Will climate change alter Arctic nitrogen budgets? Impacts of warming and fertilization on nitrogen fixing microbial communities at Alexandra Fiord, Ellesmere Island, Nunavut.

The impacts of simulated climate change (warming and fertilization treatments) on diazotroph community structure and activity were investigated at Alexandra Fiord, Ellesmere Island, Canada. Open Top Chambers were randomly placed in a dwarf-shrub, cushion-plant dominated mesic tundra site in 1995. In 2000 and 2001 20N: 20P2O5: 20K2O fertilizer was applied at a rate of 5 g m-2year-1. Estimates of nitrogen fixation rates were made in the field by Acetylene Reduction Assays (ARA). Higher rates of N-fixation were observed 19-35 days post-fertilization but were otherwise unaffected by treatments and we hypothesize that microsite variation was a greater determinant of N-fixation rate than were the treatments applied. 'Nif'H genes were amplified from bulk soil DNA and analyzed by Terminal Restriction Fragment Length Polymorphism (T-RFLP) analysis. Nonmetric Multidimensional Scaling (NMS) was used to ordinate treatment plots in 'nif'H genotype space. 'Nif'H gene communities were more strongly structured by warming treatment late in the growing season, suggesting that an annual succession in diazotroph community composition occurs. [delta]15N analysis of plant and soil material from each treatment plot suggests that evergreen dwarf shrubs will depend more heavily on organic-N derived from mycorrhizae in warmer climates and that relative importance of symbiotic nitrogen fixation to the N-nutrition of 'D. integrifolia' will decline at this site.