Context
The recent rapid growth in the global agricultural area cultivated with transgenic crops, genetically-engineered to have either or both herbicide and pest resistance, has raised public concerns over the potential impact of these crops and their management on the environment (Hoffman, 1990; Grogan and Long, 2000; Nottingham, 2002). In 2001, 52.5 million hectares of biotechnology-derived crops were planted worldwide, and during that year 46% of the world's total soybean area, 7% of the corn area and 20% of the cotton area were planted to biotechnology-derived seed (CAST, 2002). Among these transgenic crops, pest-resistant transgenic corn expressing insecticidal proteins derived from Bacillus thuringiensis (Bt) has increased in planted area from 16,000 hectares in 1996 to 6.3 million hectares or 22% of all corn planted in the USA in 2002 (USDA-NASS, 2002). Possible direct environmental hazards associated with the release of transgenic crops include movement of transgenes in the environment, escape of the whole plant, effects on nontarget organisms, and resistance evolution (National Research Council, 2002). Changes in soil and crop management over large land areas due to widespread cultivation of transgenic crops, such as changes in pesticide use and tillage practices, could also have both positive and negative environmental effects (CAST, 2002; Nottingham, 2002).
Although some research has examined the environmental impacts of the "above-ground" portion of transgenic crops, relatively less research effort has examined the effects of these crops and their residues on soil flora and fauna and biologically-mediated processes and functions in soils (O'Callaghan and Glare, 2001). Among the many essential functions of soil flora and fauna are soil organic matter decomposition and nutrient mineralization and immobilization.
Expected Outcomes
Expected outcomes of this research include an evaluation of the effects of residues from con1 rootworm resistant transgenic corn on crop growth and the soil biologically-mediated processes of decomposition and N mineralization.
Contribution to the Mission and Objectives of IMBA
This proposal fits under the Enhanced Profitability Program of IMBA, since it aims to maximize positive and minimize negative impacts of the corn and soybean industries on the environment.
Research Description
Obiectives
1. To evaluate the effects of the growth of corn rootworm resistant transgenic corn and incorporation of corn rootworm crop residues on subsequent soybean growth and the soil biologically-mediated processes of decomposition and N mineralization.
Materials and Methods
Field Experiment
A two-year field trial will be initiated at the University of Missouri Greenley Agricultural Experiment Station located in Northeastern Missouri (40°02′N, 92°14′W). The experimental site has been no-till planted in a corn-soybean rotation with nontransgenic corn at least seven years before the expected initiation of this experiment. The soil at the site is classified as a Mexico silt loam (fine, smectitic, mesic Aeric Vertic Epiaqualfs).
The study will be arranged as a split plot design with three replications. The main plots will be no-till and minimum-till soybean following corn. The subplots will be treatments of three commercially available transgenic Bt corn rootworm resistant hybrids, three respective nontransgenic isolines of those Bt hybrids, and the three nontransgenic isoline com hybrids treated with an insecticide to control root feeding by soil pests such as white grubs, wireworms, and southern corn rootworm.
Plot dimensions will be 4.56m width by 30.40m length. Corn will be no-till planted in 6 rows spaced 76cm apart. Nontransgenic soybean will be planted under no-till and reduced tillage practices (chisel plow and disking) in 19cm rows at 494,000 seeds/hectare. Rates of preplant N, P, and K fertilization for the corn and soybean crops will be based on University of Missouri soil test recommendations (Buchholz, 1992). During the second cropping season, the area previously planted to soybeans will be planted with the same corn varieties and spray treatments used during the first year. Field measurements will include determination of corn and soybean grain and total above ground biomass yields for both crops. Samples of corn stem, leaf, and root tissue will be collected at harvest to determine any differences in tissue composition. The quantity of residue on the soil surface will be determined by collecting the aboveground residues in five 1m2 quadrats using a PVC frame and a knife. Residues collected in the quadrats will be washed to remove soil, oven-dried, and weighed. The proportion of residue cover will be assessed in each plot on five subsamples using the line transect method (Sloneker and Moldenhauer, 1977). Stem, leaf and root tissue will be analyzed for total Kjeldahl N (Zellweger Analytics, 1996), soluble and total organic C (Nelson and Sommers, 1975), and cellulose, lignin, and ash contents (Goering and Van Soest, 1970).
Soil samples will be collected in the field using stainless steel push probes to a depth of 15cm at the end of the corn growing season and three times during the soybean growing season (spring before soybean planting, after planting, and after harvest). These soils will be analyzed for potential N mineralization, soil C02 efflux, particulate organic matter C and N, and total organic C and N.
Project Responsibilities of Investigators
This proposed research is linked with a complementary project being proposed to IMBA by Dr. Michelle Wander and her research group at the University of Illinois. Her proposal will evaluate methods to assess the environmental persistence of Bt endotoxin in soil utilizing samples from the field experiment proposed in this study.
