Field evaluation of the effects of cotton variety and GM status on rhizosphere microbial diversity and function in Australian soils

Despite the high level of adoption of genetically modified (GM) cultivars in the Australian cotton production system, concerns remain over the use ofGMtechnology, particularly with regard to potential non-target effects. To address the hypothesis that GM cotton causes shifts in rhizosphere microbial diversity or function, we assessed rhizosphere soil samples from a range of conventional and GM cotton cultivars for diversity of bacteria and fungi, populations of ammonium oxidisers, rhizosphere basal and selective substrate-induced respiration, and non-symbiotic N2 fixation and nitrification. Comparison of results for GM and conventional cotton cultivars, both between and within seasons, indicated that the cotton rhizosphere plant–microbial interactions are variable in nature and significantly influenced by cultivar type. The GM status of the plant did not result in rhizosphere bacterial or fungal DNA-based grouping, but MicroResp data did show some grouping based on GM status, although this was not consistent by trait, suggesting that the GM trait is not greater than cultivar selection in causing rhizosphere change, especially when measured in the field environment with all the associative management practices.