Prospects for Sphagnum bogs subject to high nitrogen deposition

S</strong> phagnum bogs harbour a wealth of rare vascular plant and bryophyte species, preserve an amazing pollen record and are long-term sinks for atmospheric carbon. Unfortunately, the relatively low production and decomposition rates, that make these bogs such important environments, also make them vulnerable to changes in atmospheric nitrogen (N) input. The research presented in this thesis had as its aim to investigate to what extent N deposition could affect bogs, and explore whether its influence could render initially successful restoration efforts futile.We conducted a set of field and greenhouse experiments, aimed at delineating the effects of N and P on the interactions between Sphagnum and vascular plants, epiphytic algae, fungi and other Sphagnum species. In addition, we paid attention to the physiological effects of a high N supply on Sphagnum , and we studied the impact of an elevated N supply on litter quality and decomposition rate, to get an impression of its long-term effects on bog vegetation.</font><font size="2"></font>Our results unambiguously showed that a simulated increase in N deposition depressed Sphagnum growth. How this decreased vitality came about is not so straightforward, however. We can distinguish two types of negative N effects on Sphagnum . A direct toxic effect that seems to be linked to the N metabolism of Sphagnum and an indirect effect brought about by intensified interactions with other organisms. Additionally, our results showed that Sphagnum originating from sites with a high N deposition decomposed faster than Sphagnum from a site with intermediate N deposition. This combination of decreased Sphagnum production and increased decomposition nudges the carbon balance of these systems towards the negative, and thus challenges the survival of bogs.A considerable part of the effects mentioned above depends on the amount of deposited N that Sphagnum can incorporate in its tissue and on the resulting tissue N concentration. As such, the impact of a high N supply is not so much determined by the level of N deposition per se than by the balance between the negative effects of N on the one hand and the supply of potentially growth-limiting factors such as water, P, CO ₂ , light and temperature on the other hand. Thus, it seems possible to circumvent an important part of the negative N effects by optimising the overall growing conditions of Sphagnum . However, we must realise that the resilience of the bog ecosystem and the range of conditions under which Sphagnum bogs can survive decrease with N deposition, and thus are limited