Plant species and global change agents as driving factors of rhizosphere processes and soil nematode communities

Abstract

The soil system harbors important ecosystem functions. Understanding the factors influencing the functioning of soil systems is crucial to react adequately on changes induced by alteration in plant species compositions and climate change. Soil food webs deliver important information on energy flux in soil and can help to understand global carbon and nitrogen fluxes. To date, the effect of tree species identity and diversity as well as the effect of species-specific root- and litter-mediated effects have not been investigated on the compartmentalization of basal resources, i.e., bacteria and fungi, and on the way carbon and nitrogen is channeled through the soil system. There is also lack of information on how human induced climate change influences processes in soil. Although soil stores at least twice as much carbon as the atmosphere and plants together, the response of the belowground system to interactively acting global change agents has not been investigated so far. Carbon stocks in soil are likely to be influenced by the response of the belowground system to global change agents thereby potentially reinforcing atmospheric CO2 concentrations by accelerating the loss of soil carbon. In Chapter 2, the effect of tree identity and diversity was investigated on the structure of soil nematodes. The results suggest that tree species identity is more important than tree species diversity in structuring nematode communities and associated soil processes. Ash increased the density of bacterial-feeding nematodes and reduced the number of fungal feeders, indicating distinct changes in regulatory forces of soil food webs. Beech detrimentally affected bacterial feeders but favored fungal feeders probably via pH-mediated increase in the fungal-to-bacterial ratio. The effect of lime was less pronounced but tended to be generally negative. The results indicate that both leaf litter and roots influence the nematode community, with the effects being driven not solely by resource quality but additionally by availability, i.e., seasonal shifts in dominant belowground resources. In sum the structure of soil food webs varies markedly with tree species pointing to the importance of variations in plant resources, i.e., leaf litter quality and root exudates, as strong bottom-up regulating factors of microbial communities and energy channels of decomposer systems. In Chapter 3, we used 13C and 15N labeled ash litter to trace the flux of carbon and nitrogen through soil influenced by beech and ash seedlings. We found that beech decreased soil pH by root exudation thereby increasing fungal biomass and decreasing carbon use efficiency of bacteria. Higher bacterial respiration induced carbon loss from soil and prevented litter-derived carbon and nitrogen to reach higher trophic levels. In contrast, ash had no effect on microbial or soil parameters although its root biomass exceeded that of beech markedly. Ash was, however, very efficient in using litter-derived nitrogen from soil. The results suggest that beech and ash differentially impact soil processes with beech affecting the belowground system predominantly via roots whereas ash predominantly via litter. In Chapter 4, the effects of increased atmospheric CO2, elevated nitrogen and reduced precipitation were investigated on the composition and diversity of belowground animal communities. Elevated CO2 increased microbial biomass by increased rhizodeposition leading to increased densities of ciliates, collembolans and gamasid mites. Reduced precipitation was of minor importance as the soil community in the sandy soils of the study site is probably adapted to drought conditions. Nitrogen addition negatively affected predatory and herbivorous nematodes, and overall nematode diversity. CO2 and nitrogen interactively affected nematode communities resulting in taxonomically and functionally altered, potentially simplified, soil communities. In Chapter 5, we used functional guilds of nematodes to inspect changes in soil processes induced by increased atmospheric CO2 concentrations, elevated nitrogen levels and reduced precipitation. We found that the decomposer community switched from a bacterial dominated to a more fungal dominated system at elevated N indicating strong changes in the microbial community and the functioning of belowground food webs. Nitrogen fertilization also reduced top-down forces and simplified soil food webs exposed to additional N input. Further, bacterial-feeding nematodes were not able to profit from increased microbial biomass suggesting that quality rather than quantity of food resources controls nematode densities. Reduced densities of root-feeding Longidoridae likely reflected increased belowground plant defense at high CO2 and N levels. Changes in decomposition processes shifting towards fungal domination at elevated N levels indicating an increase in recalcitrant and resistant compounds contributing to C sequestration thereby attenuating the increase in atmospheric CO2 concentrations. In Chapter 6, NanoSIMS is introduced as a technique to measure the isotopic signature of single nematodes. I developed embedding techniques of nematodes for measurements in high vacuum and present first results but also restrictions. Further, different fields of application for tracing elements in food webs are presented.