Effect of ammonium and nitrate on 15N 2-fixation of Azospirillum spp. and Pantoea agglomerans in association with wheat plants

Abstract

The dinitrogen fixing ability of two diazotrophic bacterial strains Pantoea agglomerons and Azospirillum spp. which are proved to express N 2-fixing activity in presence of additional inorganic nitrogen sources was tested in association with wheat plants in hydroponic experiments using 15N 2 incubation. The effect of 100 ppm nitrogen added as NH 4C1 or KNO 3 to wheat plants on dinitrogen fixing activity of native as well as inoculated bacteria was determined. Enrichment of 15N, that means fixed dinitrogen, was detected in plant growth media, in roots and shoots of wheat plants grown 26 days in 15N 2 enriched atmosphere. Highest 15N amounts were found in wheat shoots. As well as the form of nitrogen applied and the bacterial strain inoculated effected plant growth, nitrogen uptake and the amount of biologically fixed dinitrogen. Ammonia or nitrate supply to plants did not repress 15N 2 fixation. Distribution of 15N within the plant and media was mainly influenced by the inoculated bacterial strain. The detected dinitrogen fixing ability in presence of inorganic nitrogen of both bacterial strains in pure culture was confirmed even in association with wheat plants. That finding offers the possibility to select diazotrophic bacterial strains in pure culture which are able to fix dinitrogen in association with plants when additional inorganic nitrogen was fertilized.