Earthworm-mycorrhiza interactions and grassland ecosystems

Understanding what determines plant diversity and functioning of grassland ecosystems is a major goal of contemporary ecological research. While there is increasing evidence that either earthworms or arbuscular- mycorhizal fungi (AMF) play an important role in determining the diversity and functioning of grassland ecosystems, their interactive effects have greatly been neglected in the past research. This project is specifically designed to improve our understanding of the functional and structural interactions between earthworms, AMF and plant species in grassland ecosystems and their feedback effects on the diversity, structure and functioning of these ecosystems. Therefore, during 36 months full-factorial experiments at different levels of ecological complexity and spatial scales will be conducted using mesocosms in the greenhouse, trays in the laboratory and field plots. Mesocosm experiments will address basic questions whether earthworm-AMF interactions can (i) affect the diversity and functioning of plant communities, (ii) influence the establishment of seedlings in these communities and (iii) affect plant community diversity and structure by creating soil microsites enriched in nutrients and/or AMF. The laboratory experiment indirectly addresses earthworm-AMF interactions by investigating AMF-induced changes on plant tissue quality and its consequences for earthworm herbivory. The objective of the field experiment is to investigate whether different earthworm community densities can influence AMF distribution and symbiosis and consequently affect grassland diversity and functioning.

This project investigated ecological interactions between belowground and aboveground organisms such as earthworms, symbiotic arbuscular-mycorrhizal fungi and plants and their consequences for the functioning of grassland ecosystems. Therefore, we conducted a series of greenhouse experiments at different levels of ecological complexity. In two experiments we tested scientific methods aiming at tracking these interactions. As a result of these experiments we developed a new method for in-situ labelling of plant species with stable isotope tracers using foliar-brushing and a new method for in-situ isotope labelling of earthworms. These rather simple methods can easily be adopted in ecological experiments when the goal is to investigate functional links between belowground and aboveground organisms. In another experiment we found that soil sand content alters the effects of different mycorrhizal taxa on plant growth. Based on these findings we conducted a more complex experiment on the effects of earthworms and mycorrhiza on the diversity and functioning of establishing grassland communities. Results showed that earthworms altered mycorrhizal colonisation of plant species and reduced seedling emergence and plant community diversity. Moreover important ecosystem properties such as water infiltation or nutrient leaching was affected by both earthworms and mycorrhiza. Another experiment studied the role of subsurface nutrient-rich and mycorrhiza-containing earthworm casts on the growth of different grassland plant species. Results showed that minute amounts of subsurface casts specifically affect the growth of plants and provide microsites from which root mycorrhizal colonization can start and might therefore play a role in structuring plant communities. In a further experiment we applied the isotopic tracing method developed earlier and tried to track the functional relationships across trophic levels including detritivores, symbionts, primary producers and herbivores. We found that plants utilize nutrients excreted via earthworm casts or mucus and that these nutrients can be traced in roots, leaves and aphids infesting the plant. Intrigued by these results we conducted two other experiments investigating whether invasive slugs as generalist herbivores are affected by earthworms or mycorrhiza. Results showed that the feeding behavior of an invasive slug is altered by effects of earthworms and mycorrhiza on plant chemical quality and plant community structure. Moreover, effects of earthworms on slug herbivores were dependent on the species composition of plant communities. Taken together, the findings from this project provide new insights for a better understanding of belowground-aboveground linkages in grassland ecosystems by (i) bridging commonly separated scientific disciplines such as plant, animal and microbial ecology and (ii) by employing traditional as well as modern methodological approaches.