This Issue: Too Much of a Good Element
In this issue, Brian DeVore's story "Dead Zone Puzzle" points to a simple truth with wide-ranging ramifications: One can have too much of a good thing. In this case, that good thing is nitrogen--a chemical that makes up 78 percent of Earth's atmosphere and forms proteins in every living cell.
To fathom nitrogen's negative impact, we must consider its nature. Before it can be used by plants and animals, the abundant nitrogen in the air must be fixed--transformed (in most cases, by bacteria or lightning) into ammonia and nitrates. Then--like carbon, oxygen, and water--nitrogen cycles through the biosphere.
The water cycle, in turn, moves nitrate-nitrogen from land into waterways, where it fertilizes aquatic flora. The natural influx of nitrogen from soils, as well as from lightning, has kept marine systems healthy for millennia. But today, many of our waters receive an overdose.
To boost plant growth, gardeners and farmers add nitrogen to soil. Like cooks without a recipe, well-intentioned growers can easily make the mistake of thinking more is better and adding too much of this potent ingredient. Then the system loses its balance.
It turns out that humans have been more or less flooding the world with nitrogen, releasing as much of the nutrient as all natural sources do. Since the advent of artificial fertilizers in the 1950s, the amount of nitrogen leaving the land and entering the Mississippi has added up to about 12 million metric tons a year. DeVore describes the dramatic consequences a thousand miles downstream in the Gulf of Mexico.
Most of the excess nitrogen comes from agricultural lands, but other sources contribute too: fossil fuel combustion, forest fires, land clearing, untreated or ineffectively treated sewage. Even fertilized city lawns add to the load.
While many of us would be loath to give up golf-course-green lawns, we'd do well to consider the research findings of University of Minnesota ecologist David Tilman. Examining the effects of various amounts of nitrogen added to plots of native and nonnative grasses, Tilman found that the number of plant species decreased as nitrogen increased. Quack grass grew best of all, crowding out other species.
Diverse forms of life create a more resilient, productive ecosystem, Tilman says. His experimental plots with 16 plant species produced more plant material, both above and below ground, than did less diverse plots. They also better tolerated drought and additions of carbon and nitrogen. According to DeVore, more diverse crop and livestock operations also handle nitrogen more effectively.
How much nitrogen does the soil need to be productive? More precise methods for applying fertilizer may be able to reduce leaching and runoff without cutting crop yields. A landowner education project in the Zumbro River watershed suggests how. A survey of farmers showed that only 4 percent tested livestock manure for nutrients at the start of the project in 1991, but 72 percent did so in 1998. Less than 15 percent of the farmers knew how much manure they used per acre in 1991; by 1998 more than 70 percent knew the amount. As a result, 65 percent of the respondents reduced nitrogen application by at least 50 pounds per acre on fields fertilized with manure.
The watershed situation DeVore describes is complex, the outgrowth of decades of unwitting practices. Solving the dilemma will take a long time and many players who are willing to reconsider their part in the nitrogen cycle.
Kathleen Weflen, Editor