Nitrate Nitrogen: Iowa's Unintended Export
by Robert D. Libra
The Mississippi River has long been an important transportation route for Iowa products. For decades, river barges have provided a relatively low-cost way to export the bounty from the states farms, factories, and woodlands to the Gulf of Mexico and the world beyond. However, recent studies have highlighted an unintended export from Iowa and the rest of the Corn Belt to the Gulf, one that may be helping to generate a condition that limits the productivity of some of the Gulfs fisheries. This export is nitrogen, and the condition is called "hypoxia."
Hypoxia describes a zone of poorly oxygenated water that occurs along the Louisiana Gulf coast, and that may be growing more extensive. While the causes of hypoxia are still under debate, many marine scientists are convinced that an increase in the delivery of nutrients, particularly nitrogen, to the Gulf is at the heart of the problem. Nitrogen increases the production of tiny marine phytoplankton whose life cycle reduces the amount of oxygen available for fish and shrimp. Fish and shrimp populations decline in response to this less favorable environment.
The Mississippi River provides the bulk of the runoff to the Gulf, taking the drainage from 40% of the continental U.S. and funneling it southward. This impressive volume of water averages almost 140 cubic miles per year, enough to cover Iowa to a depth of 13 feet. It carries with it an equally impressive amount of nitrogen, estimated by the U.S. Geological Survey to average 1.65 million tons/year since 1980, with most of the nitrogen, about 1 million tons/year, in the form of the chemical compound nitrate (NO3). When researchers look up-river for the sources of this nitrate, they find no shortage. The list of suspects includes human waste, industrial activities, and input from rain. However, most attention has focused on agricultural practices and sources, such as fertilizer, manure, legume production, and mineralization of soil nitrogen, all of which account for most of the nitrogen added to the Mississippi basin. The largest agricultural input occurs in the Corn Belt, and the greatest delivery of nitrate to the Mississippi River appears to be from Corn Belt states. Studies by the U.S. Geological Survey suggest that over 60% of the Rivers nitrate load is derived from sources north of St. Louis.
The possible link between hypoxia in the Gulf and nitrate from the Corn Belt raises questions about Iowas nitrate export to the Mississippi. Estimating this contribution requires nitrate concentrations and stream flow rates for representative parts of the state which, when multiplied together, produce the total mass or load of nitrate. Two sets of monitoring data have been used to estimate loads for the period 1987-1996. The first data set includes weekly to bimonthly data from the Des Moines, Iowa, Cedar, and Turkey rivers, which together drain over 21,000 square miles, or 38% of the state. The average annual nitrate-N loads for the individual rivers are very similar, varying from 14 to 16 lbs/acre, with an overall average of 15 lbs/acre. This close correspondence suggests that 15 lbs/acre/year characterizes loads for the eastern part of the state as a whole. This load was adjusted downward when applied to drier western Iowa. Statewide, the results suggest nitrate-N loads from Iowa to the Mississippi River system averaged about 245,000 tons/year during the period. Additionally, these loads vary greatly year to year, ranging from 12,000 tons during the drought of 1989 to 700,000 tons during the floods of 1993 (see graph above). The average nitrate-N concentration for the Iowa rivers during the period was 5.6 mg/L. Both runoff and nitrate concentrations are greater in wetter years, resulting in the 50-fold variation in estimated N-loads between drought and flood conditions.
The second data set is provided by the DNRs surface-water monitoring network, which includes 13 smaller streams, with drainage areas of 400 to 1,200 square miles. These streams are distributed across the state and have been sampled monthly since 1987. Combining their nitrate-N concentrations and flow rates for 1987-1996 produces an average annual N-load of about 12.5 lbs/acre. The average nitrate-N concentration was 5.2 mg/L. If the loads from these streams characterize those that occurred state-wide, an estimated 225,000 tons/year of nitrate-N was exported from Iowa during the period, ranging from 25,000 tons in 1989 to 655,000 tons in 1993. These amounts are remarkably similar to those estimated from the four larger rivers. Taken together, the estimates suggest that Iowa, on average, supplies almost 25% of the nitrate-N that the Mississippi River delivers to the Gulf of Mexico, while occupying less than 5% of its drainage basin.
What is the source of this unintended export? While all nitrogen sources contribute nitrate-N to the states streams, 80% of Iowas land is agricultural, and agricultural nitrate losses appear sufficient to account for the river loads. For example, nitrate-N concentrations in shallow groundwater, field tiles, and headwater streams draining from agricultural lands are often several times higher than those measured farther downstream where the loads were estimated. Fortunately, biochemical processes that occur in the upper reaches of streams act to remove nitrate-N, resulting in lower downstream concentrations. If these processes were not occurring, downstream nitrate-N loads would be considerably greater.
What can be done about Iowas nitrate-N losses? Research, education and
demonstration projects aimed at matching nitrogen inputs to crop needs, and therefore
limiting excess nitrogen, were an important part of Iowas 1987 Groundwater
Protection Act, long before hypoxia became a household word. These efforts contributed
significantly to a decade-long trend of lower N-inputs on Iowas farms and fields,
during which producers spent millions of dollars less on nitrogen fertilizers without
sacrificing yields. Unfortunately, funding for these efforts has lagged, and nitrogen
inputs appear to be inching upwards again. A refocus on nitrogen management seems in
order. In addition, the natural processes that remove nitrate-N from streams have drawn
the attention of researchers. Can in-stream nitrate removal be enhanced, through the use
of constructed wetlands or other methods? Beyond agriculture, we need a better accounting
of other nitrogen sources. Ultimately, it is in the best interests of Iowans, and
our downstream neighbors, to improve the efficiency of nitrogen use, and to lower the
nitrate concentrations in the states surface water and groundwater.
Adapted from Iowa Geology 1998, Iowa Department of Natural Resources