Part VII – Where Does the Nitrogen Come From?
“I’m impressed—you’ve put together most of the pieces of the Dead Zone puzzle,” said Professor Gracia. Sue blushed, but nodded, as they walked together towards his office.
“Well, it really matters to my family, so I had a pretty strong motivation,” she said. “I think I understand now why the Dead Zone is temporary, getting flushed out by the turbulence of fall and winter storms. The data on nitrogen carried in by the Mississippi show a sizeable increase in early spring when the river discharge rate is at its peak. That supports the bloom of phytoplankton, and the eventual population explosion of marine aerobic bacteria. They deplete the oxygen available in deeper waters, and form the Dead Zone in early summer. The one thing I haven’t figured out is where the nutrients in the Mississippi river water are coming from, and why the Dead Zone was not a problem many years ago.”
“I think your second concern, why hypoxia is a relatively recent phenomena, is explained by this graph” said Professor Gracia. “What do you notice about nitrogen discharge in the sixties versus the last decade?”
Figure 7. Nitrate and nitrogen (in millions of tons per year) discharged into the Gulf of Mexico from the Mississippi River. (Modified from Goolsby, 2001.)
“I can see that nitrogen has increased, but what is the source of the nitrogen?” said Sue.
“That’s probably the most controversial part of this whole problem,” said Professor Gracia, opening his office door. He pushed some books off a chair and motioned towards it. “Here, have a seat. Most researchers, such as Nancy Rabalais and Don Goolsby, have argued that the most important source of nitrogen comes from the fertilization of farms on the land that drains into the Mississippi River. Nutrients not used by crops are washed into the river system and are carried downstream. The farming interests argue that you can’t rule out the possibility that it comes from other sources. Here, take a look at this letter to the editor of the journal Science by Clifford Snyder, the Midsouth Director of the Potash and Phosphate Institute.” Professor Gracia pulled out a folder from his filing cabinet, and handed Sue a sheet.
“There are also no conclusive data that identify the sources of the nitrate and nitrogen that enter the Mississippi River and ultimately reach the Gulf. In the White House Committee on the Environment and Natural Resources (CENR) Topic 3 report, Don Goolsby and others used a statistical model to conclude that agriculture was the major source of nitrogen to the Mississippi River basin. Their conclusion was not surprising, since inputs to the model were based on the assumption that many non-agricultural sources (for example, urban runoff and geological nitrate) were insignificant. Their estimates of discharge are simply proportionate to the tonnage of each input source in sub-basins of the Mississippi River basin. River monitoring clearly indicates that major nitrogen loads come from the geographic area of the Corn Belt, but the sources remain unclear. This geographic area contains naturally rich soils of the prairies, as well as agriculture. The proportion of the nitrogen that comes from agriculture and the proportion of the agricultural nitrogen that arises from fertilizer use remain uncertain.” (Snyder, 2001)
Sue read the letter quickly.
“Snyder seems to be saying that while it’s true the nitrogen is coming from the Corn Belt, it could be originating from the prairie soils, that we can’t know how much is coming from the natural soil and how much is coming from added fertilizer. I guess it would be hard to tell exactly where a nitrogen molecule came from,” Sue said.
“Ok,” said Professor Gracia. “Now take a look at this data from Don Goolsby and his colleagues, and see what you think.”
Figure 8. Nitrogen inputs to the 20-state region of the Mississippi River basin, in kilograms of nitrogen per hectare per year. Fixation refers to the nitrogen fixation that occurs in legume crops. (Modified from McIsaac et al., 2002.)
Questions
Which nitrogen source has added the greatest amount of nitrogen to the land in the Mississippi River basin in the years since 1970?
Refer to Figure 7, which shows the increase in nitrogen carried by the Mississippi into the Gulf, and to Figure 8 above. Are the data in these graphs consistent with the idea that nitrogen naturally present in rich prairie soils is the source of nitrogen carried into the Gulf of Mexico? Why, or why not?
Figure References
Goolsby, D.A., and Battaglin, W.A. 2001. Long-term changes in concentrations and flux of nitrogen in the Mississippi River Basin, USA. Hydroogical Processes 15:1209–1226.
McIsaac, G.F., David, M.B., Gertner, G.Z, and Goolsby, D.A. 2002. Relating net nitrogen input in the Mississippi River basin to nitrate flux in the lower Mississippi River: A comparison of approaches. Journal of Enviromental Quality 31: 1610–1622.
Originally published at http://www.sciencecases.org/dead_zone/case7.asp
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