Occurrence of nitrogen-fixing activity and aerobic heterotrophic N-2-fixing populations in Portuguese soils

Nitrogen-fixing activity in three different soil types was measured by the acetylene reduction method. N-2-fixation ranged from 0.38 to 6.89 nmoles C2 H4 . h(-1) . g(-1) soil dry weight. Activity of nitrogen-fixing microorgani sms was positively correlated with the moisture content of the soil samples . Populations of aerobic, heterotrophic, N-2-fixing microorganisms were est imated by dilution plate counts on three commonly used semisolid media. The different carbon sources in the media markedly influenced the counts. In g eneral a multiple carbon source medium gave the highest counts and was chos en for future work.