Symbiosis-driven links between carbon and nitrogen cycling:implications for peatlands and other ecosystems

Ecosystems, including peatlands, are threatened worldwide by human activities, like draining and fertilizing soils and burning of fossil fuels. This leads to climate change and loss of biodiversity by increased concentrations of greenhouse gases and reactive nitrogen in the atmosphere. To better understand these effects it is important to study the carbon and nitrogen cycle, in which microorganisms play a crucial role. This thesis focuses on microorganisms that are in close contact, in symbiosis, with plants, especially plants that are keystone species, determining the functioning of the ecosystem. Both eutrophic and pristine areas were studied, and in the latter, epiphytes typical of American savannahs were found to have a mutualistic interaction with their host oak tree, with benefits for both parties. This symbiosis and also that of peatmosses with their microbial community increase the input of nitrogen (with around 20%) to the (nitrogen limited) ecosystem, leading to increased productivity. In peatlands, this additional input of nitrogen is due to nitrogen fixation, the process in which microbes convert inert nitrogen gas to plant-available reactive nitrogen. This process in peatmosses, Sphagnum, is especially driven by phosphorus availability and decomposability of tissue and not down-regulated by nitrogen deposition. This means that in eutrophic systems, microorganisms still fix nitrogen even when external nitrogen input is high, depending on the relative availability of phosphorus and other minerals, leading to further increased nitrogen accumulation. Therefore, it is important to take the symbiotic microbial community of plant species into account when assessing ecosystem functioning. This thesis shows the strong link between carbon and nitrogen dynamics by symbiotic nitrogen fixation, affecting nitrogen availability and productivity in different nitrogen limited ecosystems, with important implications for ecosystem restoration. In peatlands, microorganisms associated with keystone peat-forming species can determine carbon exchange rates, affecting important environmental drivers like the global warming potential.