Variation of microbial rhizosphere communities in response to crop species, soil origin, and inoculation with Sinorhizobium meliloti L33

A greenhouse study with soil-plant microcosms was conducted in order to com pare the effect of crop species, soil origin, and a bacterial inoculant on the establishment of microbial communities colonizing plant roots. Two crop species, alfalfa (Medicago sativa) and rye (Secale cereale), were grown se parately in two soils collected from agricultural fields at different locat ions and with differing histories of leguminous crop rotation. A subset of microcosms was inoculated at 10(6) cfu g(-1) soil with the luciferase marke r gene-lagged Sinorhizobium meliloti strain L33, a symbiotic partner of M. sativa. Microbial consortia were collected from the rhizospheres of alfalfa after 10 weeks of incubation and from rye after 11 weeks. S. meliloti L33 populations were one to two orders of magnitude higher in the rhizospheres of alfalfa than of rye. In soil with previous alfalfa cultivation, 80% of t he alfalfa nodules were colonized by indigenous bacteria, while in the othe r soil alfalfa was colonized almost exclusively (>90%) with S. meliloti L33 . Three community-level targeting approaches were used to characterize the variation of the extracted microbial rhizosphere consortia: (1) Community l evel physiological profiles (CLPP), (2) fatty acid methyl ester analysis (F AME), and (3) diversity of PCR amplified 16S rRNA target sequences from dir ectly extracted ribosomes, determined by temperature gradient gel electroph oresis (TGGE). All approaches identified the crop species as the major dete rminant of microbial community characteristics. Consistently, the influence of soil was of minor importance, while a modification of the alfalfa-assoc iated microbial community structure after inoculation with S. meliloti L33 was only consistently observed by using TGGE.