Nitrogen fixation in Sitka alder by 15N isotope dilution after eight growing seasons in a lodgepole pine site

Abstract

Owing to high costs and limitations of the method, there have been no field studies using 15N isotope dilution to measure the amount of N fixed by alder in the field. As part of a long-term (8-year) study of the fate of 15N-labelled fertilizer applied to lodgepole pine (Pinuscontorta Dougl. var. latifolia Engelm.) in the field in interior British Columbia, we observed much lower 15N enrichment in Sitka alder (Alnussinuata (Reg.) Rydb.) than in lodgepole pine or other understory species. Calculations using lodgepole pine, herbs, and shrubs as nonfixing reference crops all indicated that the percent of N derived from the atmosphere in Sitka alder at this site was very high, 94–99%. The long period for stabilization of residual 15N-labelled fertilizer in organic form in the soil and for establishment of plant root systems alleviated many of the problems associated with the 15N isotope dilution method. By contrast, calculations of the percent of N derived from the atmosphere at the end of the first growing season gave erratic and unreliable results.