Effect of introduced Pseudomonas fluorescens strains on the uptake of nitrogen by wheat from N-15-enriched organic residues

The effects of an antibiotic-producing Pseudomonas fluorescens strain (F113 ) carrying the marker gene cassette lacZY and a marked, non-producing strai n (F113G22) on the uptake of nitrogen from N-15-enriched organic residues i ncorporated into a sandy soil were investigated in microcosm studies. Strai n F113 produces the antibiotic 2,4-diacetylphloroglucinol (DAPG), whilst it s modified derivative strain F113G22 has DAPG production deleted by Tn5 mut agenesis. Uptake of nitrogen by wheat (Triticum aestivum) from N-15-enriche d organic residues was estimated using stable isotope-ratio mass spectromet ry of shoot and root material of 17-day-old plants. In addition, plant grow th and active microbial biomass in soil were monitored. In contrast to resu lts obtained in our previous study on pea (Pisum sativum), it was found tha t in wheat, inoculation with either strain F113 or F113G22 decreased the pr oportion of nitrogen derived from N-15-labelled organic residues incorporat ed into soil as compared to non-inoculated controls. It is therefore sugges ted that these strains decreased mineralization of organic residues in the rhizosphere of wheat, making less inorganic N (N-15) available for plant up take. The results of this study indicate that the effects of introduced Pse udomonas fluorescens strains on nitrogen mineralization in the rhizosphere are plant-species dependent, and highlight the importance of testing microb ial inocula on a range of plant species.