Nitrogen fixation by free-living microorganisms in the soil of a mature oak-stand in Uppland, Sweden

The nitrogenase activity of free-living microorganisms was investigated in a brown forest soil beneath a mature oak-Stand outside Uppsala, Central Sweden. The effect of season, air temperature, soil depth and some soil parameters on nitrogenase activity was studied.
Only heterotrophic nitrogenase activity was detected. The main activity occurred in the upper 5 cm of the soil profile. The amount of nitrogen fixed during the growth season was calculated to 0.73 kg ha⁻¹. There was a strong seasonal variation of the nitrogenase activity with the highest level in July, August and September. The activity, when the soil was incubated at 25°C, was significantly correlated with the mean air temperature at the field site.