title: Nitrogen Fixation in Arid Western Soils
Creators: name: Rychert, Robert Charles
Publisher: name: Utah State University
Year: 1975

description: Nitrogen fixation by blue-green algae-lichen crusts from South Curlew Valley, Utah, in the Great Basin Desert, was studied using the acetylene reduction technique. A molar ratio of 3 moles C2H4 produced/mole of N2 fixed was used to estimate nitrogen (N2) fixation by acetylene reduction. Nostoc was found to be present in many of the lichen thalli examined microscopically. Crust nitrogen fixation decreased rapidly below -1/3 bar pressure (water potential) which indicated that nitrogen fixation occurs only when the crust is wet. This would suggest that most of the crust nitrogen fixation in the Great Basin Desert occurs during the fall and spring rainy seasons. Nitrogen fixation reached a maximum at 200 microeinsteins m-2 sec-1 of incandescent light intensity, somewhat comparable to natural light with a heavy grey cloud cover. In the winter months temperature would limit nitrogen fixation, and moisture would limit nitrogen fixation during the hot, dry summer months.
Ninety grams of nitrogen (N) per hectare was found to have been fixed in situ on a typical fall day, with peak in situ rates of about 14 g N fixed ha-1 hour-1.When assayed in the laboratory under optimal conditions (nitrogen fixation potential), crusts could fix nitrogen at rates as high as 84 g N fixed ha-1 hour-1. Since moisture and light were not limiting in in situ assays, in situ nitrogen fixation correlated with predictions made from the laboratory determined temperature curve. There was some dark nitrogen fixation, with values varying from 5 to 30 percent of nitrogen fixation in the light. An estimate of 10 to 100 Kg N fixed ha-1 year-1 was made, based on 120 days of fixation. The absolute value would vary from area to area depending upon the extent of blue-green algae-lichen crust cover, and with temperature, moisture availability, and duration of the rainy periods.
Glucose was found to potentiate crust heterotrophic nitrogen fixation with an optimum at 30 C. Soils beneath the crust exhibited no acetylene reduction, unless amended with glucose, and soils at the 40 to 50 cm depth showed no ability to reduce acetylene even when amended with glucose. Apparently the supply of available organic carbon in the soils is insufficient for significant heterotrophic nitrogen fixation to occur.
Ammonium ion was found to inhibit crust nitrogen fixation, pointing toward ammonium ion regulation of nitrogen fixation.
May versus September, 1973 crust transect samples from sites dominated by Atriplex confertifolia, Ceratoides lanata, and Artemisia tridentata were not significantly different. However, the Ceratoides lanata values were significantly lower and reflected the scant lichen cover in that desert shrub community. Nitrogen fixation was found to be greatly reduced under the canopies of the three desert shrubs mentioned previously, with an understory dominance by bryophytes. Aqueous leaf extracts and leaf volatile products inhibited crust nitrogen fixation, with the inhibition being particularly pronounced with Artemisia tridentata leaf extracts. Glucose was used to potentiate heterotrophic nitrogen fixation. Dried plant leaves, when added to soils, and the mixed soil and leaves moistened with glucose solutions, inhibited heterotrophic nitrogen fixation. The heterotrophic nitrogen fixation potential (glucose amended soils) was greatly reduced in soils from beneath the desert shrub canopies. Thus, shrub inhibitors may play a role in nitrogen input and blue-green algae-lichen crust distribution in desert shrub communities.