Nitrogen, Like Carbon, Also Leaving Footprint

"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

Treehugger.com

Planet Green

How Stuff Works: What is fertilizer and why do plants need it?

How Stuff Works: Nitrogen Cycle