Effects of increased nitrogen deposition on the distribution of 15N-labeled nitrogen between Sphagnum and vascular plants

To elucidate the sensitivity of bog ecosystems to high levels of nitrogen (N) deposition, we investigated the fate of N-15-labeled N deposition in bog vegetation in the Netherlands, both at ambient and increased N deposition. We doubled N deposition by adding 5 g N m(-2) y(-1) as dissolved NH4NO3 during three growing seasons to large peat monoliths (1.1 m diameter) with intact bog vegetation kept in large outdoor containers. A small amount of N-15 tracer was applied at the start of the second growing season, and its distribution among Sphagnum, vascular plant species, and peat was determined at the end of the third growing season. The N-15 tracer was also applied to additional plots at the untreated field site to check for initial distribution. One week after addition, 79% of the total amount of N-15 retrieved was found in the living Sphagnum layer and less than 10% had been captured by vascular plants. Fifteen months later, 63% of the total amount of N-15 retrieved was still present in the living Sphagnum layer at ambient N deposition. Increased N deposition significantly reduced the proportion of N-15 in Sphagnum and increased the amount of N-15 in vascular plants. Deep-rooting vascular plant species were significantly more N-15 enriched, suggesting that at higher atmospheric inputs N penetrates deeper into the peat. Our results provide the first direct experimental evidence for that which has often been suggested: Increased atmospheric N deposition will lead to increased N availability for vascular plants in ombrotrophic mires.