We experimentally added nitrogen and phosphorus in a bog on the southern Alps. We hypothesized that, alleviating nutrient limitation will increase vascular plant cover. As a consequence, more carbon will be fixed through higher rates of net ecosystem CO2 exchange (NEE). The vascular cover did increase at the expense of Sphagnum mosses. However, such vegetation changes were largely independent of the treatment and were probably triggered by an exceptional heatwave in summer 2003. Contrary to our hypothesis, NEE was unaffected by the nutrient treatments but was strongly influenced by temperature and water-table depth. In particular, ecosystem respiration in the hot summer 2003 increased dramatically, presumably due to enhanced heterotrophic respiration in an increased oxic peat layer. At the end of the experiment, the Sphagnum cover decreased significantly in the nitrogen-fertilized treatment at hummock microhabitats. In the long term, this will imply a proportionally greater accumulation of vascular litter, more easily decomposable than the recalcitrant Sphagnum litter. As a result, rates of carbon fixation may decrease because of stimulated respiration.
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