Nitrogen-fixation rates were determined by C₂H₂ reduction in intact cores taken from the top 0-1 cm of two soils. The cores were moistened and incubated under varying light (0-88 W•m⁻²) and temperature (7.5- 39.5ºC) conditions at four different times of the year to determine the effects of day temperature, night temperature and light intensity on N₂[C₂H₂]-fixation rates. Multiple regression equations were derived for each season to relate nM C₂H₄ produced with light and temperature.

A greater portion of the data was explained by each regression equation when nM C₂H₄•cm⁻² surface area was used as the dependent variable than when nM C₂H₄•g⁻¹ was used. The influence of light was pronounced with increasing light intensities producing increasing N₂[C₂H₂]- fixation rates. Therefore, it was concluded that the fixation was a surface area phenomenon with blue-green algae being responsible for most of the N₂[C₂H₂]-fixation which occurred.

The responses of the two soils to light and day temperature varied widely among the four sampling dates indicating that it was not possible to predict amounts of nitrogen fixed annually based on responses at one time of the year.

N₂[C₂H₂]-fixation rates did not appear to be related to night temperature or to the abundance of blue-green algae as extimated by the dilution plate method.