ASSESSING CHANGES IN SOIL MICROBIAL COMMUNITIES AND CARBON MINERALIZATION IN BT AND NON-BT CORN RESIDUE-AMENDED SOILS

Authors: FANG, MIN - UNIVERSITY OF MISSOURI; MOTAVALLI, PETER - UNIVERSITY OF MISSOURI; Kremer, Robert; NELSON, KELLY - UNIVERSITY OF MISSOURI

Submitted to: Applied Soil Ecology
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 9/8/2007
Publication Date: 10/1/2007
Citation: Fang, M., Motavalli, P.P., Kremer, R.J., Nelson, K.A. 2007. Assessing changes in soil microbial communities and carbon mineralization in Bt and non-Bt corn residue-amended soils. Applied Soil Ecology. 37:150-160.

Interpretive Summary: Corn genetically modified (GM) for resistance to insect pests (Bt) is planted on about 50% of the corn production area of the United States. The impact of Bt corn on the soil environment and many soil biological processes has received little attention. Effects on important soil biological processes such as decomposition (the breakdown of organic materials for carbon [C] storage in soil and release of carbon dioxide [CO2]) 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 alter soil microorganisms responsible for decomposition, interfering with this process when the 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 Bt and non-Bt corn varieties; evaluate the effect of corn residues on decomposition in different soils; and determine effects of the residues on the structure of the microbial populations in soil. Although Bt and non-Bt corn plants differed in lignin (organic compound that gives plant cells its rigid structure), no differences were observed in the decomposition based on the amount of C released when either corn residue types were mixed in soil. The greatest effect on decomposition rate was due to differences in soil texture, regardless of corn residue mixed in the soil. Decomposition was consistently highest in the silt loam soil. Also, soil microorganism composition in field-collected soils differed due to corn residue type, suggesting an effect on biological processes mediated by the microorganisms could occur during growth of the Bt corn. Because we only investigated two contrasting 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 decomposition 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 affects microbial populations and possibly some associated biological processes in the soils limited to our study; therefore, cropping systems that include Bt varieties need more examination relative to impacts on soil biological processes to better assess long-term effects on the soil resource.

Technical Abstract: The effects of Bt corn (Zea mays L.) residue on soil microbial communities and rates of C mineralization were investigated. The Bt corn residue had a higher lignin content (12%) and lignin/N (9.9) ratio compared with its non-Bt near-isoline (10% lignin; lignin/N = 8.6). We examined the relationships among the Bt/non-Bt residue properties, residue component, soil texture, sampling time, and tillage management in microcosm and field studies. Bt corn residue incorporated in soils of different textural classes (silty clay, silt loam and sandy loam) in microcosms affected bacterial substrate metabolism. Substrate utilization profiles (Biolog) of soils amended with Bt residue differed from those with non-Bt residue based on principal component analysis (PCA). Denaturing gradient gel electrophoresis (DGGE) patterns revealed only slightly altered microbial communities in the soils amended with Bt residue compared with the non-Bt isoline. Soil texture significantly (P<0.05) affected C mineralization and substrate utilization profiles. Carbon dioxide evolution rate constants (k) of 0.085 to 0.087 for non-Bt and Bt corn leaf tissue added to silt loam indicated higher rates of soil CO2 evolution compared with addition of roots and stems (k=0.06 to 0.07). However, cumulative CO2 production after 73 days was similar regardless of residue component amendment. Significant (P<0.05) interactions between soil texture, residue type (Bt vs non-Bt) and residue component illustrated the influence of soil on decomposition. In the field study, sampling time significantly correlated with Biolog metabolic activity and DGGE profiles. The field study also confirmed the effects of Bt residue on total plate count and substrate utilization profiles. Based on the results of the microcosm and field studies, we concluded that incorporation of Bt residue with higher lignin content and lignin/N ratio in soil significantly affected the structure of microbial communities compared with the residue from its non-Bt isoline. Abiotic factors including soil texture and sampling time also influenced the soil microbial communities and the decomposition of corn residues.