In a glass house experiment, GM plants increased the soil concentration of Bt proteins but this change in soil conditions had no measureable effect on nematode or soil bacteria populations.
Evidence from:
BS Griffiths et al. Plant Biotechnology Journal (2007) Vol.5 p 60 – 68
“Varietal effects of eight paired lines of transgenic Bt maize and near-isogenic non- Bt maize on soil microbial and nematode community structure”
Further detail: