A tractor sprays nitrogen-based fertilizers on a corn field in Leesburg, Virginia.

Credit Michael Kirby Smith / The New York Times

A tractor sprays nitrogen-based fertilizers on a corn field in Leesburg, Virginia.

October 2, 2017

Discovery of New Step in Nitrogen Cycle To Fundamentally Alter Fertilizer Use

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Fertilizers form the backbone of many agricultural processes worldwide. Decades worth of work has been poured into understanding the way in which fertilizers function and the ways in which they can affect the environment. In fact, the process by which bacteria break down nitrogen products in fertilizers to help provide plants with nutrients has found its way into high school textbooks, often accompanied by easy to understand diagrams.

A study led by Prof. Kyle Lancaster, chemistry, however, sheds light on a new found process that suggests that there is more to this nitrogen cycle than previously known.

According to Lancaster, existing biochemical models state that bacteria convert ammonia into an inorganic compound, Hydroxylamine, before turning that into nitrite. Nitrite can then be converted by other bacteria to form nitrate, a vital plant nutrient. Lancaster’s work, however, demonstrates that this conversion from Hydroxylamine to nitrite does not happen in one step. Instead, bacteria create an intermediate compound known as nitric oxide.

The issue with this previously unknown conversion is that nitric oxide, under imperfect conditions, is converted into the greenhouse gas nitrous oxide. Some nitric oxide accumulates in the soil while the remainder is washed off by rain or through irrigation channels into freshwater bodies. As the compound reacts with oxygen, it forms nitrous oxide.

Though the Environmental Protection Agency says that nitrous oxide accounts for only 5 percent of all greenhouse gases, it has 300 times the warming potential than carbon dioxide. The gas is also a primary ingredient in the formation of acid rain, which can severely damage foliage.

“Understanding how the model works is the key to finding a solution that maximizes crop production without much environmental consequence,” Lancaster said.

The new discovery has immense implications for the fertilization industry. A better understanding of the process of nitrification helps us pinpoint inefficiencies in current agricultural practices. Because the formation of nitric oxide reduces the amount of time that plants have to absorb nitrogen compounds, Lancaster points to future research that could create inhibitors to slow the process by which the compound is created.

Lancaster also highlighted a number of practical implications of the study. Because producers are now aware of this intermediary step, they can tweak their fertilizer application schedule to provide crops with more time to uptake vital nutrients.

On a larger scale, the study also points to the importance of revisiting the nitrogen cycle as a whole in an attempt to better understand each step. A better view of the nitrogen species that form in wastewater could, for example, be used to make water treatment methods more effective and efficient.

For now, Lancaster and his team are trying to understand the particulars of the conversion from nitric oxide to nitrate, specifically if there are other enzymes involved.

“We have spent most of our attention on carbon dioxide because our nitrogen footprint is much more complicated. There are so many different forms of nitrogen and all have dire consequences to the environment,” Lancaster said. “Nitrite, nitrate, nitrous oxide. You count them all. It’s hard to talk about nitrogen in a condensed way because all species of it are important.”

Correction: A previous version of this story incorrectly stated that nitrous oxide is 300 times more effective at depleting the Ozone layer than carbon dioxide. In fact, it has 300 times the warming potential of carbon dioxide.