Manuka joins the battle to protect waterways

New research indicates planting mānuka could reduce the impacts of farming on waterways, as well giving farmers an additional income stream.

In addition to the fertiliser applied to support pasture growth, animal urine can add nitrogen to soil at rates in excess of pasture’s ability to use it. Plant & Food Research scientists have contributed to a collaborative programme, led by University of Canterbury professor Brett Robinson which found that, after heavy application of urea, the soil around mānuka and kānuka trees contained dramatically less nitrate than around radiata pines used for comparison.

The research team planted young mānuka, kānuka and radiata in Plant & Food Research-funded lysimeters, containers set up to measure drainage and evapotranspiration from the soil. The soil was fertilised with urea at the recommended maximum rate for pasture land, 200 kilogrammes a hectare, for 15 weeks. Then the team applied the equivalent of 800 kilogrammes per hectare to each pot to similate urine patches. On grazed land, animal urine adds nitrogen at rates up to 1000kg a hectare, contributing up to 70% of nitrate leachates.

They also installed gas sampling devices to measure nitrous oxide emissions from the soil and took drainage samples at set intervals. The research found less nitrous oxide gas escaped from the mānuka and kānuka soil than from the pine pots. It also found that just 2kg of nitrates a hectare drained from the mānuka and kānuka pots, compared with 53kg a hectare from the pine tree pots.

Farmers have previously used pine as a supplementary crop but log prices have decreased, while the markets for mānuka honey and essential oils have increased, making it another option for supplementary plantings. The potential to reduce nitrogen leachate and improve downstream water quality provides an additional benefit for planting or preserving mānuka.

Research leader Lincoln University scientist Dr Juergen Esperschuetz says the results add to the growing interest in incorporating native species into farming systems to support biodiversity, and protect surface waters.

Plantings could be sited to shield waterways, and in areas known to have high concentrations of effluent, or adjacent to those areas to take runoff. He says planting or growing mānuka and kānuka as shelter would also improve animal welfare and, as animals spend more time there, those sheltered spots would naturally receive the most effluent.

“These results show mānuka and kānuka could be even more effective at protecting water systems than anyone expected,” he says.

Environmental chemistry professor Brett Robinson says better use of manuka and kanuka could offer “an all-round win”.

“Using native mānuka and kānuka as part of a farm system could support biodiversity, nutrient cycling efficiency, animal welfare and farm income,” he says. “It could contribute to sustainable agriculture.”

Plant & Food Research’s Dr Craig Anderson says the difference doesn’t come from mānuka and kānuka taking up more nitrogen but from the effect of the trees on the microbes in the soil.

“Manuka and kanuka seem to have an inhibiting effect on the microbes in the soil that mediate the nitrogen cycle,” he says.

Microbes do most of the work in the nitrogen cycle, converting urea to ammonia, ammonia to nitrites, then to nitrates, then to nitrogen gases. Reducing the activity of the microbes in the soil deprives other plants of nitrogen giving mānuka and kānuka a competitive advantage over other plants in low quality soils. Professor Robinson says it was mānuka’s antibiotic properties that prompted the experiment to see what effect mānuka had on microbes  in the soil.

The study also showed the mānuka species also responded better than the pine trees to the application of urea. They grew more vigourously, which is beneficial in a supplementary crop.

Plant & Food Research scientist Dr Simon Bulman says it’s not known what effect this might have on essential oils but this is already the subject of another study.

The collaborative study was supported by the Deutsche Forschungsgemeinschaft, Plant & Food Research, ESR and the Centre for Integrated Biowaste Research (CIBR).

Professor Robinson says the next step is to examine the effect in field trials. CIBR is setting up trials at four sites around New Zealand, testing both the environmental and economic benefits of manuka-dominated ecosystems for waterways protection.