Abstract
Peat forming Sphagnum mosses are able to prevent the dominance of vascular plants under ombrotrophic conditions by efficiently scavenging atmospherically deposited nitrogen (N). N-uptake kinetics of these mosses are therefore expected to play a key role in differential N availability, plant competition, and carbon sequestration in Sphagnum peatlands. The interacting effects of rain N concentration and exposure time on moss N-uptake rates are, however, poorly understood.We investigated the effects of N-concentration (1, 5, 10, 50, 100, 500 µM), N-form (15N - ammonium or nitrate) and exposure time (0.5, 2, 72 h) on uptake kinetics for Sphagnum magellanicum from a pristine bog in Patagonia (Argentina) and from a Dutch bog exposed to decades of N-pollution.Uptake rates for ammonium were higher than for nitrate, and N-binding at adsorption sites was negligible. During the first 0.5 h, N-uptake followed saturation kinetics revealing a high affinity (Km 3.5–6.5 µM). Ammonium was taken up 8 times faster than nitrate, whereas over 72 hours this was only 2 times. Uptake rates decreased drastically with increasing exposure times, which implies that many short-term N-uptake experiments in literature may well have overestimated long-term uptake rates and ecosystem retention. Sphagnum from the polluted site (i.e. long-term N exposure) showed lower uptake rates than mosses from the pristine site, indicating an adaptive response. Sphagnum therefore appears to be highly efficient in using short N pulses (e.g. rainfall in pristine areas). This strategy has important ecological and evolutionary implications: at high N input rates, the risk of N-toxicity seems to be reduced by lower uptake rates of Sphagnum, at the expense of its long-term filter capacity and related competitive advantage over vascular plants. As shown by our conceptual model, interacting effects of N-deposition and climate change (changes in rainfall) will seriously alter the functioning of Sphagnum peatlands.
Highlights
IntroductionBogs (ombrotrophic peatlands, i.e. predominantly fed by rainwater, from Greek ombros, rain, and trephein, to feed) are exceptional ecosystems that may show high storage rates for nutrients and carbon, while nutrient availability is very low due to ombrotrophic conditions (water and nutrient input solely by rain) limiting the growth of vascular plants
Bogs are exceptional ecosystems that may show high storage rates for nutrients and carbon, while nutrient availability is very low due to ombrotrophic conditions limiting the growth of vascular plants
In this study we address the question whether increased exposure time to elevated nitrogen inputs results in a reduction of nitrogen uptake efficiency and whether this can be related to nitrogen saturation
Summary
Bogs (ombrotrophic peatlands, i.e. predominantly fed by rainwater, from Greek ombros, rain, and trephein, to feed) are exceptional ecosystems that may show high storage rates for nutrients and carbon, while nutrient availability is very low due to ombrotrophic conditions (water and nutrient input solely by rain) limiting the growth of vascular plants. The atmospheric input of nutrients is efficiently retained in moss peat and decomposition rates are low, due to the high retention of rainwater, acidic conditions and poorly degradable organic matter [3]. A substantial part of the C losses (including CH4) are refixed by Sphagnum as growth and photosynthesis have been shown to increase upon elevated CO2 in porewater [4,5,6]. This combination of traits enables Sphagnum to avoid being outcompeted by vascular plants. Sphagnum peat may accumulate substantial amounts of nutrients, if considered m22 y21, even though nutrient concentrations in Sphagnum are lower
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