Nitrous Oxide Increase in the Atmosphere Due to Fertilizer Use in Agriculture

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N2O level in the atmosphere has increased in recent years. Photo by Jason Dunn

What’s increasing the concentration of nitrous oxide, a powerful greenhouse gas, in our atmosphere? According to a new study, this increase is due mainly to the use of N-based fertilizers. This was demonstrated by the different abundance of nitrogen and oxygen isotopes.

What is Nitrous Oxide?

Nitrous oxide (N2O) is a minor component of the atmosphere; within the atmosphere, it has a lifetime of about 120 years.

N2O is formed through bacterial activity in soils and waters, and it is then subsequently emitted into the atmosphere. Once there, it gets consumed through chemical reactions activated by sunlight in the upper layers of the atmosphere. More exactly, it can react with atoms of oxygen, to form nitric oxide (NO); NO can then react with ozone (O3), to form molecular oxygen (O2) and nitrogen (N2), two of the main components of the atmosphere itself.

Increase in the N2O Atmospheric Concentration

In the last decades, N2O concentration in the atmosphere has increased remarkably. Measurements of ice cores have demonstrated an increase of 20% in nitrous oxide concentration between the year 1750 to present day, and the global average N2O concentration at present is about 320 parts per billion (ppb). It is likely that one of the reasons for this is the increasing use of agricultural fertilizer – fertilizers cause an increased level of N2O production by bacteria in soils.

The air trapped in ice was analyzed in this study. Photo by Anne Froehlich.

New Study to Understand the Atmospheric Trend

Scientists published new research on this topic in Nature Geoscience in March of 2012. Researchers performed this study, entitled Trends and seasonal cycles in the isotopic composition of nitrous oxide since 1940, in cooperation between different institutions across the world, including the United States, Australia, Republic of Korea, New Zealand and Tasmania.

For this study, two sets of air samples were used: air trapped in Antarctic ice, and air collected at Cape Grim (Tasmania). For both sets, researchers considered many samples taken over a long time span – from 1940 to 2005 for the Antarctic air, and from 1978 to 2005 for the Cape Grim air.

The researchers performed specialized measurements to determine not just the increase in concentration, but also the N2O source; they accomplished this by considering the different isotopes for nitrogen and oxygen.

Nitrogen and Oxygen Isotopes

Both N and O can have different isotopes; these are atoms of the same element with different atomic weights, due to a different number of neutrons.

For nitrogen, 14N and 15N are the two isotopes, 14N being the most abundant (more than 99% of natural nitrogen). For oxygen, on the other hand, we can have 16O, 17O and 18O, 16O being the most abundant (more than 99% of natural oxygen).

In some specific environments, the ratio between the different isotopes can be different from the standard one; this can give indications about reactions and/or sources of the molecules containing these isotopes.

Isotope Measurements for N2O

Work previously done on this topic showed that N2O coming from bacteria is depleted in the 15N and 18O isotopes. N2O from the troposphere, on the other hand, has a relatively higher content for both these isotopes.

Considering this, researchers measured the abundance of these isotopes for the air samples over the past (from Cape Grim and Antarctic ice).

Results showed that past atmospheric N2O had a lower relative amount of 15N and 18O, which suggests that long-term accumulation of N2O is due to the use of fertilizers.

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