BIOLOGICAL NITROGEN INFLUX IN AN OHIO RELICT PRAIRIE

The principal contributors of biologically fixed N in natural grassland ecosystems appear to be asymbiotic bacteria and heterocystous cyanobacteria. The environmental factors of light, moisture, and temperature play important roles in the magnitude of the N₂ -fixation activity. Biological N₂ -fixation was measured in the Elizabeth's Prairie section of the Lynx Prairie Preserve, Adams County, Ohio, during 15 site visits beginning 29 March through 8 November 1980. In situ N₂ -fixation activity was measured using the acetylene-reduction technique. The percentage cover of cyanobacterial colonies (Nostoc sp.) was determined using Point-Frame Analysis. Soil and air temperatures and soil water potentials also were measured. Intact soil cores with a surface cover of Nostoc were collected and returned to the laboratory to quantify the effect of decreasing water potential on the N₂ (C₂ H₂ )ase activity of Nostoc. The N₂ (C₂ H₂ )ase activity of Nostoc on the intact soil cores displayed a linear response of approximately 10% decrease in N₂ (C₂ H₂ )ase activity per one bar decrease in soil water potential. The cyanobacteria contributed almost all of the biologically fixed N at the site until late June. From late June through to mid September, heterotrophic diazotrophs played the major role in the N₂ -fixation activity. These changes are attributed to fluctuations in Nostoc sp. colony cover, temperature, and soil water potentials. Extrapolation of the measured rates, and assuming an average of 10 hr per day of activity, Nostoc sp. is shown to have contributed 4.60 ± 1.17 kg N ha^-1 yr^-1 . Heterotrophic diazotrophs contributed an estimated 3.19 ± 1.18 kg N ha^-1 yr^-1 . The total biological N₂ -fixation for the site was calculated at 8.2 ± 2.55 kg N ha^-1 yr^-1 , from additional measurements which estimated total diazotrophic activity of the site. These rates of N₂ -fixation are among the highest reported for temperate grassland habitats.