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Wetland Symposium

Transformation and Fate of Nonpoint Source Nitrate Loads in Prairie Pothole Wetlands


THOMAS M. ISENHART AND WILLIAM G. CRUMPTON

Iowa State University, Department of Botany, 353 Bessey Hall, Ames, IA, 50011

The recognition of the water quality functions and values of wetlands has recently led to increasing focus on the utilization of restored or created wetlands as nutrient sinks for nonpoint source pollution in agricultural landscapes. Nitrate is a contaminant of particular concern in the Prairie Pothole Region, where nonpoint source nitrate loads are among the highest in North America. Our research uses a combination of experimental wetland mesocosms and microcosms to examine the transformation and fate of nitrate loaded to northern prairie wetlands. Results confirm the considerable capacity of freshwater wetlands to transform nitrate. Rates of nitrate loss on a sediment area basis often exceed one gram NO3--N m-2 day-1 in the presence of several mg NO3--N l-1 and are among the highest rates recorded in any wetland system. Observed nitrate loss rates were clearly a function of the concentration of nitrate in the overlying water over a wide range of concentrations. 15N tracer studies demonstrate that denitrification is the dominant loss process, generally accounting for nearly 80% of the nitrate removed. Results also suggest that decaying plant litter plays an important role in the transformation of nitrate by providing additional anaerobic microsites for denitrification.

Extrapolating nitrate loss estimates obtained within the experimental wetland mesocosms to field scale indicates the potential water quality benefits of restored wetlands in agricultural landscapes. Our results suggest that a one-ha wetland has the potential to remove nitrogen equivalent to the fertilizer nitrogen lost in drainage water from 100 ha of corn. However, in order to realize the potential of restored or constructed wetlands in the amelioration of nonpoint source nitrate contamination, careful consideration needs to be given to site selection and design criteria. As an example, consideration needs to be given to system hydrology to maximize the contact time between pollutant laden waters and biologically active surfaces.

However, because so little is known about the mechanisms of contaminant removal in northern prairie wetlands, site selection, design, and performance criteria have generally not considered water-quality functions. Recent initiatives in wetlands restoration offer a unique opportunity to utilize restored or constructed wetlands as sinks for nonpoint source pollution in regions of row crop agriculture.

Most of the published papers dealing with wetlands and water quality note the probable importance of denitrification, with resulting gaseous loss of N2O and N2. However, no thorough investigation of the mechanisms of nitrogen loss nor the controlling factors in northern prairie wetlands has been published. Denitrification is assumed to be an important process in northern prairie wetlands based largely on circumstantial evidence, first, that conditions in prairie wetlands are suitable for denitrification, and second, that nitrate disappears rapidly from water overlying marsh sediments.

A lack of research on the mechanisms of contaminant removal and nitrate loss within freshwater wetlands, however, has inhibited the scientific community from making a credible assessment of the overall role of freshwater wetlands as sinks for agricultural contaminants.


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