G . A ND R AD E, F .A .A . M. DE L EI J A N D J .M . LY NC H . 1998. Using undisturbed sandy loam soil cores heavily infested with mycorrhizae, the effects of the antibiotic-producing Pseudomonas fluorescens strain F113 and its non-antibiotic derivative Ps. fluorescens F113G22 on nodulation by introduced and indigenous Rhizobium were studied. Furthermore, the effects of the different microbial inocula on the colonization of the pea roots by mycorrhizae were studied. It was found that Ps. fluorescens F113 enhanced nodulation by Rhizobium fourfold, while the nodules produced were much larger and strongly pigmented (pink) compared with those in other treatments. The proportion of roots colonized by arbuscular mycorrhizae was not significantly affected by the different treatments.

The effects of an antibiotic‐producing Pseudomonas fluorescens strain (F113) carrying the marker gene cassette lacZY and a marked, non‐producing strain (F113G22) on the uptake of nitrogen from 15N enriched organic residues incorporated into a sandy soil were investigated in microcosm studies. Strain F113 produces the antibiotic 2,4‐diacetylphloroglucinol (DAPG), while its modified derivative strain F113G22 has DAPG production deleted by Tn5 mutagenesis. Uptake of nitrogen by pea (Pisum sativum) was estimated using isotope‐ratio mass spectrometry. In addition, plant growth and microbial activity in soil were monitored. Both strains F113 and F113G22 enhanced the uptake of nitrogen from mineralized organic residues, even though the antibiotic producing strain F113 significantly reduced microbial activity in soil. It is suggested that the effect on nitrogen uptake was due to increased mineralization of organic residues by the introduced organisms, making greater quantities of inorganic nitrogen available for plant uptake. Unlike studies assessing impact in terms of perturbation to indigenous microbial communities, this study provides direct evidence of a change in ecosystem function as a result of the introduction of strains of a genetically marked bacterium, irrespective of whether its natural antibiotic‐producing capacity has been genetically deleted.