Estimation of symbiotic dinitrogen fixation in alder forest by the method based on natural 15N abundance

Annual N2-fixation in virgin forest ecosystems has been measured using a 15N natural abundance (δ15N)procedure. This method was compared to a 15N labelled fertilizer isotopic dilution method. For young alders (5-6 years old), δ15N of leaves gave results in good agreement with the isotopic dilution of fertilizer method. Since δ15N variability was expected according to plant physiology, for alder trees, leaves were collected at various heights after the end of the growing season, and, to take account of isotopic variations coming from derived inputs, δ15N of leaves of a large number of other plants in the same area were measured to give control values. Following this procedure, the δ15N method gave reliable evaluation of the nitrogen supply, by through N2-fixation, to alders, which were found to maintain high nitrogen fixing capacity in a sequence ranging from first stage of establishment of climactic formation. Moreover, the same method is reported to discriminate various origins of Alnus glutinosa grown in natural conditions, possibly in relation to the genetic diversity of this species. (author). 22 refs., 3 figs., 2 tabs