Transgenic Bt crops are planted on over 60 million acres worldwide, yet their potential impact on soil ecology has not been fully elucidated. We are examining the effects of Bt crops on above- and below-ground carbon (C) allocation, residue decomposition kinetics, and root infection with arbuscular mycorrhizal fungi in New York, Colombia, and China.

Public concern over the use of transgenic crops and their potential effects on environmental health continues. At the same time, acreage planted with transgenic plants globally is continually increasing. Changes in the abundance, activity, or diversity of soil organisms may disrupt soil ecosystem functioning sufficiently to adversely affect soil structure, plant nutrient availability, and the natural control of pathogens or parasites by predatory organisms. Published laboratory incubation studies suggest that Bt crop residues may decompose more slowly than their non-transgenic counterparts. In our previous field work comparing MON863 Bt corn with its non-transgenic isoline, we detected no adverse effects on soil respiration, nitrogen (N) mineralization potential, short-term nitrification activity, soil microbial community composition, microbial biomass, or the abundance of protozoa or culturable bacteria and fungi (Devare et al., 2004; Devare et al., 2007). These results suggest that the decomposition of residues from transgenic Bt crops should keep pace with that of their non-transgenic counterparts, yet field data are lacking. In addition, potential effects on arbuscular mycorrhizal fungi, important mutualists of most crop plants, remain unknown. Lastly, persistence of the Bt protein in different soil types and from different transgenic events is still under debate. Clearly, more field-based information is needed to inform regulation and environmental monitoring of these crops.

We established field trials in 2004-2006 in Aurora, New York; Cali, Colombia; and Hangzhou, China on Bt corn, cotton, and rice, respectively. In 2003 we established a field trial in North Platte, Nebraska, on Bt corn. Carbon allocation to above- versus below-ground plant tissues, rates of residue decomposition, and levels of root infection by mycorrhizal fungi are being monitored. Three years of data indicate that residue mass loss is significantly lower in surface-placed residues than in buried residues. Mass loss is strongly associated with the plant parts tested, with above-ground tissues decomposing more rapidly than below-ground tissue and corn cobs being the slowest to decompose. There is no significant effect of hybrid type (Bt versus non-Bt) on residue decomposition in any of these trials. Microbial communities associated with decomposing residues were assessed with DNA fingerprinting analysis on samples taken after six months in the field in Nebraska and after 5, 15 and 25 months in the field in New York. Results indicate that the composition of residue-colonizing bacteria and fungi are strongly associated with residue location (surface versus buried), but not associated with hybrids (Bt versus non-Bt). Mycorrhizal fungi associated with corn and cotton have been assessed over the past 3 years. Spore counts from soil and root infection levels do not differ between Bt and non-Bt crops. Carbon allocation to shoots versus roots does differ between hybrids; however, we are still working to establish if these differences are more or less than those expected between other hybrids from similar maturity groups.

We have established that Bt crop residues from three different transgenic events, in three different crops (corn, cotton and rice), decompose at the same rate as their non-transgenic counterparts under field conditions. Since these crops are grown on millions of hectares of land, changes in decomposition dynamics could have broad-scale ecosystem effects. Yet, we have measured no changes in either mass loss or in the microbial communities colonizing these residues in the field that are related to crop genotype (Bt vs non-Bt). The persistence of Cry proteins in soil as residues decompose may present a risk to non-target organisms. We are currently examining the persistence of these proteins in NY soils. Information on the persistence and degradation kinetics of crop residues containing Cry proteins in field soil are essential parts of risk assessment for these crops and are needed to better inform regulatory agencies and the public.