Maquoketa headwaters council studies N management
by John Rodecap,ISU Extension project coordinator,MWP

Since April 1999, the Maquoketa Headwaters Watershed Council has reviewed water-monitoring data taken within and at the bottom of the 40,000-acre watershed entering Backbone Lake. The nitrate concentrations near Backbone Lake during April through June 1999 were: snowmelt, 8.5 milligrams per liter (mg/L); base flow (tile drainage), 12.4 mg/L; and peak flow 11.3 mg/L. (Drinking water is designated as unsafe at 10 mg/L.)

The watershed council is made up of residents of the watershed south of Arlington and Strawberry Point. They set a goal to reduce by 50 percent the nitrogen (N), phosphorus and sediment leaving the watershed and entering Backbone Lake.

Staff of the Texas Institute for Applied Environmental Research (TIAER) have developed computer modeling to aid in analyzing the watershed. The model’s baseline uses 30 years of local weather records (precipitation and temperature) and data from interviews with watershed producers in crop and livestock production.

Using accepted principles of scientific research, economic analysis and computer modeling, TIAER staff changed several management practices one by one to analyze the impact on N load. Some changes in practices result in improved profitability and improved environmental performance, as shown in table 1.

A major reduction in surface-water soluble N load can be achieved by taking credit for the N in manure, refined commercial N use and lower N rates for corn following soybean or alfalfa crops.

Maquoketa Watershed producer baseline survey interviews indicated an average use of 158 pounds of N per acre for corn following corn with a range of 100 to 318 pounds per acre. Corn following soybeans received an average N rate of 128 pounds per acre, with a range of 75 to 260 pounds per acre.

Livestock in the Maquoketa headwaters watershed during 1998 were estimated to excrete 118,000 tons of manure, containing approximately 1,300,000 pounds of N with a commercial N equivalent value of nearly $200,000. This would have provided 84 pounds of excreted manure N per acre if applied to 15,800 acres of corn.

Some changes in tillage and land management practices keep water and nutrients on the field longer, thus increasing the soluble N that will be delivered through tile lines. The sediment N lost with eroding soils and field runoff is significantly reduced with soil conservation practices. However, there is an annual projected cost (using a 30 year period) associated with these management changes (table 2).

Field-based research conducted by land-grant universities in the north central United States, part of Management Systems Evaluation Area (MSEA), has shown that leaching losses usually remain low if applied N rates are at or below the amount required for optimum crop yield. The potential for nitrate leaching increases rapidly when N from mineralization of organic matter, crop residues, manure and commercial fertilizer exceeds the optimum rate for crop production (see diagram).

The watershed council is aware that changes in production practices will be selectively adopted by producers. However, the heart of the local watershed council plan is that citizens participate in some way so collectively there will be significant and sustainable changes in the quality of water entering Backbone Lake.

Watershed residents are invited to join the council’s discussions on the environmental and economic impact of these N management changes. To receive meeting notices, contact the Maquoketa project office, P.O. Box 487, Fayette, Iowa 52142, or call (319) 425-3233.

Return to Water Watch Newsletter Homepage