Cry proteins from transgenic corn may be introduced into the soil through rhizosphere exudates (Saxena et al., 2002a) or from decomposing crop residues. Cry proteins could affect the rhizosphere or soil microbial communities directly or indirectly. Possible indirect interactions could include reductions in plant damage by insects which might alter C and N inputs to the rhizosphere and soil, top-down trophic interactions, or effects on expression of other plant genes.

Bt corn was reported to have no effects on numbers of culturable microorganisms (Saxena and Stotzky, 2001), while transient increases in culturable microorganisms were observed with Bt cotton (Donegan et al., 1995) and Bt potato (Donegan et al., 1996). Bt corn had no impact on microbial biomass, activity, or community structure (Devare et al., 2004). Certain methanogenic archaeabacteria were inhibited by Cry proteins (Yudina et al., 2004), and Bt-rice straw had transient effects on microbial numbers and activity in a flooded rice paddy soil (Wei-xiang et al., 2004).

We are investigating the impact of various Bt corn lines, expressing different Cry proteins, on rhizosphere and soil microbial communities. In this paper we summarize our results from growth chamber and field experiments.