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A field-plot experiment was undertaken to compare the effect of inorganic fertilizer with that of inoculation with an arbuscular mycorrhizal fungus, Glomus intraradices, or with a plant-growth-promoting rhizobacterium, Pseudomonas mendocina, alone or in combination with inorganic fertilizer, on plant growth and nutrient uptake by lettuce. The influence of the microbial inoculation treatments on soil physical, biochemical and biological properties was also assessed. Two months after planting, fertilizer and inoculation with G. intraradices or P. mendocina had significantly increased shoot and root biomass and foliar nutrient contents (P, Fe). The inoculation with G. intraradices or P. mendocina both increased the soil water-soluble carbohydrates and the percentage of stable aggregates. In this study, we provide the first evidence of the beneficial effect of a plant-growth-promoting rhizobacterium on soil aggregate stabilization under field conditions. Only inoculation with P. mendocina had a significant effect on the dehydrogenase and phosphatase activities, 21 and 89%, respectively, compared with the control. Inorganic fertilization alone did not increase aggregate stability or enzyme activities in soil, even though this treatment produced the largest increases in mass of lettuce.

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