Kinetics of nitrogen fixation in a glucose-amended,anaerobically incubated soil

Nitrogen fixation and acetylene reduction activities were studied in a sandy loam soil amended with glucose (2% w/w) at field moisture content and incubated anaerobically. Optimum temperature for C₂H₂ reduction was about 37°C and the maximum was 45°C. Q₁₀ values were 1.6–3.7 in the range 10–35°C. Calculated activation energies were lower than those reported for Clostridium whole cells. Apparent K_m (C₂H₂) averaged 0.006 atm pC₂H₂ and the apparent K_m (N₂) was 0.095 atm pN₂. Low concentrations of C₂H₂ competed strongly with N₂ for the soil N₂ase (apparent K[ini] was 0.0003 atm pC₂H₂ with 0.8 atm pN₂). A relatively high concentration of ethylene (0.22 atm pC₂H₄) caused 30–40 per cent inhibition of N₂ase activity (measured as ¹⁵N₂ fixation) but the lower concentrations likely to be encountered in C₂H₂ assays had no significant effect. Conversion factors (C₂H₄/N₂ molar ratios) determined under various conditions ranged from 0.75 to 3.6. A value of 2.6 was obtained using the most favourable short-term C₂H₂ assays.