Nitrogen concentration and δ15N signature of ombrotrophic Sphagnum mosses at different N deposition levels in Europe

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 δ 15 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 g m −2 yr −1 . Mean N concentration in Sphagnum capitula was about 6 mg g −1 in less polluted mires and about 13 mg g −1 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., NO 3 − +NH 4 + ). Sphagnum plants had δ 15 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 δ 15 N signature of Sphagnum plants can be used as an integrated measure of δ 15 N signature of atmospheric precipitation. Indeed, mires located in areas characterized by greater emissions of NH 3 (i.e., mainly affected by agricultural activities) had Sphagnum plants with a lower δ 15 N signature compared with mires located in areas dominated by NO x emissions (i.e., mainly affected by industrial activities).