North Island farms are testing a powder called dunite, which sucks carbon dioxide from the air and stores it. This might be a rare case where letting farming run-off enter waterways benefits the environment, reports Eloise Gibson.
When the MV Rena ran aground on Ōtaiti Astrolabe Reef off Mt Maunganui in 2011, the heavy spill of fuel oil became New Zealand’s worst ecological maritime disaster.
But out of that disaster grew a relationship between a young university scientist and iwi Ngāti Pūkenga, who are now working together in a New Zealand-first experiment attempting to undo some of the climate damage caused by fossil fuels.
There are already attempts to suck more planet-warming gases into landscapes using trees, crops and soil, however this organic carbon is prone to being released again when plants die or a drought sets in.
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This project aims to do something different – take away carbon dioxide for a very long time using a reaction between rock, CO2 and water.
Modelling suggests if the technique was used by farmers and others globally it could remove large quantities of carbon dioxide from the air.
But there are near-to-no published studies proving it works in real farming conditions, which is where Ngāti Pūkenga comes in.
At an iwi-owned food garden, just out of Tauranga, researchers have sprinkled a chalky green-grey powder made of finely-crushed volcanic rock on the soil.
Throughout the year, a team of researchers has taken samples of the soil, looking at how quickly the mineral is dissolving. That tells them approximately how much carbon dioxide has been removed from the air.
Similar trials are underway on pasture elsewhere in the Bay of Plenty and in Waikato, as well as on a carrot farm near Matamata. Although the results aren’t in – let alone peer-reviewed and published in a scientific journal, where they can be scrutinised by other specialists – the early indications are promising.
The project last week received $1m from the Ministry for Business, Innovation and Employment for projects to rapidly test “promising, innovative ideas with high potential benefit to New Zealand”.
The backstory to the iwi’s involvement began a decade ago, when Waikato University marine scientist Professor Chris Battershill happened to be in Tauranga when the Rena’s marine oil spill struck and became involved in the clean-up.
He kept in touch with representatives of Ngāti Pūkenga afterwards, which is how, last year, he ended up having a cup of tea with the chairperson of an iwi land trust and her sister, Rina Walker.
Walker is a permaculture expert who went away to study and work but returned to her Bay of Plenty whenua a few years ago, when her mother grew ill. She now manages a 17-hectare food farm (she refers to it as a food forest) for Ngāti Pūkenga just out of Rangataua (Welcome Bay).
Battershill knew Walker wanted to make the farm carbon-neutral, so he introduced her to Terry Isson, an up-and-coming researcher at the University of Waikato, who was looking for different kinds of farms to help him trial a substance called dunite.
Dunite deposits are found near Bluff, near Nelson and in other places around the world. Crucially, dunite is made up of 90% olivine, a magnesium-rich mineral named for its olive-ish colour. North Island basalt also contains olivine, but in much lower quantities.
When olivine weathers and crumbles in nature, it locks up carbon. As Isson explains, when crushed and sprinkled on land, it speeds up the weathering process. As it dissolves, the substance changes the composition of the water around it so that the water wants to hold more carbon dioxide, says Isson. That’s how carbon from the atmosphere turns into a dissolved state, meaning it can be carried away in groundwater and into streams and rivers along with other ingredients from the rock, eventually reaching the ocean.
Normally, letting farming run-off enter the ocean is damaging. But, in this case, it might be beneficial. That’s because ground olivine has an antacid effect, making both soil and water less acidic.
Since the industrial revolution, humans have been making the oceans more acidic by burning fossil fuels and pumping carbon dioxide into the air. The oceans have saved us from much worse global heating by absorbing much of that heat-trapping carbon dioxide – but doing so makes oceans more acidic, which hurts marine life.
Dunite’s anti-acidic effect could be handy for ocean PH, as well as handy for farmers, who currently use ground limestone to make soil less acidic. Dunite could replace some of that lime – possibly.
Once the run-off reaches the ocean, says Isson, phytoplankton and shellfish may eventually use it as calcium carbonate to build their shells, which eventually fall to the ocean floor, eventually becoming rock again. Although that process takes an incredibly long time, “what we’re interested in for human timescales is that as soon as you take CO2 out of the atmosphere [it’s trapped],” says Isson.
This process has been happening naturally since long before humans came along, and helped regulate Earth’s climate after CO2-releasing events such as volcanic eruptions.
“This is a process we think has kept planet Earth habitable for the last four billion years,” Isson says.
What Isson and his team want to do is speed it up, which is how the carefully-planned jumble of Walker’s permaculture food garden came to have 12 tame, orderly squares of maize.
From the air, the land at Ngapeke 7 – the plot of land that Walker plans and manages for Ngāti Pūkenga – is an orderly pattern of eye-catching swirls, tracing the shapes of plantings and swales.
There are more than 500 fruit trees and 500 native species – flowering mānuka, kowhai, feijoa, mandarin and even a pine tree (planted, ironically, to add acidity to the soil) mingling together at different heights and in different combinations.
Before Walker came back, the land was a single-crop maize plantation growing food for cows. Now, “you’ll not see monoculture around here,” she says.
“A food forest is all about diversity,” she says. “You’re trying to create a forest floor and a canopy over several years. The big trees shade and cover the ground like a korowai, so you don’t see bare soil.”
But there is one piece of monoculture at Ngāpeke 7 – the small plot of maize where Isson is running his dunite trial.
He’s applied six different rates of the powder between the 12 plots. Before the maize was cut, you could see that the plants where more dunite had been applied were taller than the others, says Walker.
This patch of farm is very far from a controlled laboratory, says Isson, and that’s exactly how he wants it.
“We’re trying to keep the trials as close to what a farmer would do as possible,” he says.
“Unlike a lot of studies, where scientists like to be in the lab and to control every single variable, this was almost the total opposite. We want to… get a sense of, if we put dunite in the ground and let nature take its course, what’s the actual rate of [carbon dioxide uptake].”
In theory, if it works, farmers would be able to prove their rates of carbon capture in much the same way as he is quantifying it in his field trials, he says.
And, unlike other climate geoengineering projects underway around the world, this could be rolled out straight away, with little change to the farming system, he says.
Fertiliser companies already grind up rock to make lime, which farmers spread on soil to improve the PH. They could do the same with dunite, Isson says.
Other benefits for farming might include releasing nutrients for crops, such as calcium, magnesium, potassium, silicon, iron and zinc, though many more field trials are needed to prove it.
The dunite being sprinkled at Ngapeke 7 comes from the fertiliser company Ballance, which has access to about 30,000 tonnes of the mineral at an existing mine, near Bluff.
Ballance has been supporting Isson’s trials through its programme of investment into low-emissions farming.
However Suzanne Young, Ballance’s Sustainable Food and Fibre’s Future programme manager, cautions that the research is at an early stage and would need more and broader testing to see how it works in different conditions. “This is a really fundamental, basic research stage,” she says. “So, we’re just trying to prove the concept at this point.”
The first phase of field trials runs into 2023, and a second phase to further refine the methods will start soon after.
If the project is successful, it may be possible to extrapolate the results to other rock types, such as basalt, which is more readily available, says Young.
While dunite has in excess of 90% of olivine by weight, basalt has only about ⅓ of the amount, so a tonne of dunite would capture roughly three times more carbon than a tonne of basalt, says Isson.
Others are looking at olivine for different climate projects. It can be processed into magnesium hydroxide, which may be used to mop up industrial carbon emissions, a concept Allan Scott, an associate professor of civil engineering at the University of Canterbury, is working on.
Yet there are practicalities to consider: it’s not entirely clear how much olivine New Zealand has, and what there is can only be extracted by mining.
When asked for an estimate, GNS Science geologist Dr Matt Sagar drew on GNS’ high-quality digital geological maps of New Zealand. He estimated that, in addition to what is in the active quarry near Bluff, there was up to 10 times as much again visible in other places, such as in the Red Hills area near Nelson (where Dun Mountain gave dunite its name). “However, these are difficult to access and/or contained in National/Forest parks,” he cautioned.
Mining conservation areas could be controversial – a drawback of which Isson and Walker are well aware.
Yet Isson points out that there are deposits of olivine overseas, as well as domestic supplies of basalt. Also, the powder might supplement or displace other soil additives, which themselves come from mining. Limestone, for example, is routinely quarried, crushed and sprinkled on farms to reduce acidity.
For Isson, addressing the climate crisis is so urgent that he has to try and see what he can do with dunite. This is one of the few techniques of its kind that may work on spread-out farmland, as opposed to concentrated sources of carbon dioxide, such industrial coal boilers, he says.
“We’re very aware we can’t sit around for too much longer and do nothing about the climate problem,” he says.
Walker agrees. She says if the powder works, she would want to use it on her iwi’s whenua.
“Somebody just asked me the other day, ‘but aren’t they mining it?’,” she says.
“But this [climate crisis] is urgent. Something’s got to be done. If this is the one of the ways we have to do it, then this is one of the ways we have to do it.”
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