May 15, 2008 -- People are getting increasingly savvy about their carbon footprint -- the ways in which carbon from fossil fuels or plants is converted into carbon dioxide, and how that CO2 in turn is taken up by the land and ocean.
Now, researchers argue, it's time we started watching our nitrogen footprint, too.
Through fertilizer use and fossil fuel-burning, humans are loading the environment with reactive forms of nitrogen that researchers say cause smog, acid rain, coastal dead zones, climate change and a growing ozone hole.
That's quite a list. But there is good news.
As two teams of scientists publishing papers in this week's issue of Science point out, the nitrogen problem is much easier to solve than the carbon one.
"Everyone is concerned with the carbon balance and increasingly with the availability of food," said study co-author Alan Townsend, of the University of Colorado at Boulder. "But those get straight to the nitrogen cycle issue."
"The biofuels explosion is intimately linked to the nitrogen cycle," he added. "Some of the main biofuels being looked at today are nitrogen-intensive crops."
Reactive nitrogen creation increased more than tenfold, from 16.5 million tons per year in 1860 to 172 million tons in 1995, the authors report. Most of this is attributable to fertilizer use.
Since the early 1900s when Fritz Haber and Carl Bosch figured out how to mimic microorganisms' ability to take inert nitrogen molecules from the air and make them into reactive forms of ammonia and nitrate that plants can use, humans have been able to supply nitrogen to soil at will -- and grow a lot more crops.
By 2005, output grew to 206 million tons, the team reports, driven by 20 and 26 percent increases in grain and meat production, respectively, since 1995.
"Fertilizer is something we absolutely need to feed a large and growing population, but our efficiencies in using this are quite poor," Townsend said.
"Right now, of all the reactive nitrogen created in food production, only about 10 to 15 percent ever makes it to a human mouth, because crop and animal systems aren't efficient in their use of nitrogen," said lead author James Galloway of the University of Virginia in Charlottesville.
Applying fertilizers more precisely to plant roots only at the time of maximum nitrogen uptake, or optimizing animal feed composition to minimize waste are examples of strategies that could minimize nitrogen waste. The authors estimate that about 30 million tons of reactive nitrogen could be eliminated per year with more efficient crop and animal management strategies.
Fossil fuel-burning contributes reactive nitrogen, too. The high combustion temperatures break apart nitrogen molecules in air, forming nitrogen oxides, which contribute to smog and acid rain. Capturing these emissions from tailpipes or power plants with the best feasible technologies could reduce reactive nitrogen by seven million tons a year, according to the authors.
The second research team points out that nitrogen is increasingly reaching the open ocean through the atmosphere. By 2000, the amount of nitrogen deposited on most areas of the ocean had increased by more than fourfold since 1860. In many areas, this number was 14 times higher.
This nitrogen acts as fertilizer in the ocean, stimulating the growth of phytoplankton and other organisms.
"We might consider that the anthropogenic nitrogen input to the sea is a good thing," said study author Doug Capone of the University of Southern California, in Los Angeles. "It's actually sequestering CO2 in the sea and mitigating some of the pH effects."
The researchers estimate that up to 10 percent of the carbon taken up by the ocean is attributable to nitrogen fertilization.
"However, through nitrification in the deep sea, we form nitrous oxide," Capone said. "Our estimate is that two-thirds of the positive effect is set back by molecules of nitrous oxide."
There are many uncertainties in this estimate, and the balance may shift as nitrogen loads change, he added. "But the effects need to be considered."
Related Links:
Jessica Marshall's blog: EnvironMental Case
How Stuff Works: What is fertilizer and why do plants need it?
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