Research note—Nitrogen fixation by cyanobacterial crusts and by associative‐symbiotic bacteria in Western Kalahari, Botswana

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Abstract
Air‐dried samples of cyanobacterial crusts and grass roots collected in the Kalahari were moistened in the laboratory, and nitrogenase activity was shown in both types of samples. The dark‐colored cryptogamic crusts, covering about 1.5% of the soil during the wet period, showed a higher potential reduction rate, about 6.8 mmol m‐2 h−1, than the sand‐colored ones, about 0.6 mmol m‐2 h−1. One species of Nostoc and one of Scytonema were isolated from the crusts. Samples of roots of eight grasses showed significant acetylene reduction. The reduction rate was higher in old roots than in young ones, and increased with extended incubation. Azospirillum sp. was recovered from the roots.