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Blue‐green algae‐lichen crusts (Atriplex confertifolia, Eurotia lanata, and Artemisia tridentata sites) from the Great Basin Desert have a laboratory potential of fixing atmospheric nitrogen at rates up to 84 g of N ha‐1 hour‐1. Nitrogen fixation is optimal when the crust is moistened to ‐⅓ bar pressure, temperature is 19 to 23C, and the light intensity is 200 microeinsteins m‐2 sec‐1 with incandescent light. The acetylene reduction technique provided a useful assay to measure in situ nitrogen fixation which was correlated with potential values obtained in the laboratory under optimum conditions. Nitrogen fixation was found to be reduced under desert shrub canopies possibly due to allelopathic effects of the shrubs. Aqueous leaf extracts of desert shrubs significantly inhibited nitrogen fixation. Annual nitrogen fixed was estimated at 10 to 100 kg of N ha‐1 year‐1, depending upon microenvironmental conditions.

N2 fixation by bacteria in associative symbiosis with washed roots of 13 Poaceae and 8 other noncultivated plant species in Finland was demonstrated by the acetylene reduction method. The roots most active in C2H2 reduction were those of Agrostis stolonifera, Calamagrostis lanceolata, Elytrigia repens, and Phalaris arundinacea, which produced 538 to 1,510 nmol of C2H14 g-1 (dry weight). h-1 when incubated at PO2 0.04 with sucrose (pH 6.5), and 70 to 269 nmol of C2H4. g' (dry weight) h-' without an added energy source and unbuffered. Azospirillum lipferum, Enterobacter agglomerans, Klebsiella pneumoniae, and a Pseudomonas sp. were the acetylene-reducing organisms isolated. The results demonstrate the presence of N2-fixing organisms in associative symbiosis with plant roots found in a northern climatic region in acidic soils ranging down to pH 4.0. A number of investigators have described bacteria, associated with the roots ofnonleguminous plants, which fix nitrogen. Beijerinckia species have been isolated from the roots of sugar cane and several tropical grasses (7), Azotobacter paspali has been isolated from Paspalum notatum (8), Pseudomonas sp. has been found in rice (1) and in Pennisetum americanum (14), and Azospirillum species have been isolated from a variety of forage grasses, grain crops, and legumes (9, 10). In Oregon and Nebraska, Bacil

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