Sources and Transport of Nitrogen in the Mississippi River Basin

By Donald A. Goolsby, William A. Battaglin, U.S. Geological Survey, Lakewood, CO and
Richard P. Hooper, U.S. Geological Survey, Atlanta, GA

Presented at the American Farm Bureau Federation Workshop
"From the Corn Belt to the Gulf...Agriculture and Hypoxia in the
Mississippi River Watershed", July 14-15, 1997, St. Louis, Missouri


The Mississippi and Atchafalaya Rivers are the primary riverine sources of fresh water and nutrients discharged to the Gulf of Mexico. The combined annual mean streamflow for the Mississippi and Atchafalaya Rivers (21,800 cubic meters per second) represents about 80 percent of the estimated freshwater discharge to the Gulf (Dunn, 1996). These two rivers account for an estimated 90 percent of total nitrogen (N) load and 87 percent of the total phosphorus load discharged annually to the Gulf (Dunn, 1996). Nitrate along with other nutrients has been implicated as a possible cause of oxygen depletion (hypoxia) in a large zone of the Gulf of Mexico along the Louisiana-Texas coast (Justic, et. al., 1993; Justic et. al., 1994; Turner and Rabalais, 1991; Rabalais, et. al., 1996). The seasonal reduction in dissolved oxygen (DO) occurs each year during late spring and summer following high inflows of fresh water and nutrients to the Gulf. For example, following the 1993 flood, the hypoxia zone (DO less than 2 parts per million) covered nearly 17,000 square kilometers, twice the size of Chesapeake Bay. In 1994, 1995, and 1996 the zone of hypoxia was reported to be as large or larger (about 18,000 square kilometers) than during the summer of 1993 (Rabalais and Turner, press release, 1996). Estimates of the size of the zone of hypoxia prior to the 1993 flood (1985-1992) averaged about 10,000 square kilometers.

Nitrogen Sources

The increased use of nitrogen and phosphorus fertilizers is being pointed to as a possible cause of water quality changes in the Mississippi River that lead to hypoxia in the Gulf of Mexico (Rabalais et. al., 1996). Fertilizer use has increased significantly over the past 25 years (Goolsby and Battaglin, 1995)(figure 1) and has similar patterns to increasing nitrate concentrations in the Mississippi River. However, there are other sources of nitrogen in the basin including animal manure, legumes (soybeans and alfalfa), domestic effluents, atmospheric deposition and soil nitrogen. Estimates of some of these inputs of nitrogen to the Mississippi basin and its major tributary basins have been made by Battaglin and others (1997) and are given in table 1. Estimates of nitrogen fertilizer inputs are for 1987 and are given both as a total and by major type, based on data provided by the U.S. Environmental Protection Agency (1990) and summarized by Battaglin and Goolsby (1995). Estimates of manure nitrogen inputs are for 1987 and were computed by Alexander (USGS, written commun., 1992) from livestock population estimates in the 1987 Census of Agriculture (U.S. Department of Commerce, 1989) and estimates of the nutrient content of daily wastes produced by livestock provided by the National Resource Conservation Service. Estimates of nitrogen input from legumes are for 1987 and were calculated using information on soybean and alfalfa acreage from the 1987 Census of Agriculture and nitrogen replacement rates (N fixed minus N in harvested crop) of 35 kg/ha for soybeans and 65 kg/ha for alfalfa (Board on Agriculture, National Research Council, 1993). Estimates of nitrogen input in wet deposition are for 1987 and were calculated from estimates of annual mean nitrate deposition at 188 National Atmospheric Deposition Program stations across the United States (Alexander, R.L., USGS, written commun., 1995). Estimates of nitrogen input from human domestic waste are for 1990 and were calculated from population estimates (U.S. Department of Commerce, 1990) and an estimated per capita loading of nitrogen in untreated municipal waste of 8.65 kg per year. Estimates of municipal and industrial point loadings of nitrogen are typical for the time period 1977-81, and were reported originally as total Kjeldahl nitrogen (Gianessi and Peskin, 1984). Estimates of industrial point sources of nitrogen were included in the total inputs reported in table 1. Estimates of municipal points sources of nitrogen were considered to represent a subset of human domestic waste and were not included in the reported total inputs. The oxidation of soil organic nitrogen also contributes nitrate to surface water and groundwater. However the annual amount contributed by this source is difficult to estimate and is not included in table 1.
Table 1
Although most of the inputs of nitrogen to the Mississippi basin can be estimated and the outputs in surface water can be measured, the actual sources of the nitrate transported by the Mississippi River are unknown. How much is from fertilizer applied this year? from fertilizer applied last year and flushed from the soil zone? from manure? legumes? natural sources? Of an estimated 11.6 million metric tons of N added annually to the Mississippi and Atchafalaya basins, approximately 51 percent is from commercial fertilizer, 30 percent is from livestock manure, 9 percent is fixed by legumes, 5 percent is from human domestic waste, and 4 percent is deposited by rainfall. Municipal and industrial point discharges of N to rivers are estimated to contribute only 2 and 1 percent, respectively, to the total annual loading of N in the Mississippi basin. However, municipal and industrial point discharges of N are often directly to rivers, whereas the other potential N sources are applied or generated at the land surface. Municipal and industrial point discharges of N to rivers could be the source of as much as 25 percent of the total nitrogen discharged to the Gulf of Mexico.

Transport of Nitrogen

The transport of nitrogen (N) from the Mississippi River to the Gulf of Mexico has averaged about 1.5 million metric tons per year since 1980. This flux represents about 13% of the estimated annual nitrogen input from all sources except soil nitrogen. About 60% of the annual N flux is nitrate and the remainder is mostly dissolved and particulate organic N. Both the concentration and flux of nitrate tend to be highest in the spring when streamflow is highest. This direct relationship between nitrate concentration and flow may result from leaching of nitrate from the soil and unsaturated zone during periods of high rainfall. Increased flows and elevated nitrate concentrations in agricultural tile drains also may contribute to this relationship.

The available data suggests accumulation of nitrate in the soil and unsaturated zone during dry years, such as the 1988-89 drought, and release of stored nitrate during wet years, such as the 1993 flood. The flux of dissolved nitrate tends to peak in the spring and early summer months when daily flux rates can exceed 5,000 metric tons per day (figure 2). The annual flux of nitrate from the Mississippi River to the Gulf has more than doubled over the last 40 years (figure 3). Prior to 1972 annual loads were less than 300,000 metric tons. In the 1980s and 1990s annual loads of 800,000 to 1 million metric tons per year were not uncommon.

The principal source areas for nitrate discharged to the Gulf are watersheds draining the cornbelt states, particularly Iowa, Illinois, Indiana, Ohio, and southern Minnesota. For example, the upper Mississippi basin, above the Missouri River, comprises about 15% of the drainage area of the Mississippi basin but contributes more than 50 percent of the nitrate discharged to the Gulf. The average annual yields of nitrate in the cornbelt states of Ohio and Indiana are typically were greater than 1000 kg/km2/yr for the 1980-96 time period. In contrast, outside of the cornbelt the annual nitrate yields for this same period ranged from less than 50 to about 300 kg/km2/yr.


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