Effects of tillage and nitrogen rate on decomposition of transgenic Bt and near-isogenic non-Bt maize residue

A great deal of research has been conducted to evaluate transgenic Bt maize (Zea mays L) effect on soil organisms and residue decomposition, but the effects of management practices, such as tillage and N applications have not been fully studied. The objective of this study was to examine the decomposition rate of genetically modified Bt maize residue compared to near isogenic non-Bt maize residue under different tillage and N rates in field and laboratory studies. The study was established at the Iowa State University Field Extension Education Laboratory Research Farm near Ames, Iowa in November of 2004 through 2007. The soil type of the site was Nicollet loam (fine-loamy, mixed, mesic, Aquic Hapludolls). The field decomposition study design was a replicated complete factorial with three tillage treatments: no-tillage (NT), strip-tillage (ST) and deep tillage (DT); and two maize hybrids of a transgenic, Bt variety and another of non-BT variety; at five times intervals. The laboratory study was conducted concurrently to examine the effect of different N rates and simulated tillage treatments on Bt and non-Bt residue decomposition. The hypothesis of this study is that alterations made to the physiological traits in Bt maize residue would result in changes in residue decomposition rate and those changes would vary by N and tillage management. The findings of field decomposition study show that there were no significant differences between Bt and non-Bt decomposition rate under NT. However, in one year out of two years, buried Bt maize residue showed slower decomposition rate than non-Bt maize residue. In the incubation study, Bt maize residue that was mixed with soil had significantly less residue decomposition (11% of added residue C) than Bt that was un-mixed (13% of added residue C). Non-Bt maize residue mixed or unmixed had on average 13% of residue decomposed after 80 days. Additions of N significantly lowered decomposition rates, but did not result in significant differences between Bt and non-Bt maize residue decomposition. The relevance and application of these findings are critical in providing practical information that Bt corn residue did not significantly differ in majority of time from non-Bt residue with different tillage systems in field conditions. Also, the addition of N for the purpose of increasing residue decomposition is not warranted in this study. These findings can have value in promoting conservation practices and keeping residue on soil surface rather than incorporating it with tillage. Published by Elsevier B.V.