How crushed rock can capture carbon

The battle against climate change could have a new soldier - ground-up rocks.

New research from the University of Waikato suggests that finely ground mafic rock could have significant carbon-capturing qualities when spread on agricultural land.

Trials in the Bay of Plenty in collaboration with Ngati Pukenga and Ballance Agri-Nutrients are going nationwide, and a farming leader says it could be a workable solution - if it works.

Modelling suggests the process has the potential to reduce Aotearoa's carbon emissions on the million-tonne scale, but the idea needs to be rigorously tested in the field.

Scientist Terry Isson, the project's lead researcher, said the study could bring us "one step closer to becoming a carbon-neutral nation".

Isson said New Zealand issues 55 million net tonnes or 80 million tonnes (gross) of carbon each year - 40 million tonnes gross is from the agricultural sector.

"We can't be emitting at the same rate that we have without having irreversible change ... this helps push us in the right direction."

Isson said the ground-up rocks pulled carbon out of the atmosphere by changing where carbon dioxide wanted to live.

"[The mineral] basically shuttles carbon from the atmosphere into groundwater and seawater, and then eventually, it locks that away as a mineral phase, and as a rock on longer time scales."

The basis of the research was harnessing ancient processes already existing in nature to capture carbon faster.

"We're calling on a process that has been operating for billions of years, and then just trying to accelerate that ... this is naturally how the earth's system sequesters carbon.

"We've been accelerating carbon being put from the earth into the atmosphere, so basically what we're trying to do now is ... accelerate the opposite of that."

Agricultural land was "the obvious place" to use the method, as "it's very flat, and ... we're good at putting material down on farmland because we add fertilisers, we add lime".

Lime was used by farmers to control the acidity of the soil, and Isson said the minerals in his trial could also perform this function, but with added benefits. This method was "extremely easy" for farmers to use without any new infrastructure, vehicles, or technology - "all you're doing is swapping out limestone or fertiliser for a different mineral".

The trials aimed to be "as natural as possible" and replicate how the method would work in practice.

"It's important to have natural trials because you can do the laboratory experiments, but that's not representative of the natural environment."

Isson said the mafic rocks involved in the process, such as dunite and basalt, were common in New Zealand - "why not make use of the two things we have a lot of in New Zealand if we have an abundance of these rock types, and also an abundance of agricultural land?"

Unlike other carbon-capture solutions that were decades away, Isson said this method was "a process that's scalable today".

"Other carbon-capture methods, you're sort of sitting waiting around for development in the way of making it affordable and having the technology to do that."

Another advantage of the method, he said, was "empowering the public to be able to actually participate in carbon capture".

"For the large part, individuals sit around as sort of bystanders and climate issues, but with this method ... we're calling on the people to participate, and people are engaging."

The next step was to begin testing the method outside the Bay of Plenty, with trials around the country aiming to start early next year. This would involve testing where the method worked best and under what conditions, and how much carbon could be captured on a national scale.

Federated Farmers Bay of Plenty president Brent Mountfort thought that, if the research proved this method effective, farmers would get on board with it.

"If there's proof that it helps absorb a whole lot of carbon while at the same time ... sorting out pH in the soil, surely people would go for it ," Mountfort said. He said for there to be uptake from farmers, the resources would need to be "economically viable" and practical to implement.

He thought the gross amount of carbon emissions from farming needed to account for the whole picture, including the carbon absorbed in the soil and by vegetation on farmland. It was sometimes hard for farmers to figure out the best option when experts disagreed, but "these days, [farmers] are quite progressive".

"They will look at different, alternative things ... and they're not just thinking about for themselves, we actually think about the wider picture as well.

"I think they'd be keen, but the science has to be spot-on." He added: "It's quite exciting. It's great to hear that these sorts of things are going on."