21 Sep 2017

Mānuka roots found to assist water quality

4:15 pm on 21 September 2017

A new study has found that mānuka and kānuka plants reduce nitrate leaching into waterways.


Manuka Photo: Supplied / Bayleys

Researchers planted young mānuka, kānuka, and radiata pine trees in containers, called lysimeters, which measure drainage and evapotranspiration from the soil, and then fertilised the plants with urea for 15 weeks.

They applied the equivalent of 800kg per hectare to each pot to simulate urine patches, because research shows that on grazed land, animal urine adds nitrogen at rates up to 1000kg a hectare, contributing up to 70 percent of nitrate leachates.

The study found that after heavy application of urea, the soil around mānuka and kānuka trees contained a lot less nitrate than around the pines.

The programme is led by University of Canterbury Environmental chemistry professor Brett Robinson and is a collaboration between the German Research Foundation, Plant and Food Research, the Institute of Environmental Science and Research and the Centre for Integrated Biowaste Research.

The lysimeter buckets used to measure leeching.

The lysimeter buckets used to measure leaching. Photo: Supplied

Professor Robinson said they had found that mānuka and kānuka have interesting and unusual properties that reduce the amount of contaminants that move through soil and into waterways.

"They significantly reduce nitrate leaching, they attenuate pathogens, and we have good evidence that they also reduce run-off - including phosphorus and sediment that would otherwise contaminate waterways."

Professor Robinson said because native plants are commonly used for waterway protection, the researchers decided to look at the root zones.

"We thought about mānuka and kānuka because these are plants adapted to very low fertility soils and we thought these plants might have a selective advantage to holding nutrients, particularly nitrogen in their root zone."

He said mānuka and kānuka significantly reduced nitrate leaching in both high and low nutrient environments.

"We were surprised, we didn't expect plants such as mānuka, which is adapted to low soils, would have this effect when there is a lot of nutrients going in, for example, on a farm margin."

The researchers don't want people to only plant mānuka and kānuka, but suggest ecosystems dominated by these plants.

"The cost of planting can be offset by honey production or indeed essential oils."

Professor Robinson said there was a lot more research to be done and the next step would be to examine the effect in field trials.

People could plant mānuka and kānuka not just beside waterways, but also in 'critical source areas' where contaminants collect, such as dairy farm tracks, Professor Robinson said.

He said the aim of the programme was to improve water quality in rural and urban areas and find affordable solutions for land owners.

See the full study here

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