Nitrogen concentration and d15N signature of ombrotrophic Sphagnum mosses at different N deposition levels in Europe

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 d15 N 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 gm2 yr 1 . Mean N concentration in Sphagnum capitula was about 6mg g1 in less polluted mires and about 13mg 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 1NH4 1). Sphagnum plants had d15 N 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 d15 N signature of Sphagnum plants can be used as an integrated measure of d15 N 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 d15 N signature compared with mires located in areas dominated by NOx emissions (i.e., mainly affected by industrial activities)