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Indicator of the Month: September 2001- Nitrogen in the Environment: Global and Local Considerations

Nitrogen is an important element in our environment, comprising 78 percent of the earth's atmosphere. Nitrogen is an essential element for all life. With rapidly expanding populations and increasing consumption of fossil fuels and land, humans have dramatically altered the natural cycle of nitrogen. There are potential risks from an imbalance of nitrogen in the environment and these warrant our consideration. In this Indicator of the Month, we first examine global nitrogen releases and environmental impacts that may be associated with them. We then focus on Minnesota's contribution to the "nitrogen issue".

Global considerations

The release of nitrogen to the environment by humans has increased steadily in the past century and now exceeds natural sources by 50 percent. Figure 1 indicates that fertilizers account for much of the human-derived nitrogen. Other important sources include legumes (such as soybeans), biomass burning, fossil fuel combustion, land clearing, and drainage.

Figure 1: Annual global release of nitrogen, by source

(Source: Peter M. Vitousek, et. al., "Human Alteration of the Global Nitrogen Cycle: Causes and Consequences," Issues in Ecology, No.1 (1997), pp. 4-6.
Adopted from:: http://www.wri.org/wri/wr-98-99/nutrient.htm)

Nitrogen contributes to eutrophication of lakes, rivers, and oceans; contamination of ground water; development of "dead zones" in oceans and seas because of hypoxia; acidification or other modifications of terrestrial and aquatic ecosystems; and global warming. Figure 2a, for example, shows the estimated size of the bottom-water hypoxic zone in the Gulf of Mexico, 1985-2001. Figure 2b is a map showing the frequency of occurrence of mid-summer hypoxia during the same period. The size of the hypoxic zone depends on many factors, one of the most important of which is weather conditions. For example, 1988 was a hot and very dry year, resulting in low flows in streams draining to the Gulf and a corresponding reduction in areal extent of the hypoxic zone. The mid-to-late 1990's saw some very wet years (1993 was a year of record flooding on the Mississippi River), resulting in higher flows and nutrient loadings (including nitrogen) to the Gulf and an increase in the size of the hypoxic zone.

Figure 2a: Estimated Size of Bottom-water Hypoxia in Mid-summer

Figure 2b: Frequency of occurrence of mid-summer hypoxia, 1985-2001

(Figures 2a and 2b show updated hypoxia data, including estimates for 2001,
reprinted with permission of the Louisiana Universities Marine Consortium;
data of Rabalais, Turner and Wiseman)

Minnesota's contribution

Minnesota is a major contributor to environmental releases of nitrogen. Consider the following:

1. We are the sixth largest user of nitrogen fertilizers in the United States. Over 600,000 tons of nitrogen fertilizer are applied annually in Minnesota-roughly 5.5 percent of the national total. Figure 3 shows the Minnesota River Basin is one of the largest contributors of nitrate-nitrogen to the Mississippi River and hence, to the Gulf of Mexico.
   
   
2. Release of nitrogen to the atmosphere increased 14 percent in Minnesota during the past ten years. Meanwhile, national emissions have decreased by 5.1 percent since 1980. Much of the national decrease is the result of reduced emissions from power plants, particularly in the Northeastern United States, while the increase in Minnesota is largely the result of increased emissions from automobiles.
   
   
3. Since 1800, over 11 million acres of wetland have been drained in Minnesota. Wetlands are important "sinks" that hold and utilize nitrogen and other nutrients.
   
   
4. While acreage of forested land has actually increased 0.7 percent in the state since 1980, much forested land has been cleared and developed, particularly adjacent to the State's metropolitan areas. Impervious surfaces like roads and parking lots increase surface water runoff to rivers and streams. Application of fertilizers containing nitrogen and other nutrients to lawns in subdivisions may also contribute increased loads to surface and ground water.

Figure 3: Nitrogen yields of streams during the period 1980-1996

(Source: http://ks.water.usgs.gov/Kansas/pubs/fact-sheets/fs.135-00.fig6.gif)
(Reprinted with permission of the U. S. Geological Survey; data of Goolsby and
Battaglin)

Minnesotans see few direct environmental effects from nitrogen, with the exception of nitrate concentrations in ground water. Figure 4 shows widespread occurrence of nitrate above presettlement concentrations, including many exceedances of drinking water standards. Elevated nitrate concentrations in shallow ground water systems (aquifers), can often be related to human activities at the land surface including application of fertilizers and nitrogen inputs from unsewered developments.

Figure 4: Distribution of nitrate concentrations exceeding presettlement concentrations

(Source: Adapted from data collected by the
MPCA's Ground Water Monitoring and Assessment Program)

What to do

Several "common sense" actions could diminish the environmental risk from nitrogen. Conservation alone, including implementing environmentally-friendly and sustainable agricultural methods, preventing further loss of wetlands, using land efficiently, and using energy wisely would likely decrease nitrogen releases significantly. Specific "best management practices" (BMPs) that will minimize excess input of nitrogen to the environment include:

• Applying fertilizer at recommended rates;
• Accounting for nitrogen credits associated with manure;
• Sound planning and zoning to minimize nitrogen inputs from unsewered developments; and,
• Incorporating green space including wetlands and forested areas into development plans.

For more information

• EPA National Air Quality and Emissions Trends Report, 1999:
  http://www.epa.gov/oar/aqtrnd99/
  
  
• NOAA Hypoxia Web Site:
  http://state-of-coast.noaa.gov/bulletins/html/hyp_09/regional.html
  
  
• Nitrogen in Minnesota Ground Water
  http://www.pca.state.mn.us/water/groundwater/gwmap/index.html