Seven years of continuously planted Bt corn did not affect mineralizable and total soil C and total N in surface soil

Genetically engineered corn (Zea mays L.) containing a gene from the soil bacterium Bt (Bacillus thuringiensis) constitutes a large proportion of all corn planted in the United States. In a number of studies, Bt plant residues have been reported to have higher lignin content and to decompose slower than those of non-Bt plants, possibly due to the presence of the Cry endotoxin. We hypothesize that after multiple years of continuous cultivation of Bt corn, the combined results of alleged differences in Bt corn residues and of Cry endotoxin presence will be reflected in soil characteristics, specifically in an increase in soil total carbon (C) and nitrogen (N) levels. We collected soil samples at 0-7.5 cm depth in 2006 from continuous Bt and non-Bt corn treatments in an randomized complete block design experiment with four replications. The experiment was established in 1999 at the Kellogg Biological Station long-term ecological research (LTER) site in southwest Michigan. We found that, after 7 years of Bt cropping, neither total soil C and N nor soil C mineralized during a 35-day incubation were significantly different between Bt and non-Bt corn treatments (P > 0.05). Total soil C was equal to 7.3 g kg⁻¹ and 7.4 g kg⁻¹, in Bt and non-Bt corn, respectively, with a standard error of the means (SEM) = 0.2, and total N was 0.67 g kg⁻¹ (SEM 0.02) in both treatments. Post-hoc power analysis indicated that, given the number of samples collected in this study and the observed level of variability, the minimal differences between the Bt and non-Bt treatments that could be detected as statistically significant at α = 0.05 with a power of 0.80 were equal to 1.0 g kg⁻¹, 0.14 g kg⁻¹ and 0.125 g kg⁻¹ for total C, total N, and soil C mineralized during a 35-day incubation, respectively. The results indicate that continuous Bt corn production during a 7-year period did not lead to sizeable changes in total soil C and N.