Nitrogen fixation by free-living cyanobacteria from different coastal sites in a high arctic tundra, Spitsbergen

Nitrogen fixation rates were measured throughout the summer of 1994 in samples from seven coastal sites at the north side of Brøggerhalvøya, Spitsbergen (78.5°N, 11.6°E) using the acetylene reduction method on intact soil cores. All the sites had a crust containing cyanobacteria, which were considered to be the major nitrogen-fixing organisms. Nostoc commune was the most abundant cyanobacterium. The samples were incubated at 19°C and a constant light intensity of 200 μE m-2s-1. The potential nitrogen fixation activities ranged from 0.22 to 23.7 nmol ethylene produced cm-2h-1when the samples were incubated under constant conditions. The spatial differences in nitrogen fixation were greater than the seasonal. No correlation between nitrogen fixation activities and ecosystem succession could be demonstrated at the sites emerged from the sea, but the nitrogen fixation activity decreased with time since the glacial retreat. There was a significant correlation between temperature at time of sampling and the nitrogen fixation activity measured under constant conditions. Lowest nitrogen fixation activities were measured at sites with the lowest concentrations of extractable magnesium and calcium and the lowest C:N ratios.