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The nitrogen fixation by biological soil crusts with a dominance of cyanobacteria was studied using the acetylene reduction assay in the territory of the Subpolar Urals (65A degrees 11’ N, 60A degrees 18’ E), Russia. The field measurements of nitrogen fixation activity were conducted in situ for two different types of soil crusts dominated by Stigonema (V1 type) and Nostoc with Scytonema (V2 type). The nitrogen fixation process had similar dynamics in both crusts but nitrogen fixation rates were different. The crusts of the V2 type showed a significantly higher acetylene reduction activity, with ethylene production rate of 1.76 +/- 0.49 g C2H4 m(-2) h(-1) at 15A degrees C, compared with V1-type soil crusts, with a rate of 0.53 +/- 0.21 mg C2H4 m(-2) h(-1) at 15A degrees C. The daily value of acetylene reduction activity in V2-type soil crusts was 32.7 +/- 6.2 mg C2H4 m(-2) d(-1) and in V1-type crusts, 12.3 +/- 1.8 mg C2H4 m(-2) d(-1). After recalculation for N, the daily values of nitrogen fixation were in the range 3.3-22.3 mg N m(-2) d(-1), which is a few times higher than the values of N input from the precipitation to the soil in the studied regions. The dependence of nitrogen-fixation activity on temperature and light intensity of biological soil crusts was investigated. On the basis of temperature models obtained from the dependence, the nitrogen balance was calculated for the growing season (approximately 120 days). The crusts dominated by Stigonema species were fixing 0.3 g N m(-2) (ethylene production rate, 1.10 g C2H4 m(-2)) and crusts dominated by Nostoc and Scytonema were fixing 1.3 g N m(-2) (4.10 g C2H4 m(-2)).The nitrogenase activity of two types of biological soil crusts with domination of different cyanobacteria genuses has similar dynamics but distinct rate as a result of dominant species composition (Stigonema vs Nostoc and Scytonema), cyanobacteria abundance, and environmental conditions of mountain tundras on territory of Subpolar Urals.The nitrogenase activity of two types of biological soil crusts with domination of different cyanobacteria genuses has similar dynamics but distinct rate as a result of dominant species composition (Stigonema vs Nostoc and Scytonema), cyanobacteria abundance, and environmental conditions of mountain tundras on territory of Subpolar Urals.