The ecological impact of bt cotton on soil microbial communities in East African soils

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University of Dar es Salaam
There is a concern that transgenic Bt crops carry genes that could have undesirable effects on natural and agro-ecosystem functions. The aim of the experiment was to investigate the impact of Bt cotton (Bollgard II) event 15985 (expressing both Cry1Ac/Cry2Ab2 protein) on some microbial and biochemical indicators in a clay/red nitric soils of central Kenya. Bt cotton (06Z604D), its non transgenic near isoline (99M03) and its conventional counterpart (HART 89M) were grown in a confined field trial (CFT) site at Samuru-Thika. Rhizosphere soil samples were collected at the planting, maturity, flower and boll, and boll-opening stages, harvesting and post harvesting stages of cotton. The soil samples were analyzed for microbial biomass carbon (MBC), microbial basal activity (MBA), microbial genetic structure (RAPD) and decomposition and persistence of Cry1Ac/Cry2Ab2 in the soil. The averages between plots in six sampling events revealed that slightly 99M03 leading in MBC and 06Z604D leading in MBA over 9903M and HART 89M cultivars. Hence the Cry1Ac/Cry2Ab2 protein appeared not to influence the composition of the microbial community. The data presented here showed no consistent statistically significant differences in MBC and MBA between plots in rhizosphere soil of 06Z604D, 99M03 and HART 89M cotton lines in the confined field site, and no obvious trends in the yields (values) measured during growth duration. The 16S rDNA sequence pattern of the three microbial groups in three different soil samples showed slightly different profiles of bacterial genomic diversity. In the RAPD-PCR sufficient polymorphic bands were generated indicating no or transient genomic variability among the three bacterial groups within the three cotton lines. Furthermore, the RAPD result revealed that the individuals among the three microbial populations within the rhizosphere of the three cotton lines were genetically not distinct from one another. Numbers of soil microbial groups (Proteobacteria, actinomycetes, pseudomonas and fungi) involved in decomposition and nutrient recycling were measured at CFU and population levels. Cry1Ac/Cry2Ab2 proteins did not show immediate effects on bacterial, actinomycetes and fungal counts and populations possibly as a result of adsorption of the proteins on soil particles, which could have rendered the proteins inaccessible for microbial utilization. Microbial population and colony counts arranged in decreasing order were 06Z604D>99M03>HART89M, similar to the amounts of MBC and clay in the soils. Moreover, bacteria and fungi counts were higher at 110 DAS in 06Z604D than in 99M03 and HART89M plot soils. My observations suggest that insecticidal proteins (Cry2Ac) produced by Bollgard II Bt cotton may or may not persist in tropical soils as a result of adsorption on clay soils but that there were no observable effect on the studied culturable microbial groups.The data presented here showed no consistent statistically significant differences in the numbers of different groups of culturable bacteria, actinomycetes and fungi between rhizosphere soil of Bt, isoline and Hart 89M cotton lines in the same field, and no obvious trends in the numbers of the various group of culturable bacteria and fungi with the increasing growth duration. The findings of this study support the existence of neutral Bt-induced ecological shifts in the MBC and MBA of the nitric red cotton soils of central Kenya. Moreover, this study suggests that a single-year cultivation of transgenic Bt cotton may not affect the functional bacteria and fungi populations in rhizosphere soil. Keywords: Transgenic Bt cotton, Bacillus thuringiensis (Bt), bacteria, fungi, Cry1Ac/Cry2Ab2, Microbial populations, microbial biomass carbon, microbial activity.
Available in print form, East Africana Collection, Chagula Library, Class mark Dr. Wilbert (THS EAF QR82.B3S94)
Bacillus thuringiensis, Soil microbiology, Soils, Africa, East
Swilla, J.D. (2013) The ecological impact of bt cotton on soil microbial communities in East African soils, Doctoral dissertation, University of Dar es Salaam, Dar es Salaam