Nitrous oxide no laughing matter
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Once a fashionable party drug, a gas produced by cow’s urine is a growing climate problem. Eloise Gibson talks to the scientists and farmers who are leading efforts keep nitrogen in the ground and out of the atmosphere.
Laughing gas was all the rage at fashionable 18th century parties, and, according to people younger and hipper than this writer, modern party-goers still make themselves giddy by inhaling the gas from whipped cream canisters.
Nitrous oxide – or, to give it its chemical formula, N2O – is also sometimes administered to women in labour for temporary pain relief.
But if you want to find the biggest source of laughing gas in New Zealand, you’ll need to get yourself to a farm paddock.
It seems unlikely that anyone has tried to inhale this particular source of laughing gas.
But, whenever you see cows on pasture, they’re producing nitrous oxide when their urine hits the soil.
The gas comes about because of an imbalance between the amount of nitrogen cows excrete and how much nitrogen grass can absorb to fuel new growth.
When cows wee, their urine contains concentrated nitrogen from all the plentiful grass they’ve eaten. When urine hits the ground, it lands in a patch, which now contains much more nitrogen than the small piece of pasture beneath the patch can re-absorb. That leaves a lot of nitrogen “sloshing around the place”, as Waikato University soil scientists Louis Schipper puts it.
Some of this excess will be converted by microbes to nitrate, which can run off into water and harm fish and seed algal blooms. This is the form of pollution that most people probably most closely associate with farming, with headlines like those following last year’s river stock-take by the Ministry for the Environment and Statistics NZ, which found nitrogen levels were worsening at more than half (55 per cent) of monitored river sites.
But there’s another, lesser-known problem that occurs, one that affects the climate. A smaller amount of nitrogen joins with oxygen atoms and enters the atmosphere as N2O —laughing gas.
Each molecule of nitrous oxide is about 300 times as powerful as a molecule of CO2 at warming the planet, and, unlike methane (the other major greenhouse gas created by farming), it sticks around in the atmosphere for a very long time. That makes it both potent (like methane) and annoyingly persistent (like CO2). In all nitrous oxide makes up about 11 percent of New Zealand’s total greenhouse gas emissions, maybe more – our third most prevalent greenhouse gas after carbon dioxide and methane. Eighty percent of New Zealand’s total nitrous oxide emissions come from urine patches.
According to the Parliamentary Commissioner for the Environment, our production is rising. Nitrous oxide emissions have increased by almost half since 1990, a by-product of increasing use of nitrogen-based fertilisers and more intensified dairying. As farmers applied more nitrogen to grow more grass, they increased dairy production successfully. But they also increased both emissions of nitrous oxide and nitrate run-off to water.
Nitrate’s evil twin
You might call nitrous oxide nitrate run-off’s evil twin. But since the two problems are connected, scientists hope that shrinking one issue will also help solve the other. None of the solutions so far would solve either problem entirely – far from it. But there have been some intriguing finds.
For once, it’s not hyperbole to say that New Zealand is leading the world when it comes to measuring and finding ways to reduce nitrous oxide from farm paddocks, says Schipper. He’s part of a team running tests at a farm at Waharoa, near Matamata, where the farmers have tried some cutting-edge solutions.
Ben and Sarah Troughton volunteered their farm to be a greenhouse gas research hub several years ago. The Troughton family farm is now one of the most-closely monitored working farms for greenhouse gases in the world, according to Schipper.
Together, scientists and the Troughtons track every bit of feed that goes in, monitor where the cows graze, the milk that comes out, and the associated greenhouse gases from the animals and the paddocks they graze on. Waharoa in the Waikato is a perfect place to do it: In 2011, Statistics NZ said the Waikato region had the largest proportion of nitrates being leached from agriculture in New Zealand. Approximately 74.5 percent of farming leachate was from dairying; and much of the rest was from beef farming.
Some of the statistics coming from the Troughton farm have been sobering, despite the fact the Troughtons make a big effort to farm gently. Ben and Sarah Troughton try to treat their paddocks and their Jersey cows respectfully, designing their milking sessions to relax the cows, treading lightly on the soil as much as possible, and, most crucially, operating with a lower stocking rate than most dairy farms in the area.
Still, over a year of measurements, scientists calculated the paddocks were making about 7kg of nitrous oxide per hectare, which translates to two tons of CO2 equivalent. At an open day in April, farmers and consultants gathered at Troughton farm to hear about the latest research. One man raised his hand to ask what others were probably thinking: Given the Troughtons' comparatively low stocking rate, wouldn’t a typical dairy farm lose significantly more nitrogen than this one? “Yep,” said Schipper. The rule of thumb is that farms lose about 1 percent of their nitrogen inputs as nitrous oxide, he says. A typical, more intensive farm would almost certainly be producing more greenhouse gas than the Troughtons.
Another notable finding, which would apply to any farm, was that N2O spiked alarmingly after rainfall, when the soil microbes became busy and caused a big pulse in greenhouse gases. In summer “the strength of the relationship between nitrous oxide and rainfall just blew us away,” Schipper told farmers at the open day. “I didn’t expect a relationship as clean as this.”
Now the scientists are concentrating on finding solutions, and the Troughtons are determined to help, even when it means allowing the scientists to tinker with their busy operation. Recently the researchers persuaded a slightly reluctant Ben Troughton to let them rip up a perfectly good paddock and plant a new crop that might cut the nitrous oxide leakage: plaintain. This green, broad-leafed forage can be planted alongside other grassy species for cows to eat, and so far, the results look promising. It’s been trialed by researchers at several universities and by Agresearch, the crown-owned farm science company. The scientists we spoke to agreed, it’s one of the most promising nitrogen-cutting options they’ve seen. But there’s a way to go before farms like the Troughton’s could adopt it permanently.
When it comes to stemming nitrogen losses, DairyNZ’s Bruce Thorrold divides the possible solutions into three groups. There’s the well-proven, the promising, and, lastly, the you-never-know-it’s-worth-a-go.
In that last, worth-a-shot, category is breeding lower nitrogen cows, a project in its early infancy. “We are just working out now if there is genetic variability,” says Thorrold. “If we can select cattle with lower nitrogen in their urine or smaller urine patches… it’s simple to roll out because 95 percent of dairy cows are from artificial insemination, so it’s just a matter of choosing nitrogen-efficient bulls and in 7-10 years we can change the whole national herd.”
Like a parallel bid by other scientists to breed low-methane cows, breeding for low-N cattle might only make a tiny percentage of difference to the emissions of each animal. But, if implemented across the country, the savings could be significant. Successful breeding is still very much in the “maybe” category, however.
The only things in the “certain-to-work” category, says Thorrold, are shrinking the amount of nitrogen coming on to the farm (by using less fertiliser or less pre-cut cow feed) or getting cows off pastures during wet periods by laying concrete pads (at a cost of hundreds of thousands of dollars per farm).
The concrete pads catch the cows’ wee so it can be distributed evenly around the paddocks later, fostering greater nitrogen absorption and lower losses. The other proven tactic, bringing in less nitrogen, means making less milk, says Thorrold. But farmers might still find they make a similar profit as before, because they’re spending less on feed and fertiliser. For farmers in places like Rotorua or Canterbury, who need to act fast to meet incoming environmental rules, these proven tactics are their best options.
But there are also other, promising discoveries that might work in exciting ways. These aren’t as proven as, say, cutting fertiliser use, but they are considerably closer to reality than low-nitrogen breeding. This is where plaintain fits in, with so-called catch-crops that absorb urine. While there’s still work to do to make sure plaintain works in a real farming environment, Thorrold and Schipper agree that it’s one of the most significant discoveries to date.
There are three ways in which plaintain may help cut nitrogen losses, says Cecile de Klein, from Agresearch. The first is that eating the plant reduces the concentration of nitrogen in cows’ urine. “There is definitely evidence that the concentration of nitrogen in the urine patch reduces, although the animals urinate more so there are more patches. But the reduction on concentration is larger than the increase in volume,” she says.
“The second part is, if we reduce the concentration, we might reduce the percentage of nitrogen lost as nitrous oxide (because the grass can absorb more of what the cow excretes.)”. That part is still inconclusive, says de Klein, with studies going either way.
But there’s a third, more intriguing possibility. “It seems the plant itself may create conditions in the soil that reduces nitrogen loss by affecting the microbes in the soil, and affecting the naturally-occurring processes that create nitrous oxide,” says de Klein. “It’s only a hypothesis, at the moment. We’re trying to disentangle the effects. But what the evidence seems to point to, is that the roots release a natural compound in the soil that acts as a natural nitrification inhibitor. The results are hinting that the plant itself is causing this effect.”
Crucially, this nitrous-cutting effect seems to work regardless of what the cow eats. In other words, the reduction in greenhouse gas is independent of the fact that eating plantain reduces nitrogen in the cattle’s urine. For example, scientists in an Agresearch study funded by the New Zealand Agricultural Greenhouse Gas Research Centre poured identical cow urine on pasture they’d planted with, variously, ryegrass, plaintain and other crops. In autumn and winter, they found that nitrous oxide emissions from the plaintain-covered ground were 39 to 74 per cent less than emissions from the ground covered in ryegrass.
Greenhouse gas research-watchers might remember DCD – the nitrogen-fixing fertiliser product that was once one of the great hopes for reducing nitrate leaching and greenhouse gases. A few hundred New Zealand farms started using it, hoping to save money on fertiliser as well as cutting their greenhouse impact. But DCD was rapidly withdrawn in 2013 after traces were found in milk. While there was no reason to suspect any health risk from the residue, there was no agreed safe level in milk so DCD use was put on hold noticed and, so far, it hasn’t returned to farm paddocks.
Early indications are that plaintain might release a compound that’s akin to a natural equivalent to DCD, in this case from the plant itself. But if – and it’s still an if – scientists can prove it works they would still need to show farmers that plantain was cost-effective. Farmers would want to know it lasts as long in the field as ryegrass, so they don’t need to re-sow their fields too often.
If it does work, plantain could slot into our grass-based farming system without needing major changes to the way New Zealand cows graze, says de Klein. She’s cautious of getting out the party banners just yet, after the lessons of DCD. But that doesn’t mean we can’t feel a little excited.
“With DCD we were too quick to say ‘this is the answer’ without understanding the full consequences,” she says. “But at the moment this (plaintain) is the most promising alternative.”
This story was written with help from the Aotearoa New Zealand Science Journalism Fund.
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