Assessing changes in soil microbial communities and carbon mineralization in Bt and non-Bt corn residue-amended soils

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 0.085-0.087 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 = 0.06-0.07). 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 versus 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.