Seasonal fluctuations in nitrogen fixation (acetylene reduction) by free-living bacteria in soils contaminated with cadmium, lead and zinc

Acetylene reduction by non-symbiotic, heterotrophic micro-organisms in a range of soils containing different concentrations of heavy metals was determined using intact soil cores. The suitability of this method for the soils used in this investigation was established. Samples were collected seasonally, and were incubated under standard conditions (darkness: 15°). Mean values of metal concentrations in the soil (μg g^?1) were: Cd: 1–200; Pb: 60–8000; Zn: 70–26000, Cu: 20–40. Rates of acetylene reduction were generally low, from 2800 to 50000 nmol C₂H₄, m^?2 day^?1. Assuming a 3:1 ratio of C₂H₂ reduction to N₂ fixation, this represents a rate of 0.3 to 5.0 g N fixed ha^?1 day^?1 in the surface 150 mm of soil. No consistent effect of heavy metal concentration was found. The most important factors determining activity were soil moisture content and possibly inorganic nitrogen concentration. It thus appears that the bacteria in polluted soils are capable of adapting to potentially toxic concentrations of heavy metals, or that these metals are present in the soils tested in unavailable or non-toxic forms.