Nitrogen content and d15N signature of ombrotrophic Sphagnum plants in Europe: to what extent is the increasing atmospheric N deposition altering the N-status of nutrient-poor mires?

L. Bragazza, J. Limpens, R. Gerdol, P. Grosvernier, M. Hajèk, P. Hajkova, P. Lacumin, L. Kutnar, H. Rydin, T. Tahvanainen

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Abstract

Alteration of the global nitrogen (N) cycle because of human-enhanced N fixation is a major concern particularly for those ecosystems that are nutrient poor by nature. Because Sphagnum-dominated mires are exclusively fed by wet and dry atmospheric deposition, they are assumed to be very sensitive to increased atmospheric N input. We assessed the consequences of increased atmospheric N deposition on total N concentration, N retention ability, and 15N isotopic signature of Sphagnum plants collected in 16 ombrotrophic mires across 11 European countries. The mires spanned a gradient of atmospheric N deposition from about 0.1 up to about 2 g m2 yr1. Mean N concentration in Sphagnum capitula was about 6 mg g1 in less polluted mires and about 13 mg g1 in highly N-polluted mires. The relative difference in N concentration between capitulum and stem decreased with increasing atmospheric N deposition, suggesting a possible metabolic mechanism that reduces excessive N accumulation in the capitulum. Sphagnum plants showed lower rates of N absorption under increasing atmospheric N deposition, indicating N saturation in Sphagnum tissues. The latter probably is related to a shift from N-limited conditions to limitation by other nutrients. The capacity of the Sphagnum layer to filter atmospheric N deposition decreased exponentially along the depositional gradient resulting in enrichment of the mire pore water with inorganic N forms (i.e., NO3+NH4+). Sphagnum plants had 15N signatures ranging from about 8 to about 3. The isotopic signatures were rather related to the ratio of reduced to oxidized N forms in atmospheric deposition than to total amount of atmospheric N deposition, indicating that 15N signature of Sphagnum plants can be used as an integrated measure of 15N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH3 (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower 15N signature compared with mires located in areas dominated by NOx emissions (i.e., mainly affected by industrial activities).
Original languageEnglish
Pages (from-to)106-114
JournalGlobal Change Biology
Volume11
Issue number1
DOIs
Publication statusPublished - 2005

Keywords

  • n-15 natural-abundance
  • atmospheric deposition
  • isotope composition
  • n-15/n-14 ratios
  • vascular plants
  • wet deposition
  • bog vegetation
  • carbon-dioxide
  • nitrate
  • growth

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