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- Bragazza, Luca, et al.
(författare)
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Atmospheric nitrogen deposition promotes carbon loss from peat bogs
- 2006
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Ingår i: Proceedings of the National Academy of Sciences of the United States of America. - : Proceedings of the National Academy of Sciences. - 0027-8424 .- 1091-6490. ; 103:51, s. 19386-19389
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Tidskriftsartikel (refereegranskat)abstract
- Peat bogs have historically represented exceptional carbon (C) sinks because of their extremely low decomposition rates and consequent accumulation of plant remnants as peat. Among the factors favoring that peat accumulation, a major role is played by the chemical quality of plant litter itself, which is poor in nutrients and characterized by polyphenols with a strong inhibitory effect on microbial breakdown. Because bogs receive their nutrient supply solely from atmospheric deposition, the global increase of atmospheric nitrogen (N) inputs as a consequence of human activities could potentially alter the litter chemistry with important, but still unknown, effects on their C balance. Here we present data showing the decomposition rates of recently formed litter peat samples collected in nine European countries under a natural gradient of atmospheric N deposition from approximate to 0.2 to 2 g center dot m(-2)center dot yr(-1). We found that enhanced decomposition rates for material accumulated under higher atmospheric N supplies resulted in higher carbon dioxide (CO2) emissions and dissolved organic carbon release. The increased IN availability favored microbial decomposition (i) by removing N constraints on microbial metabolism and (ii) through a chemical amelioration of litter peat quality with a positive feedback on microbial enzymatic activity. Although some uncertainty remains about whether decay-resistant Sphagnum will continue to dominate litter peat, our data indicate that, even without such changes, increased N deposition poses a serious risk to our valuable peatland C sinks.
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| 2. |
- Wiedermann, Magdalena, 1975-
(författare)
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Responses of peatland vegetation to enhanced nitrogen
- 2008
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Human alteration of the global nitrogen (N) cycle has had major impacts on naturally N-limited ecosystems worldwide. Peatlands, dominated by peat mosses, Sphagnum species, represent one such sensitive ecosystem. I have studied how this ecosystem is affected by increased N availability, using a small-scale N fertilization experiment in combination with a gradient study of three peatlands with varying N deposition.I found both in the experiment and in the gradient a similar pattern of Sphagnum decline accompanied by an increase of vascular plants associated with enhanced N supply. For one common Sphagnum species - both in the experiment and in the gradient study - I also found an identical, linear increase in soluble amino acid N (NAA) accumulation. As soluble amino acids function as N storage compounds among Sphagna, NAA is a suitable measure for Sphagnum N status, and indicates accumulation of excess N not used for growth. My results show that NAA can be used as a sensitive indicator to signal N pollution before the slow, and gradual, regime shift from Sphagnum to vascular plant dominance is visible. In an N-uptake experiment using Sphagnum specimens from the three peatlands varying in N deposition, I found a reduced N-uptake by both investigated Sphagnum species from a high N deposition site, in south-western Sweden. This potential of Sphagna to adjust to high N loads through N uptake regulation will, however, not prevent tissue N accumulation, and as a result a shift from Sphagnum to vascular plant dominance.In general I found similar patterns of N induced changes both in Sphagnum tissue chemistry and vegetation structure in the experiment and along the gradient study. Thus, I conclude that long-term, small-scale field experiments seem to offer reliable estimates of both the direction and strength of key vegetation responses in Sphagnum dominated peatlands. This is likely related to the key role of Sphagna as ecosystem engineers.In the experiment I found a marked time lag in vegetation response to N application treatments. The closed Sphagnum carpet did not collapse until after eight years of continuous treatments. Another result was that dwarf shrubs, e.g. cranberry Vaccinium oxycoccos, first increased, but later declined due to severe attacks by fungal diseases. One important conclusion is that long-term, manipulative field experiments are necessary for our ability to understand how ecosystems will respond to environmental change.
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