When farmers drained peat bogs to create “productive land”, they unwittingly produced greenhouse gases. Peat is made mostly of carbon, and it releases carbon dioxide when exposed to air – for decades until the peat is gone. Now, the government is proposing to rewet peatlands as part of the proposed Emissions Reduction Plan to 2030. Consultation on this is set to close on Wednesday, August 21. But it might be harder than it sounds.
Peat farmland looks like the flat paddocks that feature in the Waikato and Hauraki Plains. They are patchworked, with ditches that keep draining the land. Only 1.3% of our agricultural land is drained peatlands, but they produce a whopping 8-10% of our net emissions a year.
David Campbell, of the University of Waikato, says that is “absolutely eye-watering. If there’s dairy on peat, the amount of carbon dioxide coming from the peat is much greater than the methane and nitrous oxide from the cows and fertiliser.” The emissions reduction proposal talks about using “well-known practices, such as rewetting drained wetlands”.
Campbell specialises in measuring fluxes of water and greenhouse gases between ecosystems and the atmosphere, especially in intact peat bogs and drained peatlands. He says rewetting to slow down or reverse emissions is an uncertain science.
For a start, not just any wetland type will do. Only peat bogs store meaningful amounts of carbon. Rewetting them has been shown internationally to reduce carbon dioxide emissions and that’s likely to be the case here. With funding from the New Zealand and Irish governments, Campbell and colleagues have recently begun studying the feasibility of increasing the water table to reduce emissions.
A missing link in drained peatlands is unique peat-adapted plants that trap and hold water. They disappear when bogs are drained, but are crucial to maintaining a useful water table. Over-wetting any wetland type can generate methane, but in some places, there’s unlikely to be enough rain to raise the level sufficiently. “It’s very difficult to control the water table. You need to hold it really shallow – maybe 20cm below the soil surface,” says Campbell. To achieve that, peat needs specific bog-adapted plants, he says. Other vegetation, such as pine trees, lose too much water into the air.
The government’s plan describes rewetting as a way to remove carbon from the atmosphere rather than just slowing its release. But again, it’s the missing plants that transform carbon dioxide into peat – very slowly. They are abundant on the Hauraki Plains’ internationally recognised Kopuatai Peat Dome. Its huge carbon store of peat is about 11m deep and took 11,000 years to accrue. “We still don’t know how to restore these ecosystems at scale,” says Campbell. (The emissions reduction plan discussion document states that, “Activities such as peatland restoration are viable now.”)
Campbell collaborates with peatland carbon specialist Jordan Goodrich of Manaaki Whenua – Landcare Research. Goodrich agrees that there’s years of work to do, including measuring how rewetting changes emissions. “Internationally, it’s accepted to use default emission factors from the IPCC [Intergovernmental Panel on Climate Change] for rewetted areas.” IPCC reviewers expect that countries then research what happens locally.
Campbell says current agricultural practices are likely incompatible with rewetting and that farmland’s high value is a challenge.
The government proposes unspecified incentives for landowners and notes the need to develop ways to validate and certify greenhouse gas removals. But in some places, peatland is already hard to manage as pasture because it is subsided to near sea or river level, so it’s seasonally flooded.
