Abstract
In arid and semiarid Mediterranean regions, an increase in the severity of drought events could be caused by rising atmospheric CO 2 concentrations. We studied the effects of the interaction of CO 2, water supply and inoculation with a plant-growth-promoting rhizobacterium (PGPR), Pseudomonas mendocina Palleroni, or inoculation with an arbuscular mycorrhizal (AM) fungus, Glomus intraradices (Schenk & Smith), on aggregate stabilisation of the rhizosphere soil of Lactuca sativa L. cv. Tafalla. The influence of such structural improvements on the growth of lettuce was evaluated. We hypothesised that elevated atmospheric CO 2 concentration would increase the beneficial effects of inoculation with a PGPR or an AM fungus on the aggregate stability of the rhizosphere soil of lettuce plants. Leaf hydration, shoot dry biomass and mycorrhizal colonisation were decreased significantly under water-stress conditions, but this decrease was more pronounced under ambient vs elevated CO 2. The root biomass decreased under elevated CO 2 but only in non-stressed plants. Under elevated CO 2, the microbial biomass C of the rhizosphere of the G. intraradices-colonised plants increased with water stress. Bacterial and mycorrhizal inoculation and CO 2 had no significant effect on the easily-extractable glomalin concentration. Plants grown under elevated CO 2 had a significantly higher percentage of stable aggregates under drought stress than under well-watered conditions, particularly the plants inoculated with either of the assayed microbial inocula (about 20% higher than the control soil). We conclude that the contribution of mycorrhizal fungi and PGPR to soil aggregate stability under elevated atmospheric CO 2 is largely enhanced by soil drying.
Published Version
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