Nitrogen deposition drives loss of moss cover in alpine moss–sedge heath via lowered C: N ratio and accelerated decomposition

• Published in: The New phytologist 2018-04, Vol.218 (2), p.470-478
• Main Authors: Britton, Andrea J., Mitchell, Ruth J., Fisher, Julia M., Riach, David J., Taylor, Andy F. S.
• Format: Article
• Language: English
• Subjects: bryophytes; carbon pool; community composition; C : N ratio; litter decomposition; nitrogen pool; nitrogen saturation; Racomitrium heath
• ISSN: 0028-646X; 1469-8137
• DOI: 10.1111/nph.15006

In alpine ecosystems, nitrogen (N) deposition has been linked to plant community composition change, including loss of bryophytes and increase of graminoids. Since bryophyte growth is stimulated by increased N availability, it has been hypothesized that loss of bryophyte cover is driven by enhanced decomposition. As bryophyte mats are a significant carbon (C) store, their loss may impact C storage in these ecosystems. We used an N deposition gradient across 15 sites in the UK to examine effects of N deposition on bryophyte litter quality, decomposition and C and N stocks in Racomitrium moss–sedge heath. Increasing N deposition reduced C : N in bryophyte litter, which in turn enhanced decomposition. Soil N stocks increased significantly in response to increased N deposition, and soil C : N declined. However, depletion of the bryophyte mat and its replacement by graminoids under high N deposition was not associated with a change in total ecosystem C stocks. We conclude that decomposition processes in Racomitrium heath are very sensitive to N deposition and provide a mechanism by which N deposition drives depletion of the bryophyte mat. Nitrogen deposition did not measurably alter C stocks, but changes in soil N stocks and C : N suggest the ecosystem is becoming N saturated.