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Fixation of Isotopic Nitrogen on a Semiarid Soil by Algal Crust Organisms¹

ABSTRACT

Semiarid desert algal crust organisms were found to fix N₂ when exposed to an atomosphere which contained isotopically enriched N. Significant quantities of the N isotope were detected in the total crust N after 3 days of incubation under field-simulated conditions.

Net N₂ fixation rates by the algal crust organisms were 0.16 and 0.10 lb of N/acre of crust surface per day under continuous wet and cycling wet-dry conditions, respectively. Net fixation of N under field-simulated conditions adequately compensated for the removal of N by livestock. The rate of N₂ fixation under field-simulated conditions increased linearly for at least 520 days. The amount of N in the algal crust was doubled during this time. No net N change was observed in dry crusts.

Growing algal crusts contained 1% to 2% of the total N as extracellular NH₄-N. Excretion of some fixed nitrogen was suggested by the isotopic enrichment of the extracellular N fraction and uptake of labeled N by grass seedlings (Artemesia sp.) growing on incubated crusts.

¹ Journal Paper No. 1022 of the Arizona Agr. Exp. Sta., Tucson. This research was supported in part by the Cooperative Western Regional Research Project W-31. Presented before Div. S-3, Soil Science Society America, Nov., 1963, Denver, Colo.

² Formerly graduate assistant, now Research Soil Scientist, Soil and Water Conservation Research Div., ARS, USDA, Kimberly, Idaho.

³ Assistant Professor, and Professor and Head, Department of Agricultural Chemistry and Soils.

Received for publication July 12, 1965. Accepted for publication September 17, 1965.