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Acid N depositions in the Bohemian Forest during the second half of the last century caused enormous soil acidification which led to the leaching of essential nutrients including nitrates. We investigated the effect of dissolved organic matter (DOM) and pH on the abundance of 16S RDNA, nirK and nirS gene copies in four spruce forest sites. Soil samples for molecular based quantification (qPCR) were taken from the organic litter and humus layers. The amounts of dissolved organic carbon (DOC) and dissolved nitrogen (DN) were much lower in highly acidified soils. We found a strong correlation between nirK denitrifiers and the amount of available P (r = 0.83, p \ 0.001), which suggested a higher nutrient sensitivity of this group of denitrifying bacteria. Additionally, we found that correlations between the amount of nirK denitrifiers and DOC and pH are exponentional showing two important threshold values, being 4.8 mol kg -1 and 5, respectively. The amount of nirK denitrifiers rapidly decreased below these values. The amount of nirK and nirS denitrifiers was higher in the organic litter horizon than the organic humus horizon at all sampling sites.

Rootless aquatic carnivorous Utricularia exude up to 25% of their photosynthates into the trap lumen, which also harbours a complex microbial community thought to play a role in enhancing Utricularia nutrient acquisition. We investigated the composition of organic carbon in the trap fluid, its availability for microbial uptake, the influence of plant nutrient status and trap age on its biodegradability, and the composition of prokaryotic assemblages within the traps of three aquatic Utricularia species. Using ion chromatography and basal respiration rate measurements we confirmed that up to 30% of total dissolved organic carbon in Utricularia trap fluid in oligotrophic conditions was easily biodegradable compounds commonly found in plant root exudates (mainly glucose, fructose and lactate). The proportion of these compounds and their microbial utilisation decreased with increasing mineral nutrient supply and trap age. Fluorescence in situ hybridisation analyses showed that microbial trap assemblages are dominated by alpha and beta Proteobacteria, and that the assemblage composition is affected by changes in the ambient mineral nutrient supply. We suggest that organic carbon dynamics within the traps, involving both the plant and associated microbial assemblages, underlies the acquisition of key nutrients by Utricularia and may help explain the evolutionary success of the genus.

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