Ratio of fixed and assimilated nitrogen in a black alder (Alnusglutinosa) stand

Abstract

The ratio of fixed and assimilated nitrogen was determined in a black alder (Alnusglutinosa (L.) Gaertn.) stand throughout a growing season by measuring symbiotic dinitrogen fixation, soil mineral nitrogen concentration, and soil nitrifying capacity. The symbiotic dinitrogen fixation evaluated by the method based on natural isotopic abundance, δ15N) showed that most nitrogen present in alder leaves was derived from fixation (94%). Nitrogenase activity, measured by acetylene reduction assay, began at bud break and continued over the growing season, but showed great variation. The upper soil layer (0–15 cm) beneath black alder contained ammonium and nitrate nitrogen on sampling dates. Nitrification, determined by biological nitrous oxide (N2O) production after a short insitu incubation, occurred throughout the season. Black alders satisfied most of their nitrogen requirements from an atmospheric origin.