NONPOINT-SOURCE POLLUTION IMPACTS OF ALTERNATIVE AGRICULTURAL MANAGEMENT-PRACTICES IN ILLINOIS - A SIMULATION STUDY

The 1987 National Resources Inventory (NRI) was used to provide a random sample of 100 Illinois cropland sites growing corn and soybeans. These were used with the Erosion Productivity Impact Calculator (EPIC) model to simulate responses of soil erosion and nutrient exports to several different tillage and crop rotation practices. The NRI and other data sources were used for site specific characterizations that reflected the multidimensional variability in site conditions in Illinois. Four alternative management schemes were simulated for each site: continuous corn and soybean/corn rotations under conventional tillage and no-till. Average crop yields varied little under the four management schemes and closely corresponded to expected yields from the SCS Soils-5 Soil Interpretation Records. However, no-till significantly reduced soil erosion rates and loss of soil organic carbon compared to conventional tillage. This reduced erosion under no-till also led to reduced losses of nitrogen (N) and phosphorus (P) in eroded soil. Percolation below the root zone represented the greatest N export and is of concern for the issue of groundwater contamination in this region. Nitrate N concentrations in percolate substantially exceeded the drinking water standard of 10 mg/l. Soybean/corn rotations generally bad 10-30 percent lower soluble N losses in surface runoff, subsurface flow, and percolation than the corresponding conventional tillage or no-till practices under continuous corn. No-till resulted in a 24 percent increase in soluble N runoff losses compared to conventional tillage in soybean/corn rotations. P losses in surface runoff were larger under no-till, likely as a result of the lack of immediate incorporation of surface P fertilizer into the soil. The model predictions for changes in soil erosion and N and P losses under different management practices were in line with measured changes in site-specific field studies under similar conditions. Our study results take into account the effect of site-to-site variability on the responses and the relative frequency of different site conditions state-wide. This indicates the potential magnitude for changes in nonpoint source pollution by sediment, N, and P state-wide by adoption of alternative corn and soybean tillage and rotation schemes.