Evaluation of intact soil‐core microcosms for determining potential impacts on nutrient dynamics by genetically engineered microorganisms

Nutrient export from intact soil-core microcosms in leachate or by plant uptake was evaluated as a means to assess the ecosystem impacts from the environmental release of genetically modified root-colonizing bacteria. Intact cores of two soil types, a Burbank sandy loam and a Palouse silt loam, were excavated for use as microcosms, seeded with corn and wheat, respectively, and inoculated with Azospirillum lipoferum transposon Tn5 mutants. Microcosms were leached at 33 d and 67 d after seeding and the leachate analyzed for sulfate, phosphate, ammonium, nitrite, nitrate, dissolved organic carbon and inorganic carbon. Statistically significant differences in leachate nutrient concentrations between treatments were due to differences in soil types, rather than inoculation with the bacteria. The high variation in the concentrations of nutrients in the leachates from replicate microcosms, attributed to real field variation, suggest that the measurement of nutrient export in leachate from microcosms may not be a sensitive indicator of ecosystem impacts. One exception was dissolved organic carbon (and total carbon). A statistically significant difference (p ≥ 0.05) was observed between the microcosms receiving live inoculum and those receiving heat-killed cells for leachate organic carbon concentration. Plant uptake of N, P, S, B, Cu, Fe, Mg, Mn and Zn was not affected by inoculation but the coefficients of variation were considerably lower than for the leachate analyses. It is suggested that, although the leaching of organic carbon from microcosms may be used as a holistic indicator of ecosystem impacts from the release of genetically engineered microorganisms (GEMs), measurements of plant assimilation of nutrients and specific rates of microbial transformation of nutrients may provide more sensitive effects end points.