Subsurface drainage research in the North Central Region has produced findings that will further agriculture's quest to meet the national and global demand for abundant, affordable food, as well as clean water and a healthy environment. An increased understanding of the interactions between drainage systems and soil, hydrologic and ecological systems is contributing to the development of modern drainage system technologies.
Research results in the North Central Region clearly show that drainage improves yields for crops grown on poorly drained soils. Average yield increases for long-term studies conducted in Indiana and Ohio report annual increases in corn yields of 14 to 23 bu/acre, and 20 to 30 bu/acre, respectively, for crops grown with subsurface drainage versus without. Even larger increases in yield were obtained where subsurface drainage system function included the capability for water table management. Michigan results show corn yield increased 9% in an above average rainfall growth season, and 58% in a below average rainfall growing season. Soybean and corn yields in Ohio were on average 43% and 30% greater, respectively, for subirrigation/drainage systems compared to conventional subsurface drainage alone.
In addition, research in Ohio and Minnesota shows that subsurface drainage promotes mellower soil conditions and reduces compaction potential. Where soil infiltration is promoted by practices such as subsurface drainage and conservation tillage, significant reductions have been found in total and dissolved phosphorus and sediment concentrations of surface waters. At an Ohio site evaluated over a 14-year period, subsurface drainage reduced the losses of sediment, phosphorus and potash by 40, 50 and 30%, respectively, compared to surface drained cropland.
Improved agricultural stewardship of land and water resources has led to reductions in phosphorus and sediment loadings from agricultural land, however nitrate-N losses to surface waters have continued and are increasing in some areas, for reasons that are not completely understood. Elevated concentrations of nitrate-N have long been considered a source of drinking water pollution. Recently, N fertilizer has received attention for its contribution to hypoxia in the Gulf of Mexico. Hypoxia is a condition of low dissolved oxygen occurring in vital Gulf of Mexico fisheries. Oxygen depletion occurs when nutrients exported from upstream Mississippi Basin watersheds cause increased algal growth in the nutrient-limited marine ecosystem. The decomposition of dead algal matter by oxygen-using organisms results in depleted oxygen levels that kill animals in the sediment and force shrimp and fish out of preferred habitats.
Although the relative loading of nitrate-N to surface waters from different sources and the mechanisms by which nitrate-N loss occurs remains a matter of debate requiring more scientific research, agricultural production in the North Central Region is probably a key contributor. Certainly, subsurface drainage acts as a conduit for the movement of nitrate-N to surface waters. Research in Illinois showed that nitrate-N concentrations in drainage water from fertilized crop fields was 10 times greater than concentrations measured in drainage from a meadow.
In addition, there is a great potential for nitrate-N loss when available N exceeds plant requirement. Minnesota results reveal that interactions between N management and weather patterns play an important role in loss of nitrate-N to surface water. Nitrate-N losses occur when large rainfall and flow events flush nitrate-N that has yet to be used by plants. In Iowa and Illinois, stream flow and instream N loading have been linked to subsurface drainage flow patterns, indicating nitrate-N loss is highly related to climate and hydrology. Long-term research in Minnesota has demonstrated a strong relationship between precipitation, subsurface drainage volume, and nitrate-N losses. Indiana findings show that nitrate-N is present in drainage water at all times that drainage occurs. Since most drainage flow occurs during the off-season when crops are not growing (November-May), most of the nitrate-N losses occur then. This contrasts with the timing of pesticide loss which typically occurs immediately after application.
These findings indicate innovative methods of drainage water management, possibly altering the seasonal pattern of drainage discharges or eliminating discharge and thus associated nitrate-N, are required in addition to N application management strategies.
Fortunately, innovative water table management technologies are being developed, tested, and increasingly used. These technologies are developed at universities and MSEA sites throughout the region and communicated to producers through educational programs. Research in Ohio and Michigan found that water table management leads to improved ground water and drainage water quality. At several sites in Michigan, subirrigation reduced nitrate-N delivered through subsurface drainage to surface water by 64 and 58%. Research in Iowa and other sites shows corn and soybean yields increase, and nitrate-N concentrations decrease in drainage outflow when shallow water table depths are maintained. Cropland with controlled drainage may provide reduction in nitrate-N losses by raising the water table during the off-season.
Systems that link subsurface drainage with wetlands serve to further reduce soluble pollutant loadings to surface water. Wetlands that intercept agricultural drainage are being evaluated in Illinois as a method to reduce agrichemical inputs to rivers and reservoirs used as drinking water supplies while maintaining agricultural production. Preliminary results indicate that most of the nitrate-N can be removed by wetlands during spring and summer periods. A great potential exists to reduce pollutant losses and increase agricultural production using Wetland Reservoir Subirrigation Systems. These systems that recycle drainage are being tested and demonstrated at sites in Ohio. The system supplies water to crops, eliminating drought stress; uses the wetland system to reduce sediment and plant nutrients in drainage water; and increases wetland acres, vegetation, and wildlife habitat.
In summary, significant progress has been made through research and educational efforts toward the goals of increasing and maintaining agricultural production in the North Central Region and reducing nonpoint source pollution. Problems remain, but vital and active research and educational programs have already contributed greatly to mitigation of water quality impacts and will continue to balance key food production, economic, and environmental objectives.