A field study with genetically engineered alfalfa inoculated with recombinant Sinorhizobium meliloti: effects on the soil ecosystem.
Three genotypes of alfalfa [lucerne] (Medicago sativa) plants (parental, transgenic α-amylase-producing and transgenic lignin peroxidase-producing) were planted in an agricultural field plot at Oregon State University. Immediately prior to planting, the roots of the alfalfa plants were left uninoculated or were inoculated with a wild-type strain (PC), a recombinant strain with antibiotic resistances (RMB7201), or a recombinant strain with antibiotic resistances and enhanced nitrogen-fixation capability (RMBPC-2), of Sinorhizobium meliloti. Analyses of the alfalfa plants and field plot soil were made over two growing seasons and included: metabolic fingerprints and DNA fingerprints of soil bacterial communities; soil microbial respiration; population counts of indigenous soil bacteria, fungi, nematodes, protozoa and micro-arthropods; identification of nematodes and micro-arthropods; plant shoot weight and composition; and soil chemistry and enzyme activities. The lignin peroxidase transgenic plants had significantly lower shoot weight, and higher nitrogen and phosphorus content, than the parental or transgenic amylase plants. Distinct metabolic fingerprints, based on patterns of substrate utilization in Biolog plates, were exhibited by the soil bacterial communities associated with the three alfalfa genotypes, and those for the lignin peroxidase plants were the most unique. Significantly higher population levels of culturable, aerobic sporeforming and cellulose-utilizing bacteria, lower activity of the soil enzymes dehydrogenase and alkaline phosphatase, and higher soil pH levels, were also associated with the lignin peroxidase transgenic plants. Significantly higher population levels of culturable, aerobic spore-forming bacteria were also measured in the treatments containing the recombinant RMBPC-2 S. meliloti. Population levels of protozoa, nematodes and micro-arthropods. DNA fingerprints of indigenous soil bacteria, and rates of microbial substrate-induced respiration were not significantly affected by the transgenic alfalfa and recombinant S. meliloti treatments. Results suggest that the genetically engineered organisms caused detectable changes in some components of the soil ecosystem.