NITROGEN FIXATION BY LEGUMES AND BLUE-GREEN ALGAL-LICHEN CRUSTS IN A COLORADO DESERT ENVIRONMENT

Potential sources of fixed nitrogen in a Colorado desert environment were examined by the acetylene reduction method at the Deep Canyon Desert Research Center, near Palm Desert, California. In field and greenhouse studies all members of the genera Astragalus, Dalea, Lotus, Lupinus, Melilotus, and Prosopis examined formed active nodules (acetylene reduction) with indigenous soil bacteria. No evidence of nodulation was found for Acacia greggii, Cercidium floridum, or Hoffmannseggia microphylla. Lotus tomentellus was estimated to fix 0.1 kg N ha⁻¹ by the time of flowering under field conditions. Several members of the genus Dalea showed substantial rates of acetylene reduction in the greenhouse: D. emoryi, 16.1 + 3.5, D. mollissima, 11.4 + 3.7, D. schottii 2.9 + 1.7, D. spinosa 2.5 + 0.4 μmoles ethylene plant⁻¹ hr⁻¹. In greenhouse assays where water was supplied continuously, blue-green algal-lichen crusts reduced acetylene at an average rate of 11.0 + 5.7 nmoles ethylene cm⁻² hr⁻¹ with a maximum of 57.1. But when in situ assays were done following irrigation of a field plot with 2.3 cm of water, much lower activities were observed with a maximum activity of only 6.4 nmoles cm⁻² hr⁻¹.