How can nitrous oxide be reduced

These technologies are coming or are already out there. Which means less N in the system and that will have a positive effect on nitrate leaching as well. So it's a win win scenario. In cropping systems the main source of nitrous oxide is the fertiliser that is applied to the crops. The most reliable option is to cut back the amount of nitrogen hitting your soil — by lowering stock numbers, reducing supplementary feeds or using less fertiliser. Increasing outputs relative to inputs won't necessarily reduce absolute emissions, but it will improve emissions per unit of product.

It's been of great benefit to New Zealand already - and that's likely to continue. Reducing the use of nitrogen-based fertiliser is a measurable way of reducing the amount of nitrogen available in the soil to be transformed into nitrous oxide. Once-a-day milking could reduce emissions but maintain profitability, if reduced input costs balance the reduction in total milk production. Some supplementary feeds reduce methane emissions per unit of feed intake, while others help reduce nitrous oxide emissions by decreasing the amount of nitrogen excreted onto pastures.

Some practices and technologies have been promoted as options to reduce emissions, but there's currently insufficient robust scientific evidence to be confident of their efficacy.

Significant research is underway in New Zealand, exploring new technologies for reducing on-farm emissions.

The science is complex, but some promising options are on the horizon. As well as being highly effective at trapping heat, nitrous oxide is a long-lived greenhouse gas, like carbon dioxide. So, from a climate change perspective, the warming caused by every new nitrous oxide emission today adds to the warming caused by past emissions.

Each emission lasts in the atmosphere for over a century, and the warming it causes continues for several more centuries after it has disappeared.

By far the largest source of nitrous oxide emissions is livestock urine deposited onto soils. Under the Climate Change Response Zero Carbon Amendment Act, the Government has set a target to reduce long-lived greenhouse gas emissions carbon dioxide and nitrous oxide to net zero that is, emissions are matched by removals by Nitrous oxide emissions come from a range of sources, including fossil fuel combustion.

In agriculture, nitrous oxide is emitted into the atmosphere when micro-organisms act on nitrogen introduced to the soil via animal urine and dung, synthetic fertilisers and legumes.

Many farmers use nitrogen-based fertilisers, legumes or animal manures to enrich their soil with nitrogen and help crops and pastures flourish. When grazing ruminant livestock eat nitrogen-rich pastures or crops, they use only a fraction of the nitrogen consumed to support the production of meat and milk. They excrete most of it in urine and dung, which creates very concentrated nitrogen patches in the soil.

For comparison, a urine patch can contain the equivalent of up to kg N per hectare, while fertiliser application rates are typically kg N per hectare although there may be several applications per year.

Nitrogen in the soil undergoes a range of transformations by micro-organisms see diagram below. Some of the nitrogen is given off as ammonia through a process called volatilisation.

Over time, this is redeposited as nitrogen in rainfall and eventually gives rise to nitrous oxide emissions. Finally, some of the nitrate produced during nitrification isn't taken up by plants or converted by denitrifiers into nitrous oxide and just sits in the soil as nitrate. This can leach or run off in irrigation or rainwater and result in nitrous oxide emissions from rivers and lakes. Currently, there are no simple solutions to reduce nitrous oxide emissions that will work consistently on all farms.

Right now, reducing emissions essentially comes down to reducing the amount of nitrogen in the farming system, and minimising the proportion of the excess nitrogen in the soil that is transformed into nitrous oxide.

Considerable research is under way in New Zealand and overseas to identify and verify new approaches to reducing on-farm emissions of nitrous oxide. While none of these options is fully proven yet, a number are showing promise. You can find information on these approaches on the Future actions page. Nitrous oxide arises from nitrogen transformations in the soil: the more nitrogen there is, the more nitrous oxide is produced.

Therefore, efforts to reduce the amount of nitrogen in the soil will reduce both leaching and nitrous oxide emissions. Scientists are looking closely at practices they know will help reduce nitrate leaching to see how they affect nitrous oxide emissions. George and Sharon Moss have been dairy farming in Tokoroa for nearly 40 years. Their impact on the climate wasn't a consideration back then, but it's at the forefront of how they farm now.

Scientists are looking at various ways to do that. One strategy under investigation is to use precision agriculture techniques that use remote sensing technology to determine where and when to add nitrogen to fields, and how much. Another is to use nitrification inhibitors, chemicals that suppress the ability of microbes to turn ammonia into nitrate, impeding the creation of N2O and keeping the nitrogen in the soil for plants to use over a longer span of time.

But the authors say it will take more than that to help meet greenhouse gas targets such as those set forth in the Paris Agreement. So, scientists are exploring additional strategies. One option involves harnessing the potential of certain microbes to directly supply nitrogen to plants, much as nitrogen-fixing bacteria already do in partnership with beans, peanuts and other legumes.

In that vein, since the company Pivot Bio has marketed a microbial product called Pivot Bio Proven that, they say, forms a symbiosis with crops' roots after an inoculant is poured in the furrows where corn seeds are planted.

The company plans to release similar products for sorghum, wheat, barley and rice. The microbes spoon-feed nitrogen a little at a time in exchange for sugars leaked by the plant, reducing the need for synthetic fertiliser, says Karsten Temme, chief executive of Pivot Bio. Microbes in the soil break ammonia down through a series of reactions, releasing N2O in the process, which can be measured in the field Credit: Getty Images.

Temme says that company scientists created the inoculant by isolating a strain of the bacterium Kosakonia sacchari that already had nitrogen-fixing capabilities in its genome, although the genes in question were not naturally active under field conditions. Using gene editing technology, the scientists were able to reactivate a set of 18 genes so the bacterium makes the enzyme nitrogenase even in the presence of synthetic fertiliser.

Steven Hall, a biogeochemist at Iowa State University, is now testing the product in large, dumpster-sized containers with corn growing in them. Researchers apply the inoculant, along with different amounts of synthetic fertiliser, to the soil and measure corn yields, nitrous oxide production and how much nitrate leaches from the base of the containers.

Though results of the trial are not yet out, Hall says there's "good initial support" for the hypothesis that the microbes reduce the need for fertiliser, thereby reducing nitrous oxide emissions. But some soil scientists and microbiologists are sceptical of a quick microbial fix.

In one field study of wheat, for example, inoculating the crops with beneficial microbes enhanced growth of the plants but only resulted in slightly greater yields. Unknowns abound, Mafa-Attoye's Guelph colleagues wrote in February in Frontiers in Sustainable Food Systems — such as whether the microbes will negatively affect the soil ecology or be outcompeted by native microbes.

Instead of adding in a microbe, it may make more sense to encourage the growth of desirable microbes that already exist in the soil, says Caroline Orr, a microbiologist at Teesside University in the UK. She has found that cutting back on pesticide use led to a more diverse microbial community and a greater amount of natural nitrogen fixation. In addition, production of nitrous oxide is influenced by the availability of carbon, oxygen and nitrogen — and all are affected by adjusting fertiliser use, irrigation and ploughing.

Take tillage, for example. An analysis of more than studies found that nitrous oxide emissions increased in the first 10 years after farmers stopped or cut back on ploughing their land. But after that, emissions fell. View Author Information. Cite this: Environ. Article Views Altmetric -. Citations Supporting Information. Cited By. This article is cited by 53 publications.

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