Marie Dam:
Global change effects on plant-soil interactions

Global change is expected to increasingly affect composition and functioning of soil communities. In terrestrial ecosystems, the plant-soil interactions will be of particular importance for the ecosystem response, including feed-back responses that may further increase climate change. The aim of this dissertation has been to determine how soil food web structure and function is affected when the quantity and quality of plant input is altered under global change.

By studying the abundance and composition of soil organisms, particularly those in the rhizosphere, closely associated with living plants, we are able to determine effects of global change on the plant-soil system. By extraction and microscopy of nematode communities, we are able to characterize the trophic structure of a significant part of the rhizosphere community. The work compiled for this dissertation is based on field experiments in temperate heathland and grassland. This includes characterization of a decomposer system under global change as defined by the plants present (Paper I), understanding the mechanism shaping the system response by manipulating input from living plants (Paper II) or by manipulating plant input by burning (Paper III). Furthermore, by way of meta-analysis, the role of organisms in global change effects on ecosystem function is modelled (Paper IV).

Among CO₂, warming and summer drought, CO₂ is the factor most consistently impacting soil organisms. CO₂ increases abundance of microorganisms and nematodes, and increase flow through the fungal decomposition pathway, most likely due to an increase of plant resources allocated belowground and an increased C:N ratio of the plant input. In line with this, CO₂ increase is found to generally correlate with plant biomass in the meta-analysis. Hence, plant allocation of resources is of significant impact belowground, but the allocation is also influenced by other factors: When grasses are defoliated in the growing season, there is an increased shoot regrowth at the expense of the belowground system. Both microorganisms and nematodes are reduced, despite positive CO₂ effects. Furthermore, the plant functional type (shrub or grass) is more strongly determining the rhizosphere community structure than any global change factor. Frequent burning of prairie vegetation changes the soil community to an extent that alters the decomposition rate.

Together, these results suggest that not only the global change effects on established ecosystems, but also the global change effects on plant community composition as well as land use management may determine the composition and function of soil food webs in the future.