Seasonal patterns of nitrogen fixation in biological soil crusts from British Columbia's Chilcotin grasslands.

This study provides estimates of N2 fixation in biological soil crusts (BSCs) from shrub-steppe grasslands in British Columbia's Chilcotin plateau. We identify key seasonal periods and environmental conditions during episodes of BSC nitrogen-fixation activity. Predicted nitrogen-fixation activity showed two peak periods: one during late winter and early spring snowmelt events, and the second during summer precipitation events. BSC nitrogen fixation, especially summer activity, was quite variable from year to year. The magnitude of predicted nitrogen fixation at the landscape level was dependent upon the ratio used to convert rates of acetylene reduction to nitrogen fixed. Using a theoretical ratio of 3:1, estimates of average annual BSC nitrogen fixation were 1.0 kg N·ha−1. However, using a conversion ratio of 0.06, obtained from side-by-side measurements of acetylene reduction and 15N uptake, annual estimates are 52 kg N·ha−1. These estimates are based on assessments of the aerial extent of dark BSC communities, which at the Farwell Canyon study site exceeded 50%. The fate of fixed nitrogen, especially during the late winter period, when underlying soils are frozen, remains unclear. [ABSTRACT FROM AUTHOR]