Effects of genetically modified maize (MON810) and its residues on the functional diversity of microorganisms in two South African soils

Genetically modified (GM) crops are commercially cultivated worldwide but there are concerns on their possible negative impacts on soil biodiversity. A glasshouse study was conducted to determine effects of Bt maize residues on soil microbial diversity. Residues of Bt maize (PAN 6Q-308B) and non-Bt maize (PAN 6Q-121) were incorporated into the soil and corresponding maize seeds planted. The treatments were replicated three times. Fertilizer and water application were similar for both treatments. Rhizosphere and bulk soil was destructively sampled from each treatment and analyzed for microbial community level physiological profiles using Biolog plates with 31 different carbon substrates. Absorbance in the Biolog plates was recorded after 72 h of incubation at 20oC. Arbuscular mycorrhizal fungi spore counts were also determined. Field studies were conducted at the University of Free State and University of Fort Hare Research Farms to determine the effects of growing Bt maize on soil microbial diversity. One Bt maize cultivar (PAN6Q-308B) and non-Bt maize (PAN6Q-121) were grown in a paired experiment at University of Free State farm, while two Bt maize (DKC61-25B and PAN6Q-321B) and their near-isogenic non-Bt maize lines (DKC61-24 and PAN6777) were grown in a randomized complete block design with three replicates. Fertilization, weed control and water application, were similar for both Bt maize cultivars and their non-Bt maize counterparts. Rhizosphere soil samples were collected by uprooting whole plants and collecting the soil attached to the roots. The samples were analysed for microbial diversity and for arbuscular mycorrhizae fungal spore counts. Principal component analysis showed that soil microbial diversity was affected more by sampling time whereas genetic modification had minimal effects. Presence of residues also increased the diversity of microorganisms. Mycorrhizal fungal spores were not affected by the presence of Bt maize residues. Growing Bt maize had no effect on the soil microbial diversity in the rhizosphere