Not for sale

Key to improving water quality is increasing NZ's wetlands - after 90 per cent were drained.

'Drain the swamp' might be a popular rallying cry for a certain politician, but New Zealand's farmers are being urged to do the opposite.

They are being asked to restore, increase or construct wetlands on farms. DairyNZ water quality scientist Aslan Wright-Stow describes wetlands as "the kidneys of the land" – they filter, absorb and transform water contaminants, particularly run-off and shallow groundwater from farms, and help reduce the amount of nasties reaching streams, rivers and lakes.

A recent review of scientific studies, undertaken by NIWA for DairyNZ, found seepage wetlands can reduce the amount of nitrate – a problematic form of nitrogen – in waterways by up to 75-98 per cent.

They filter, absorb and transform water contaminants, helping to reduce excess reaching waterways. In particular, wetlands can be highly efficient at removing excess nitrogen in run-off from dairy farms. Wetlands are also efficient at trapping sediment and sediment-bound phosphorus, reducing faecal bacteria, providing a habitat that improves biodiversity and lessens the risk of flooding.

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However, over 90 per cent of the country's wetlands have been drained in the past 200 years, with North Island regions most affected.

"The drainage has been undertaken to make land farmable," Wright-Stow says, "and there's a prevalence of dairy farming in flatter, lower-lying areas. Often that is the terrain where you've got the right conditions for re-establishing wetlands – the place where they were removed.

"So there's a good reason for pushing the protection of these areas and for increasing their number and size."

The main process of treating nitrogen entering a wetland is de-nitrification – bacteria converting nitrate into harmless nitrogen gas before it can reach a waterway. Wetlands work by creating the right environment for these bacteria to thrive.

Nitrate removal is maximised by increased soil-water contact periods; water travelling deeper into the soil where oxygen is absent; and plentiful sources of carbon, which comes from decaying leaves and sticks. Some nitrate removal also occurs through uptake by plants.

The first stage of the restoration effort is to collect and collate data; DairyNZ and Landcare Research have developed a free online tool, the Riparian Planner (DairyNZ.co.nz/riparian-planner). Farmers use it to record the location of waterways and wetlands and plan restoration efforts.

"It's a planning tool that enables farmers and other users to identify and map out where their waterways are – then put a plan in place to protect them," Wright-Stow explains. "It gives estimates of costing and helps people plan on an annual basis what they're going to do, where and when, and it's a mechanism for driving the protection of wetlands."

In many cases, it's a straightforward exercise - pinpointing boggy, wet, seepage areas which are often of lower productive value - and protecting them.

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"That simply means, in the first instance, keeping stock out, fencing them, and then there's the potential to extend their function by additional planting. Often they are found where the appropriate vegetation is going to grow anyway."

The Riparian Planner gives regional planting guides – generally reeds, sedges and Carex for wetlands.

"We're pushing the information out to farmers in terms of what the science tells us and we're moving into a phase where we want to increase the extent of wetlands, because we now know just how efficient and effective these things can be," says Wright-Stow.

DairyNZ and partners are also working on the 'build it and they will come' principle: "The constructed wetland is essentially a tool by which we can mimic what a natural wetland does.

"We are doing a whole bunch of research in this space – and we are embarking on a national study in conjunction with NIWA , regional councils and landowners to better quantify the performance of constructed wetlands," he says.

"It's a pretty large-scale study; we're going to use that not only to give more confidence in how well these things perform, but also we're intentionally doing the work in conjunction with regional councils."

That's because water quality limits will come into force across every catchment in New Zealand by 2025 and many farms will be under pressure to reduce the amount of nutrients released into waterways.

Wright-Stow sees additional incentive for farmers to protect and extend wetlands: "Decisions on how to manage nutrient discharge is currently being addressed by the government and nutrient budgeting is part of good farming. Formal recognition of their performance there is an opportunity to further incentivise wetlands protection.

"We ultimately want to better recognise these things in in nutrient budgets so farmers are getting credit for them. Farmers want to do the right thing for the environment and part of this is reducing their footprint. Incentivising wetlands through nutrient credits is win-win and likely to promote further adoption."

Robust science on wetland performance is needed if farmers are to be recognised in the regulatory and policy process, Wright-Stow says, with the wetlands project aims to do that through partnership science with the regional councils: "Ultimately, it's about stimulating as many farmers as possible to protect wetlands and further their enhancement."

It's not only farmers, their stock and their businesses that stand to benefit from more and cleaner wetlands; some of the country's least-known, least-seen residents will, too.

"Wetlands have massive biodiversity value and benefit," Wright-Stow says. "They're an extremely diverse system in terms of the number of plant, bird, animal and insect species they host – as well as being a wonderful carbon sink."