Phosphorus fertilization in a phosphorus-limited fen: effects of timing

Abstract. Phosphorus availability in low P-soils is primarily controlled by soil processes that are subject to seasonal fluctuation. There is evidence that summer drought causing low fen-water levels induces temporal high P-availability. We investigate here whether and how fen vegetation responds to P-pulses at different times in the season. Plots were fertilized with equal P-doses at three different times in the season. Four conclusions can be drawn from the experiment: 1. The vegetation is able to increase P-uptake independent of the timing of fertilizer application; 2. Early-season fertilization stimulates growth and increases P-concentration in above-ground tissue while late-season fertilization does not stimulate growth but strongly increases above-ground tissue concentration; 3. Timing effects cause differential species responses: the response of Carex demissa, a stress-tolerant species, is truly independent of timing, while Juncus articulatus, a CSR-type species (sensu Grime) profits more from early-season fertilization; 4. Timing effects persist over several years. The differences between the experimental treatment and the events it aimed to simulate are discussed. We expect that higher frequencies of drought events, that may be induced by climate change, will cause a shift from nutrient-stress tolerant towards stress-tolerant-competitor fen species.