Nitrogen availability to Pseudomonas fluorescens DF57 is limited during decomposition of barley straw in bulk soil and in the barley rhizosphere

The availability of nitrogen to Pseudomonas fluorescens DF57 during straw d egradation in bulk soil and in barley rhizosphere was studied by introducin g a bioluminescent reporter strain (DF57-N3), responding to nitrogen limita tion, to model systems of varying complexity. DF57-N3 was apparently not ni trogen limited in the natural and sterilized bulk soil used for these exper iments. The soil was subsequently amended with barley straw, representing a plant residue with a high carbon-to-nitrogen ratio (between 60 and 100). I n these systems the DF57-N3 population gradually developed a nitrogen limit ation response during the first week of straw decomposition, but exclusivel y in the presence of the indigenous microbial population. This probably ref lects the restricted ability of DF57 to degrade plant polymers by hydrolyti c enzymes. The impact of the indigenous population on nitrogen availability to DF57-N3 was mimicked by the cellulolytic organism Trichoderma harzianum Rifai strain T3 when coinoculated with DF57-N3 in sterilized, straw-amende d soil, Limitation occurred concomitantly with fungal cellulase production, pointing to the significance of hydrolytic activity for the mobilization o f straw carbon sources, thereby increasing the nitrogen demand. Enhanced su rvival of DF57-N3 in natural soil after straw amendment further indicated t hat DF57 was cross-fed with carbon/energy sources. The natural barley rhizo sphere was experienced by DF57-N3 as an environment with restricted nitroge n availability regardless of straw amendment. In the rhizosphere of plants grown in sterilized soil, nitrogen limitation was less severe, pointing to competition with indigenous microorganisms as an important determinant of t he nitrogen status for DF57-N3 in this environment. Hence, these studies ha ve demonstrated that nitrogen availability and gene expression in Pseudomon as is intimately linked to the structure and function of the microbial comm unity. Further, it was demonstrated that the activities of cellulolytic mic roorganisms may affect the availability of energy and specific nutrients to a group of organisms deficient in hydrolytic enzyme activities.