Decomposition of genetically engineered tobacco under field conditions: persistence of the proteinase inhibitor I product and effects on soil microbial respiration and protozoa, nematode and microarthropod populations.
To evaluate the potential effects of genetically engineered (transgenic) plants on soil ecosystems, litter bags containing leaves of non-engineered (parental) and transgenic tobacco plants were buried in field plots. The transgenic tobacco plants were genetically engineered to constitutively produce proteinase inhibitor I, a protein with insecticidal activity. The litter bag contents and surrounding soil, as well as soil from control plots without litter bags, were sampled over a 5-month period at 2- to 4-week intervals and assayed for proteinase inhibitor concentration, litter decomposition rates, carbon and nitrogen content, microbial respiration rates and population levels of nematodes, protozoa and microarthropods. The proteinase inhibitor concentration in the transgenic plant litter after 57 days was 0.05% of the sample day 0-value and was not detectable on subsequent sample days. Although the carbon content of the transgenic plant litter was comparable to that of the parental plant litter on sample day 0, it became significantly lower over the course of the experiment. Nematode populations in the soil surrounding the transgenic plant litter bags were greater than those in the soil surrounding parental plant litter bags and had a different trophic group composition, including a significantly higher ratio of fungal feeding nematodes to bacterial feeding nematodes on sample day 57. In contrast, Collembola populations in the soil surrounding the transgenic plant litter bags were significantly lower than in the soil surrounding parental plant litter bags. The results demonstrated that under field conditions the proteinase inhibitor remained immunologically active in buried transgenic plant litter for at least 57 days and that decomposing parental and transgenic plant litter differed in quality (carbon content) and in the response of exposed soil organisms (Collembola and nematodes).