated independently. Saplings of four deciduous tree species, differing in litter quality and associated mycorrhizal fungi were planted in a full-factorial design in a 150 year old mountain oak forest. The response of soil microorganisms (Chapter 2) to variations in resources was measured by substrate-induced respiration (SIR) and phospholipid fatty acid (PLFA) analyses, while the response of mesofauna invertebrates (Chapter 3) was measured by changes in community composition and abundance of groups or species. Generally, neither soil microorganisms nor mesofauna invertebrates responded to the type of mycorrhizal fungi or root identity and diversity. In contrast, aboveground litter affected basal respiration and community composition of soil biota. However, there was no beneficial leaf litter mixture effect; rather soil microorganisms and mesofauna invertebrates responded to the quality and identity of litter, with the effects varying between microarthropod groups and species. Overall, the results point to a stand age dependent importance of aboveground and belowground resources for decomposer food webs of forest ecosystems, with effects of litter quality and identity on mineralization processes and feedbacks to plants being most significant for young trees, i.e. during stand regeneration. In Chapter 4 I investigated interactions between four detritivores species differing in body size or habitat association, i.e. two species of earthworms (Lumbricus terrestris, Aporrectodea caliginosa) and two species of collembolans (Heteromurus nitidus and Protaphorura armata). I tested if species with similar traits exert negative effects on their respective performance or their effects on ecosystem processes, i.e. leaf litter mass loss and 15N cycling. Litter-associated species had more pronounced effects on litter decomposition and N cycling, but their effects depended on the presence of other soil invertebrates. Detrimental but also facilitative interactions between soil animal species occurred, independent of trait similarity, indicating other factors than dissimilarity of traits to be important for complementarity effects of soil organisms. Interactions in part were mutually dissimilar with one species benefitting and the second being detrimentally affected in presence of the other. Furthermore, some effects needed more than two species to occur, suggesting that the identity of soil animal species and the composition of the soil animal community override the importance of diversity for ecosystem processes. This suggests that soil fauna interactions are complex and difficult to predict, with predictions of their effects requiring knowledge on the identity of soil animal species that interact. Overall, the results of this thesis indicate aboveground and belowground communities to be intimately linked and to closely depend on each other. Effects of plants on decomposer systems of deciduous forests vary with tree species identity and thereby tree species may drive feedbacks of soil detritivores to plants. Combining our approaches with compound-specific stable isotope analysis, molecular gut content analysis and real time PCR ultimately may allow the understanding of trophic relationships in soil food webs. It also may help to predict the relevance of individual species and community composition on ecosystem processes and hence on aboveground - belowground interactions in forest ecosystems.