Invisible secret beneath the soil
A little-known source of carbon dioxide has a climate impact comparable to the Huntly power station. You have probably never heard of it. Eloise Gibson reports.
New Zealand’s biggest-emitting region, Waikato, has two big sources of greenhouse gas, beyond the obvious cars and cows. One you will have heard of and one you almost certainly have not.
Huntly’s twin smoke stacks are hard to miss if you are driving on State Highway 1 between Hamilton and Auckland. The power station, just across the Waikato River from the highway, produced about
2.5 million tonnes of carbon dioxide in the last financial year from burning coal and gas – about 3 per cent of the country’s emissions. Virtually every serious plan to get carbon-neutral involves closing Huntly’s coal generators, though its gas generators may still be needed.
Yet there is another source of carbon dioxide nearby that seldom rates a mention. Nationally, it probably produces more than 2 million tonnes of carbon dioxide-equivalent a year, although estimates vary widely.
You cannot see the carbon dioxide it makes. And you won’t find it tallied in the greenhouse gas estimates the Government uses to track whether it’s meeting its international climate commitments.
But drive the back roads and you will see signs. On the Hauraki Plains, you might see telephone poles on a drunken lean. Roads in parts of Waikato have to be reshaped every 5-20 years and more frequently during droughts. Farmers face extra bills for draining and fencing.
The source of these problems is the draining of peat, a cork-like substance made from the corpses of a strange, superresourceful plant. Empodisma robustum isa spindly, rush-like species whose name translates as ‘sturdy tangle-foot’.
After evolving to cope with extreme dry in Australia, it arrived here, says Dave Campbell, a Waikato University associate professor who has been studying the species for many years. Empodisma is the self-reliant type. It uses sponge-like, upturned roots to catch water and nutrients straight from the sky when it rains, controlling the rate of evaporation to maintain boggy conditions around it. It has been able to create wetlands in highish, dryish places where you might not expect to find them.
‘‘Their roots have the consistency of moss, so if you squeeze them it is just completely saturated,’’ says Campbell. ‘‘They have formed most of the peat in New Zealand and they have done it so incredibly well.’’
As the plants died, their bodies were preserved in the saturated peat below. Over a period of 10,000-14,000 years, they accrued in layers up to 14 metres deep, pulling up the water table with them.
Almost half this spongy, dripping peat was in Waikato, which is now the centre of the nation’s dairy boom.
Unfortunately for the wetlands, sodden bogs are no good for dairy cows.
Having built themselves up over thousands of years, it took only decades for threequarters of the former peat wetlands to be drained.
While peat soils cover less than 1 per cent of the country, they pack a powerful climate impact.
Intact, they are by far New Zealand’s densest natural carbon stores.
You might think it would be old-growth native forests, which hold approximately 350 tonnes of carbon per hectare in both the trees and soil.
But, as astonishing as forests’ storage is, intact peat can pack quadruple as much carbon – or more – into the same land area.
A Parliamentary Commissioner for the Environment report once estimated peat holds on average 1348 tonnes per hectare at its average depth of 3.9m. Many peat soils are much deeper than that.
Once drained, peatlands turn from a gradual sink of carbon dioxide to a potent source.
Previous generations of farmers weren’t to know that, though.
Where today’s farmers and researchers see rich stores of carbon, their parents and grandparents saw unprofitable bogs.
Diggers sank, livestock foundered. The soil was acidic and too wet for grass roots.
From about a hundred years ago, and particularly from the 1950s onwards, farmers set about draining and aerating the peat, digging ditches and churning the soil to work in lime. The pace with which they drained it outstripped almost any other peatland conversion in the world.
Unknowingly, they set about undoing virtually everything that had protected the carbon for thousands of years.
When air-loving microbes get into peat, they break down the carbon-rich plant remains and release carbon dioxide to the atmosphere. The thick, deep peat dries and shrinks, says Louis Schipper, a soil science professor at Waikato University.
Animals and machines compact it further. Where we see crooked poles and messed-up roads, the microbes see an opportunity.
Opportunity is what farmers saw too.
Over the past 50 years, Waikato’s peat has gone from being considered waste ground to being considered ‘‘kind soils to farm’’, says Harry Snell, a farmer who has seen the area change.
These used to be ‘‘giant, saturated sponges’’, he says. These days, ‘‘You can pretty much
drive a car on them any time of year ... It might need to be a 4WD, though.’’
While draining peat is less obviously damaging to the climate than burning coal at Huntly, the end result is the same – carbon dioxide to the atmosphere.
In fact, coal is essentially ancient, compressed peat: peat deposits eventually form coal if they get compressed far underground.
‘‘Everyone now is getting really sensitive to the damage that coal does but really peat is no different,’’ says Campbell. ‘‘All we are doing when we are draining it is we are slowly burning fossil fuel.’’
If carbon emissions from peat are hard to see compared with a puffing smoke stack, they are also often obscured in government greenhouse gas tallies.
Although farms will soon be required to track methane and nitrous oxide from their businesses, carbon emissions from soil are not currently accounted for in the emissions-tracking software farmers will use. The popular government statistic that agriculture makes up 48 per cent of New Zealand’s greenhouse gas emissions includes methane and nitrous oxide but not carbon dioxide escaping the soil.
Nor are emissions from drained peat counted when New Zealand tracks its progress under the Paris Agreement, the major international deal to curb the planet’s heating. The Environment Ministry says New Zealand’s Paris target from now until 2030 counts the climate benefit of forests planted since 1989 but not the climate impact of peat drainage.
Peat emissions are recorded in a domestic emissions tally, known as the national greenhouse gas inventory, which is often reported by the media. But because the carbon dioxide from soil is lumped with the land sector, it is easy to miss. Peat’s losses are counted in the same sector as forestry, which sucks in much more carbon than peat releases - overall, the sector is always in credit.
But the fact that you can’t see peat’s gases doesn’t stop them heating the climate. According to the latest inventory, drained peat soils produced 2.4 million tonnes of greenhouse gases nationally in 2019 – mainly carbon dioxide but also quite a bit of nitrous oxide. That is a smidge under Huntly’s latest emissions tally. Most drained peat has been converted to farm pastures, though a decent-sized chunk is under crops. Only a tiny proportion is under housing.
More importantly, the rates of emissions used to reach the national inventory’s estimates are drawn from overseas, so the figures are not specific to New Zealand conditions. Even the ministry says the figures are uncertain. Other, New Zealandspecific estimates are either vague or hyperlocal.
Drawing on work by Landcare ResearchManaaki Whenua’s Anne-Gaelle Ausseil and others, the Parliamentary Commissioner for the Environment put national losses from drained wetlands under farming from the whole country at 0.5 to 2 million tonnes of carbon dioxide a year. But peat researchers
GRAPHIC: ELLA BATES-HERMANS think that might be conservative.
Campbell has directly tracked carbon dioxide losses using measuring towers at a few drained peat farms. His results suggest Waikato peat soils alone could produce as much as 1.5 million tonnes of carbon dioxide-equivalent per year.
Intriguingly, some farms released vastly more carbon dioxide than others.
At one, Schipper and a co-author found the land was releasing 14 tonnes of carbon dioxide per hectare every year – almost three times the average annual emissions of a household, from every hectare.
Over 40 years, the land surface had dropped more than a metre.
At another, Campbell measured losses of about 11 tonnes of carbon dioxide per hectare per year.
In 2019, Campbell compared two farms near Te Awamutu, less than 3 kilometres apart, with the same owner and similar management.
To his surprise, one lost almost 31 tonnes of carbon dioxide per hectare in a year, the other 8 tonnes.
Campbell thinks one farm’s peat may have gotten so dry the carbon-releasing microbes shut down for the summer.
Short of a truly crushing drought, there is no end in sight, once the losses begin.
Schipper says emissions ‘‘will continue for as long as that peat is drained, until we get to the basement, and in some places that could be 8m or 9m or more [deep], so we are talking for decades to centuries.’’
Waikato Regional Council has just started what will be five-yearly helicopter surveys over a dozen Waikato properties to track the rate of peat subsidence under different management styles. The results are not in.
New Zealand badly needs to ‘‘frame the size of the problem’’, Schipper says. ‘‘Then the question becomes, what do we do about it?’’
The question of what to do about peat is a ‘‘wicked’’ one, says Campbell. Waikato’s dairy land is some of the most valuable in the country.
‘‘You can’t easily abandon economic investments and it would not necessarily solve the problem anyway,’’ he says.
‘‘It is really just about incrementally finding out about different solutions, without any of them being totally good.’’
As pressure comes on to reduce the gases burped by cows, some farmers have started lobbying for the Government to count the climate benefits of trees and soil. These farmers figure that if they’re going to be penalised for emissions from their farms, they may as well get credit for the carbon sucked in by their land.
But while trees and soil can certainly act as carbon sinks, drained peat is a different matter.
Current evidence suggests that most of Aotearoa’s farmed soils are typically sucking in about as much carbon as they lose, though it is worth noting many proponents of regenerative farming believe that, with the right management, they could pack in even more.
Peat is the exception.
‘‘Overall, for most agricultural soils, it is fairly neutral – you lose a bit, you gain a bit … until you include peat soils, then it is all bad news,’’ says Campbell.
Typically, a dairy farm’s main climate impacts are cow burps (methane) and fertiliser (nitrous oxide).
But, on the peat farms Campbell measured, carbon dioxide leaking from peat soil made up, on average, more than 60 per cent of the climate impact.
Campbell is sympathetic to farmers’ calls to be allowed to take credit for the climate benefit of trees. ‘‘It makes sense. But I could only support that if the losses for peat soils
Professor Louis Schipper, from Waikato University, says the precise amount of peat emissions is uncertain but ‘‘we know it is a very big number’’.
were included in the farm budget,’’ Campbell says.
‘‘You can’t have your cake and eat it too.’’
Overseas, there is mounting pressure to reflood peat. In the United Kingdom and Ireland, people are rewetting peat bogs. Sales of dug-up peat to British gardeners were recently banned, to halt wetland destruction.
Schipper says trying to bring back New Zealand’s healthy, carbon-slurping peat wetlands would be tricky. The paddocks have changed and reflooding the land would not, on its own, remake a wetland.
But reflooding the peat would at least stem the carbon dioxide losses, he says.
A recent study published in the science journal Nature estimated drained peatlands worldwide emit 5 per cent of human-made greenhouse gas – more than aviation – on only 0.3 per cent of the land surface.
‘‘To reach the Paris goal (of limiting heating), carbon dioxide emissions from (almost) all drained peatlands have to be stopped by rewetting,’’ the study concluded.
The Parliamentary Commissioner for the Environment, Simon Upton, broached the topic in his 2019 report: ‘‘Reflooding some of these landscapes and restoring the wetlands could halt these losses and, in the long term, potentially return far more carbon to the soils than forests could ever hold,’’ he said, adding: ‘‘But carbon accumulation in peat soils is very slow.’’
Campbell and Schipper say reflooding peatlands would not be as simple as blocking drains.
Some sites are good candidates for reflooding, says Campbell. A clutch of properties close to sea-level around Muggeridge’s pump station on the Hauraki Plain were recent beneficiaries of Government ‘‘climate resilience’’ funding to boost their pumping and draining. If the $18m investment in stronger pumping and deeper drains for 1100 hectares of peat soil was abandoned, the land would reflood on its own, Campbell says.
But much of the Waikato’s drained peatlands are not low-lying. ‘‘There are large areas here in the Hamilton basin that are 20m-30m above the Waikato River and it would be nearly impossible to rewet them just by blocking drains,’’ he says. ‘‘Once the vegetation was removed, it [changed] to a whole new place and you can’t go back very easily.’’
Also: ‘‘These are good, productive dairy farms,’’ says Schipper. ‘‘There is an economic and social cost [from flooding them].’’
‘‘Those people living on that land and farming it, it is not their fault,’’ adds Campbell. ‘‘The land was drained either by their parents or ancestors. When you think about the price of land, who is going to pay to revert them to wetland?’’
That question might be easier to answer if someone was accurately measuring the climate damage.
This month, the Climate Change Commission recommended the Government urgently start working out how to account properly for peat’s carbon losses and take steps to reduce the flow, calling it ‘‘one of the most significant sources not covered’’ by emissions targets.
A study by Manaaki Whenua–Landcare Research is looking into wet agriculture and whether European methods could work here.
Other studies are looking at how to reintroduce Empodisma to some of its old haunts and bring back carbon-sucking wetlands.
Campbell is part of an international collaboration with Europe, looking at how to continue agriculture on peatlands, with lower emissions.
To Snell, a veteran farmer and farm director, ‘‘what is done is done’’.
Dairy is paying a lot of the country’s bills, Snell says.
But while he does not believe anyone ‘‘wants to stop commerce’’ on peat, he is very keen to shrink emissions, while keeping farming. The question is, how?
On a farm near Cambridge, he and the farm’s staff are trying something that could save both money and emissions.
Using a machine imported from Brazil, they are applying lime directly into the soil, near the grass root zone, rather than churning up the ground.
One issue plaguing peat farmers is that pasture does not last, Snell says. But churning up paddocks to replant pasture, or to plant maize to feed cows when grass is scarce, very likely boosts the rate of carbon loss.
‘‘The conventional wisdom is to recultivate and work more lime in,’’ says Snell.
Now, ‘‘if we have to regrass or crop, we want to do it without cultivating the peat, because cultivation tends to release carbon, and it is expensive’’, he says. The trial started in 2020 and the results are not in yet. Snell stresses the idea is still unproven.
But the idea makes sense to Campbell. ‘‘The shorter time you keep the soil bare, the less the emissions,’’ he says.
Other things that might help stem the losses are keeping drains as shallow as possible, and finding ways to keep the pasture hydrated and growing during summer.
New Zealand is behind on the research, he says, but he, Schipper and farmers like Snell are trying to change that. The topic has increasingly been cropping up at agricultural conferences. ‘‘They have got some very enthusiastic and smart minds on it, and that is good,’’ says Snell.
For now, the only certainty is that the problem is big, and hard to get a grip on.
In time, the shrinking peat might be as talked-about as coal.