Conversion factor between acetylene reduction and nitrogen fixation in soil: Effect of water content and nitrogenase activity

The conversion factor between C₂H₂ reduction and N₂ fixation was studied in two soils. In one study small cores from (he two soils were drained to three water tensions: 0.20, 1.17 and 4.89 kPa. At each tension the N₂ase activity was measured with both O.1 aim ¹⁵N₂ and 0.1 aim C₂H₂. The conversion factor was different for the two soils. 1.0 and 3.1. respectively. The water content had no influence on the value of the conversion factor in this first study, in which the fixation corresponded to about 1mg N m^?2 day ^?1 at the depth 0–3 cm.In another study glucose was added to one of the soils to enhance the N₂ase activity. The activity was measured at 75 and 100% water saturation with both 0.9 atm ¹⁵N₂ and 0.1 atm C₂ H₂. At the lower water content the conversion factor was 2.6 and at water saturation the factor was 15.7. The fixation rates were high in this study. 98 mg N m^?2 day^?1 at the lower water content and 42 mg at water saturation.By theoretical calculations it was shown that the concentration of dissolved N₂ restricted the rate of fixation in the water-saturated samples of the second study, thus giving the high conversion factor. The critical level of N₂asc activity in water-saturated soil, above which the actual C₂H₂ to N₂-ratio will be higher than usual, was estimated to about 10mg Nm ²day_?1, under the experimental conditions used in these studies.