Research suggests that wetlands may act as sinks for a variety of agriculturally important chemicals, and there is considerable interest in the potential of wetlands for reducing pollution loads in agricultural drainage water. A model of wetland nitrate-N consumption was combined with estimates of watershed nitrate-N loads to determine the potential for integrating constructed or restored wetlands as part of agricultural drainage systems. The model predicted that a 2.5 acre wetland would remove approximately 60% of the annual nitrate-N load draining from 250 acres of corn. Although wetland inlet concentrations averaged around 18 mg/L nitrate-N, outlet concentrations fell below the drinking water standard of 10 mg/L nitrate-N. The model was also used to predict changes in nitrate-N removal by wetlands with relatively low vegetation densities, representing the first year or two after construction, and higher vegetation densities, representing more mature systems several years after wetland construction. Over a three-year period, the wetland's capacity for nitrate-N removal increased approximately four fold.
Tillage is one management factor that can potentially affect agricultural chemical concentrations and losses by affecting erosion rates and hydrology, as well as the need for and method of chemical application. Studies conducted at Iowa State University's Northeast Research Center in Nashua have shown that tillage practice has little influence on nitrate-N and pesticide losses to the subsurface drainage water in a corn-soybean rotation. However, ridge-till and no-till resulted in larger losses of atrazine than the moldboard plow and chisel-based systems under continuous corn. Simulations of nitrate-N concentrations and losses made with a modified Root Zone Water Quality Model (RZWQM) for the growing season of three years (1990-1992) generally followed the same pattern as the observed data.
Practices that increase infiltration tend to improve water quality, particularly for runoff. However, increased infiltration can increase chemical leaching. A new fertilizer applicator was designed and tested at Iowa State University for its ability to protect fertilizer from infiltrating water and thus reduce the potential for leaching. The device uses a localized soil surface compaction and doming method directly above the fertilizer band. Nitrate-N applied with the applicator moved approximately 60% as deep as nitrate-N applied by a conventional knife. The method appears to present a simple yet effective strategy to reduce nitrate-N leaching losses and possible impact on drainage and ground-water quality.
Research at farms in Ames and Ankeny examined the effects of water table depth on crop yield and shallow ground-water quality. These studies help in optimizing drainage and irrigation requirements. The results show that corn and soybean yields increased significantly when shallow water table depths were maintained. In one field experiment, average soybean yield obtained for a 0.5 ft water table depth was 42% lower than for a 2.0 ft depth. In another experiment where water table depth was lowered from 0.7 to 3.6 ft, the largest corn yields were obtained at the 3.6 ft depth and the smallest at the 0.7 ft depth. However, the average concentrations of water soluble agricultural chemicals increase with lower water table depths. Results show that significant reductions in nitrate-N, metolachlor, atrazine and alachlor concentrations can be achieved by maintaining shallow water table depths. Nitrate-N and metolachlor concentrations in drainage outflow were 54 and 45% lower for a 0.5 ft water table than for a 2.0 ft depth. A depth of 1 ft was found most suitable for ground-water quality control. Based on these results, researchers at Iowa State University have recommended a water table depth between 2.0 and 3.0 ft as a Best Water Table Management Practice for crop productivity and ground-water quality control.
Nitrogen applied to fields before crop establishment is susceptible to losses by many mechanisms (i.e., denitrification, leaching, runoff, volatilization and immobilization) in the time interval between application and crop uptake. Research results show lower concentrations of nitrate-N in drainage water for reduced-rate split application treatments (an initial treatment at planting followed by applications later in the growing season).
The Iowa Management Systems Evaluation Area (MSEA) is one of five research programs comprising the President's Initiative. In 1989, the President's Initiative on Enhancing Water Quality was formed to address rising concern over contaminated ground water. MSEAs were also selected in Minnesota, Missouri, Nebraska, and Ohio. As part of this cooperative program, the Iowa project has been quantifying the levels and movement of nitrate and pesticides in soils according to soils, climate, crops, and varying management practices. Research teams are assessing the impact of Iowa's current farming systems on water quality as well as developing and evaluating new farming systems. The implications of this assessment are leading to socioeconomic adaptations of farming systems which are sensitive to the environment. These solutions are arrived at through: (1) extensive monitoring of surface and ground water; (2) comparison of farming systems impacts on water quality; and (3) evaluation of new and improved farming systems for their environmental and economic impact. The Iowa MSEA project involves three primary agencies--USDA-ARS, Iowa State University, and USGS working at four research sites in three different hydrogeologic settings: thin till over bedrock, thick till, and thick loess.
The Walnut Creek Nitrogen Initiative addresses one of the most prevalent environmental issues of the Midwest: nitrate-N contamination of surface waters. The investigation has evolved from an on-going USDA-ARS National Soil Tilth Laboratory water quality assessment study of an extensively subsurface drained agricultural watershed, Walnut Creek, located near Ames, IA. The goal of the project is to determine the potential of a conservation-based N fertilizer program to serve as a management tool for corn production. Endpoints are reduced nitrate-N contamination in surface water and maintenance of economic viability when compared to conventional farming practices at the watershed scale. Project objectives are to: (1) quantify the change in stream flow nitrate-N content as a result of implementing the conservation-based N fertilizer program; (2) develop a N balance for the conservation-based N management system; and (3) determine the economics and farmer-cooperator perspectives of the conservation-based N management system.
Composite samples are analyzed for nitrate content and side-dressed
nitrogen fertilizer rates are determined: (a) Sidedressing of nitrogen
fertilizer for corn during late spring; (b) Soil samples are 6-12 inches
tall from ground level to centrer of the whorl.