Field assays of N2(C2H2)ase activity were performed with intact nodules from a pure alder site (alder) and a mixed alder‐aspen site (aspen). Assays were performed between 12 June and 12 August 1980 and in May 1981. N2(C2H2)ase rates are expressed as g N g nodule oven‐dry wt−1hr−1(g N g−1hr−1). Diurnal N2(C2H2)ase activity showed an increase in both sites between 0600 and midday, then decreased to a low by 1800. Nighttime activity in the May 1981 assay was approximately 25% of the daytime peak. Mean (±SE) 1200 hr N2(C2H2)ase activity (μg N g−1hr−1) for all sizes in the alder stand rose from 24.56 ± 6.56 on 12 June to 73.96 ± 28.37 on 26 June and declined to 9.20 ± 2.56 by 12 August. In the aspen stand activity decreased from the 12 June rate of 21.81 ± 4.59 to 3.64 ± 1.87 on 24 July but then increased to 30.00 ± 7.39 by 12 August. Based on diurnal assays, the seasonal mean N influx (μg N g−1hr−1) is statistically higher (P0.05) in the alder stand with a value of 26.70 compared to 14.63 in the aspen stand. Small size class shrubs had significantly higher (P<0.05) N2(C2H2)ase activity (μg N g−1hr−1) in diurnal assays than medium or large class shrubs. The estimated mean (±SE) N2(C2H2)ase activity (mg N g−1season−1) for all sizes was 44.4 ± 18.6 in the alder stand compared to 16.2 ± 5.2 in the aspen stand. Nodule excavations showed the g shrub−1in the alder stand to be 16.48 ± 10.29, 38.57 ± 12.34 and 29.11 ± 7.15 for small, medium and large size shrubs and 12.73 ± 3.23, 28.21 ± 4.36 and 56.45 ± 16.23 for respective sizes in the aspen stand. Seasonal N influx was 4.69 kg ha−1in the alder stand and 0.84 kg ha−1in the aspen stand, representing 17.9% of the alder stand. Nitrogen feedback inhibition from uric acid‐N influx and allelochemic interference from aspen are discussed as explanations for the differences in N influx in the two stands.