Abstract:Alnus mandshurica and A. sibirica var. hirsuta are tastgrowing species with ability of N2-fixation widely distpihuted in northeast in China. Assay of N2-fixing activity and carbon cost by nodule of the two species was conducted in field and laboratory conditions. Techniques of acetylene reduction and infrared gas analyzer were used in the experiments.Results show: 1) Soil temperature was the major environmental factor affecting N2-fixing activity of alder nodule. 2. The optimum temperature in N2-fixation was 20℃ for A. mandshurica and 25℃ for A. sibirica var. hirsuta. 3. Ratio of value of N2 fixed to CO2 evolved was maximum at 20℃ for A. mandshurica and 25℃ for A. sibirica var. hirsuta. Efficiency of N2-fixation at temperature bellow 20℃ was higher for A. mandshurica than for A. sibirica var. hirsuta. But it was reverse at the temperature above 25℃.Measures for increasing soil temperature of site are useful in extending the potential ability of N2-fixation of alder.