DIFFERENCES IN YIELDS, RESIDUE COMPOSITION, AND N MINERALIZATION DYNAMICS OF BT AND NON-BT MAIZE

Authors: MUNGAI, NANCY - UNIV OF MO; MOTAVALLI, PETER - UNIV OF MO; NELSON, KELLY - UNIV OF MO; Kremer, Robert

Submitted to: Nutrient Cycling in Agroecosystems
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 3/1/2006
Publication Date: 4/14/2006
Citation: Mungai, N.W., Motavalli, P.P., Nelson, K.A., Kremer, R.J. 2006. Differences in yields, residue composition, and n mineralization dynamics of bt and non-bt maize. Nutrient Cycling in Agroecosystems. 73(1):101-109.

Interpretive Summary: Corn genetically modified (GM) for resistance to insect pests (Bt) is planted on about 40% of the corn production area of the United States. Although GM plants including Bt corn undergo rigorous assessment for effects on health of humans and other organisms, the impact of these plants on the soil environment and many soil biological processes has received little attention. Effects on important soil biological processes such as nitrogen mineralization (the release of nitrogen bound in organic materials as a form readily used by plants) have been largely neglected in environmental assessments of Bt corn. Previous research by other scientists suggests that chemical composition of Bt corn stalks, leaves, and roots differ from conventional corn and may interfere with biological mineralization of nitrogen when these plant parts remain on the soil surface or are incorporated into the soil. Our objectives were to determine differences in chemical composition of the vegetative parts (residues) of five Bt corn varieties; evaluate the effect of the Bt corn residues on nitrogen mineralization in different soils; and determine effects of the residues on inorganic soil nitrogen under different tillage practices. We found little consistent differences in corn residue composition between the Bt corn varieties and their respective non-Bt corn isolines (identical varieties but not genetically modified). Largely because Bt and non-Bt corn plants were chemically similar, no differences were observed in the amount of nitrogen mineralized when either corn residue type was mixed in soil. The greatest effect on nitrogen mineralization rate was due to differences in soil texture, regardless of corn residue mixed in the soil. Nitrogen mineralization was consistently highest in the silt loam soil. Also, levels of plant-available nitrogen applied as inorganic fertilizer to soil were not affected by the type of corn residue mixed in the soil or by tillage practice. The overall results indicate that Bt corn does not affect available nitrogen in soil either through disrupting nitrogen mineralization or altering the levels of fertilizer nitrogen applied to soils. Because we only investigated five Bt corn varieties, follow-up research is required to verify that these results are representative of the hundreds of Bt varieties currently available for production in different soils and environments. Also, our results suggest only that the biological process of nitrogen mineralization was not affected; we did not study effects of other specific biological processes mediated by soil microorganisms, which should be pursued in the future. Nevertheless, the current information has important implications for scientists, extension personnel, producers, and environmental stakeholders because it demonstrates that growth of Bt corn has little effect on nitrogen mineralization and subsequent availability for corn growth in soils limited to our study; therefore, cropping systems that include Bt varieties will likely be as productive with similar impacts on the soil environment.

Technical Abstract: Cultivation of genetically modified crops may have several direct and indirect effects on soil ecosystem processes, such as soil nitrogen (N) transformations. Field studies were initiated in Northeast Missouri in 2002 and 2003 to examine the effects of application of crop residues from five Bt maize hybrids and their respective non-Bt isolines on soil inorganic N under tilled and no-till conditions in a maize-soybean rotation. A separate aerobic incubation study examined soil N mineralization from residue components (leaves, stems, roots) of one Bt maize hybrid and its non-Bt isoline in soils of varying texture. Three Bt maize hybrids produced 13 to 23 % greater grain yields than the non-Bt isolines. Generally no differences in leaf and stem tissue composition and biomass were observed between Bt and non-Bt maize varieties. Additionally, no differences were observed in cumulative N mineralization from Bt and non-Bt maize residues, except that N mineralization from non-Bt maize roots was 2.7 times higher than Bt maize roots in silt loam soil. Incorporation of Bt residues in the field did not significantly affect soil inorganic N under tilled or no-till conditions. Overall Bt maize residues did not differ in their effect on N dynamics in laboratory and field studies.