Winter decomposition of transgenic cotton residue in conventional-till and no-till systems

Abstract

Current research suggests that genetic modification of commercial crops may lead to indirect effects on ecosystem function (i.e. decomposition and nutrient cycling processes). We investigated residue decomposition of cotton that was genetically modified to express an endotoxin insecticide isolated from Bacillus thuringiensis ( Bt) and/or glyphosate tolerance (Roundup Ready ®). Decomposition of the genetically modified residue was compared within agricultural systems under conventional-tillage (CT) or no-tillage (NT) management. We tested for variation in decomposition dynamics under the two tillage regimes because there are intrinsic differences in environmental and biotic conditions between them, and that both management methods are employed in cotton production. We hypothesized that decomposition dynamics would be affected by the presence or absence of the Bt endotoxin and that the degree of variation would be more distinct between tillage regimes. Decomposition dynamics were determined by change in mass remaining and nutrient content (C and N) of cotton litter material contained in mesh litterbags collected over a 20-week period from December to May. Rate of decomposition and change in nutrient content of decomposing litter within either tillage regime was not significantly different between the two cotton types examined. Percent mass remaining, total N and total C decreased over time and were significantly different between tillage regimes only. Over the 20-week experiment, mass loss with subsurface decomposition in the CT reached 55% but surface decomposition in the NT reached only 25%. We observed that cotton genetically modified to express Bt endotoxin and glyphosate tolerance decomposed similarly to cotton modified for glyphosate tolerance only.