Nitrogen fixation (C2H2) by free‐living heterotrophic microorganisms in the upper part of the forest soil (0–3 cm) was, on average, twenty times higher below birch than below pine or spruce. The highest fixation that occurred was 1.4 kg Nha−1during the growth period. In most cases the fixation of nitrogen was concentrated to older leaf litter. No activity could be detected in green leaves taken from the canopy or in recently fallen yellow leaves. Relatively low activities in the soil were obtained at sites in the southern part of Sweden, which corresponded to elevated concentrations of lead and copper in the green birch leaves.