Effect of Bacillus thuringiensis (Bt) maize cultivation history on arbuscular mycorrhizal fungal colonization, spore abundance and diversity, and plant growth

• The effect of Bt maize on arbuscular mycorrhizal fungi (AMF) is poorly understood.
• We examined the impact of Bt maize cultivation history on AMF and plant growth.
• One year of Bt maize cultivation reduced AMF spore density in soil.
• A positive feedback growth effect was detected based on cultivation history.
• Bt maize cultivation may have nontarget effects on symbiotic soil fungi.

Greenhouse studies have reported that maize expressing Bacillus thuringiensis (Bt) insecticidal toxins may have nontarget effects on symbiotic arbuscular mycorrhizal fungi (AMF), however, field studies have not detected the same pattern. This may be due to the short-term nature of previous field experiments, differences in soil properties between studies, or plant-soil feedbacks that influence AMF communities in roots and soil over time. In this field experiment, we used split plots to evaluate the effect of Bt or non-Bt maize cultivation history on AMF spore abundance, diversity, root colonization, and growth of seven different genotypes of Bt maize and five corresponding non-Bt parental (P) isolines. We found that Bt plants had higher leaf chlorophyll content when they were grown in plots that had been cultivated with Bt maize the previous year, and similarly, non-Bt plants had higher chlorophyll content when they were grown in plots with a non-Bt cultivation history, indicative of a positive feedback effect. There was a lower density of AMF spores in plots with a Bt maize cultivation history than in plots where P maize had been grown in the previous year, but no difference in spore diversity. Despite the differences in spore density, we found no significant differences in AMF colonization or root or shoot biomass between plots with a cultivation history of Bt and P maize. This study presents the first evidence of an effect of Bt maize cultivation on the soil ecosystem, but also provides further evidence that this effect is not necessarily large or easily detectable within the range of normal environmental variation. Management of agroecosystems will need to consider the potential effects of reduced numbers of AMF propagules in soil as this could have an effect on ecosystem processes including carbon sequestration, nutrient cycling, drought tolerance, soil aggregation, and plant resistance to pathogens. Taken together with greenhouse experiments, we can now make predictions on how Bt maize cultivation may affect AMF under different environmental conditions.