Nitrogen fixation activity in biological soil crusts dominated by cyanobacteria in the Subpolar Urals (European North-East Russia)

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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 (65°11′ N, 60°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 15°C, compared with V1-type soil crusts, with a rate of 0.53 ± 0.21 mg C2H4 m−2 h−1 at 15°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).

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