A better understanding of the organisms that are able to catalyse the reduction of N2O to N2 could open up new opportunities within the arena of agricultural soil management.
Dr Frank Loeffler
An international team of researchers has discovered that the range of microorganisms that combat nitrous oxide (N2
O) is broader than expected. N2
O – a greenhouse gas which is 300 times more potent than carbon dioxide (CO2
) – is released into the atmosphere through agricultural practices, sewage treatment and fossil fuel combustion, and has the capacity to destroy sections of the Earth’s ozone layer. The researchers, however, whose results have been published in the journal Proceedings of the National Academy of Sciences (PNAS)
, discovered that nature wields an unexpectedly large arsenal capable of fighting this damaging pollutant.
Some microorganisms, known as denitrifiers, help to mitigate the harmful effects of N2
O by transforming it into innocuous nitrogen gas (N2
). However, previous studies have identified an inconsistency between the predicted and actual levels of N2
O being emitted into the atmosphere. Essentially, lower quantities of the gas are being released than had been expected, indicating the existence of an unknown N2
The latest study, conducted by researchers from the University of Tennessee, Knoxville (UT)
, the University of Illinois at Urbana-Champaign
, Georgia Institute of Technology (Georgia Tech)
, the US Department of Agriculture
, the University of Puerto Rico (UPR)
and India’s National Institute of Abiotic Stress Management (NIAM)
, seems to account for this apparent discrepancy. Moreover, the team’s findings could facilitate efforts to further reduce N2
To find out more about this natural army of denitrifiers, I spoke to lead researcher Dr Frank Loeffler, Oak Ridge National Laboratory
Governor’s Chair for Microbiology at UT. I began by asking how global emissions of N2
O compared with those of other greenhouse gases such as CO2
"The most important thing to note is that N2
O is much more potent than CO2
," Dr Loeffler answered. "Its potential to trap solar radiation is far greater; small concentrations of N2
O in the atmosphere can have a significant impact on global warming. Another issue to consider is that N2
O – unlike CO2
– contributes to the destruction of the ozone layer; it poses a threat on two levels. One of the most pressing concerns, therefore, is that N2
O emissions are likely to increase even more rapidly than those of CO2
. The main source of N2
O is the use of agricultural fertilisers. The global human population is growing, and in order to feed all of these extra people, agricultural activity will have to increase. Fertilisers are also used to produce crops for bioenergy. As the utilisation of fertiliser increases, N2
O emissions will rise."
In light of this worrying assessment, Dr Loeffler and his colleagues were pleasantly surprised to discover that the number of microorganisms capable of denitrification was larger than previously anticipated. After screening the microbial genomes of enzyme systems that catalyse the reduction of N2
O to N2
, the team uncovered an unexpectedly broad distribution of denitrifying enzymes across different groups of microbes.
"Scientists have studied the nitrogen cycle for a long time, and denitrifiers were the only group of organisms known to be capable of reducing N2
O to N2
," explained Dr Loeffler. "It came as quite a surprise, therefore, to find that many other microorganisms outside of this group act as N2
Of course, the fact that a larger natural army than expected is combating this harmful greenhouse gas comes as welcome news. However, whilst it accounts for the aforementioned discrepancy, it does not help our current situation. I asked Dr Loeffler whether in the future, his findings might be used to further reduce N2
"I think that this is, at least in theory, a possibility," he replied. "N2
O fluxes are being measured in fields and forests across the globe. A better understanding of the organisms that are able to catalyse the reduction of N2
O to N2
could open up new opportunities within the arena of agricultural soil management. For example, we could potentially increase the activity of these types of organism and thus precipitate larger levels of N2
O consumption before the gas is released into the atmosphere."
To conclude our conversation, I asked Dr Loeffler whether he had further research planned in this area. As he explained, he and his colleagues are currently investigating some of the more perplexing observations that have been made.
"We have funding from the US Department of Energy
to continue our research," he explained. "Denitrification is thought to be an anoxic process – it requires a dearth of oxygen in order to take place. However, somewhat inexplicably, researchers have observed N2
O production and consumption under conditions in which oxygen has been available. If these observations are corroborated, they may help us to devise a strategy to manage soil in a way that reduces N2
O emissions. There is more work to be done, but I think that our findings will motivate others to study the abundance and activity of the atypical enzyme systems that we have identified."