Too much of a good thing
by Eric Sorensen | © Washington State University
Science has been predicting and measuring our warming planet for more than a century now. But it was only in the last two decades that most Americans came to believe the earth’s temperature was indeed rising and that the main culprit is the growing amount of carbon dioxide in our atmosphere.
Now scientists are giving a lot of thought to another culprit: nitrogen. Like carbon dioxide, it’s seemingly benign—colorless, odorless, tasteless, and a foundation of life on our planet. Left alone, it tightly binds to itself in inert, two-atom molecules, or N2. It’s ridiculously commonplace, making up four-fifths of our atmosphere. It’s also a modern miracle: synthesized into fertilizer, it has revolutionized agriculture, often with the help of Washington State University researchers.
But it’s easy for a lot of good nitrogen to go bad, as happens most often when nitrogen-based fertilizer is not assimilated by a plant and starts roaming the planet in other forms.
As nitrate fertilizer, or NO3, it can end up in drinking water and contribute to multiple health problems. In surface waters, it stimulates blooms of aquatic life that die and decompose, removing oxygen and creating hundreds of “dead zones” devoid of marine life.
NOx, the collective term for nitric oxide and nitrogen dioxide, contributes to low-altitude ozone, a significant greenhouse gas. Nitrous oxide, or N2O—yes, “laughing gas”—has more than 300 times the global warming potential of carbon dioxide and is equal to one-tenth of CO2’s contribution to global warming.
This makes it “a key player” in the global warming discussion, says Brian Lamb, an atmospheric scientist and principal investigator for a $3 million program that teaches nitrogen-cycle science and public policy to graduate students in a range of disciplines. “And the human part of that is we are big users of nitrogen fertilizers. The more nitrogen you apply to an ecosystem, even an agricultural ecosystem, the more N2O gets released.”
Before 1913, nitrogen cycled through the living world with the help of microbes that broke the N2 bonds and “fixed” nitrogen for use by plants. Other microbes balanced this out, converting nitrogen compounds back to N2 through a process called denitrification.
Fritz Haber and Carl Bosch’s process converting nitrogen to ammonia changed that, as can be seen when Lamb calls up a chart showing the planet’s nitrous oxide budget. The earth’s atmosphere can process about 14 of the nearly 18 million metric tons of N2O produced each year by natural and human sources. That leaves 4 million tons added each year. It’s almost the exact amount coming from agricultural soils as fertilizer is not absorbed by plants.
“When it comes to N2O, ag is the place people are looking to make changes,” says Lamb.
That will be tough. The Haber-Bosch process helped the world population grow from less than 2 billion to nearly 7 billion, with more on the way. All those people will need amino acids to thrive, and almost all our amino acids come from crops and the animals that eat them.
“The biggest effect is on the enzymes needed for photosynthesis,” says ecologist Dave Evans (’90 PhD Botany). “They take a lot of nitrogen. So when farmers are applying nitrogen, they’re trying to increase photosynthesis so they get more growth. They want greener plants.”
But for all the nitrogen farmers use, less than one-third ends up in the grain. One study at WSU’s Cook Agronomy Farm estimated 5 to 20 percent of the applied nitrogen ended up as nitrate in nearby surface waters.
USDA-ARS soil scientist Dave Huggins (’91 PhD, Soil Fertility and Plant Nutrition) notes that the most efficient way to use nitrogen is to apply none at all, an “untenable” option because yields would be too low. Instead, farmers estimate their yield and apply the amount of nitrogen needed to get a theoretical “maximum return to N.”
In reality, yields and nitrogen demands are incredibly inconsistent across a farm. On the 92-acre Cook farm, wheat yields can double from one spot to another, with fertilizer in the low-yield places largely going to waste.
Compounding the problem are recent lab trials, published in the journal Science, showing that wheat will pick up and assimilate nitrate even less efficiently as carbon dioxide levels rise.
Asaph Cousins, a co-author and assistant professor in the School of Biological Sciences, says plants are generally limited by the availability of CO2, which has risen 40 percent since 1800. By the end of the century, it could rise another 35 to 150 percent.
“But like everything,” says Cousins, “with too much of a good thing, you then become limited by some other factor in the environment.”
In the Science paper, one new limit appears to be the plants’ assimilation of nitrate.
“In some ecosystems the dominant form of that nitrogen is nitrate,” says Cousins. “And so now there could be this additional limitation. The nitrogen could be in there, but it’s not readily being assimilated.”
Usually, says Huggins, wheat plants are fed anhydrous ammonia, which breaks down into ammonium and nitrate. The ammonium attaches to soil particles near the surface, but much of it is converted in a matter of days to nitrate, which is carried by water deeper into the soil and tapped by plants’ roots that can reach as deep as six feet.
Over time, as much as 80 percent of a wheat plants’ nitrogen is from nitrate.
One solution could be the development of wheat plants that use nitrogen in a different way. In the meantime, says Cousins, we may see farmers needing to use more nitrogen to get the best yields.
Comments are temporarily unavailable while we perform some maintenance to reduce spam messages. If you have comments about this article, please send them to us by email: email@example.com