Pulpit

Spotlight on nitrous oxide

In the first of two articles about nitrous oxide, AgResearch principal scientist Dr Cecile de Klein explains where this potent and long-lived greenhouse gas comes from and how it influences the climate.

WHILE it might not capture headlines like methane and carbon dioxide, nitrous oxide (N2O) is nevertheless a significant greenhouse gas that we need to address.

As is the case for methane, agriculture is the largest contributor of nitrous oxide emission in New Zealand. In 2019, this sector produced about 90% of NZ’s total methane and nitrous oxide emissions. Within the agricultural sector, biogenic methane contributes almost 80% of the emissions, with nitrous oxide contributing about 20%.

The NZ government has set a target of reducing nitrous oxide emissions to net zero (that is, emissions are matched by equivalent removals of carbon dioxide) by 2050.

Nitrous oxide emissions, like biogenic methane, are not currently included in the NZ Emissions Trading Scheme (ETS). They are, however, likely to be part of any pricing scheme being developed by the He Waka Eke Noa partnership between government, the agriculture sector and iwi/Maori under the Climate Change Response (Zero Carbon) Amendment Act 2019. This scheme is scheduled for implementation by 2025.

So, where does nitrous oxide come from, and why is it concerning from a climate perspective?

Nitrous oxide is emitted into the atmosphere when fossil fuels burn, when solid waste is treated and during some industrial processes. However, in NZ, the largest proportion of nitrous oxide emissions by far result from microbes acting on nitrogen in the soil.

Nitrogen is present in the soil naturally. It’s a vital macronutrient that all plants need to grow and stay healthy.

However, some agricultural practices significantly increase the amount of nitrogen in the soil.

Many farmers and growers enrich their soil by applying synthetic nitrogen fertilisers such as urea or by planting legume crops such as white clover that fix nitrogen from the atmosphere. These are common practices designed to encourage fastgrowing and resilient pastures and/or maximise crop yields.

In grazing systems, most of this plant nitrogen is returned to the soil when grazing livestock urinate and – to a lesser extent – deposit manure. This is because the pasture plants and feed crops that the animals consume contain far more nitrogen than animals need for protein synthesis. The excess nitrogen – between 75% and 90% of what is consumed – simply passes out the other end. This would not be a problem if the nitrogen was returned to the soil evenly, as plants could simply reuse it. But urine patches are highly concentrated; they contain much more nitrogen than plants can utilise in the short-term, meaning the excess is at risk of being lost.

Down in the soil, naturally occurring microbes get to work on nitrogen from all of these sources.

The physical and chemical processes are complex – and essential.

Plants wouldn’t be able to use the nitrogen if not for the actions of these microbes. However, plants don’t use all of the nitrogen that’s made available to them, and microbial actions on what remains cause direct and indirect emissions of nitrous oxide.

Two principal actions are responsible. Nitrification is an aerobic process where microbes known as nitrifiers oxidise ammonium (a form of nitrogen) to nitrate, creating a small amount of nitrous oxide as a by-product. Denitrification is an anaerobic process where another group of microbes then reduces nitrate to nitrogen gas, with nitrous oxide produced as an intermediate step.

I won’t get into detail about these processes here, but if you’re interested there’s a further explanation and diagram on the Ag Matters website.

Overall, the majority of NZ’s agricultural nitrous oxide emissions come from urine and dung (62%) compared to 23% from nitrogen fertiliser. Cultivation of organic soils contributes 10% of the emissions, while the remaining 5% is made up of very small sources, like management of animal waste, crop residues and pasture renewal.

Incidentally, nitrate produced by nitrification can also be leached out of the soil in drainage water. If it finds its way into rivers and lakes it can cause excessive weed growth and algal blooms and be toxic to fish and invertebrates. It can also lead to further nitrous oxide emissions from those waterways.

Between 1% and 2% of all nitrogen in the soil is converted by microbes to nitrous oxide and emitted into the atmosphere.

So, what effect does nitrous oxide have once it’s up there?

Fortunately, the amount of nitrous oxide present in the atmosphere is small compared to carbon dioxide and methane.

However, tonne-for-tonne, nitrous oxide is nearly 300 times more effective at trapping heat than carbon dioxide and around 10 times more effective than methane, when averaged over a 100-year period. This comparison is made using Global Warming Potential (GWP), the ‘common currency’ used by the United Nations Framework Convention on Climate Change (UNFCCC) for comparing the warming effects of different greenhouse gases.

In addition, each emission of nitrous oxide stays in the atmosphere for over a century, making it a long-lived greenhouse gas. The key question then, is what can farmers do to reduce nitrous oxide emissions?

There are several management steps that farmers can consider right now, which I’ll explain in next week’s article.

In addition, research funded by the NZ Agricultural Greenhouse Gas Research Centre (NZAGRC) is exploring a number of practices and technologies that are showing real promise for the future to reduce nitrous oxide emissions.

This research is following two avenues: finding ways of reducing nitrogen inputs into the soil and finding ways of manipulating the microbial processes in the soil that convert nitrogen to nitrous oxide.

In next week’s article, I’ll share what we know already about reducing farm-sourced nitrous oxide emissions and look at the new practices and technologies that the scientists are working on.

ThePulpit

CLARIFIED: In a two-part series, AgResearch principal scientist Dr Cecile de Klein explains why NZ should focus on reducing nitrous oxide emissions, as well as other greenhouse gases.

Who am I?

Dr Cecile de Klein is a principal scientist with AgResearch, based at Invermay near Dunedin. She is an internationally recognised expert on nitrous oxide emissions from soils, leading work to develop methodologies for estimating and measuring nitrous oxide emissions at the paddock and farm scale. Cecile co-leads the NZAGRC’s Plants and Greenhouse Gases research programme.

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