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Huge reduction in nitrogen leaching when Miscanthus is used

19 September 2018

Huge reduction in nitrogen leaching when Miscanthus is grown as an effluent disposal crop.

One of the negative effects that pastoral agriculture can have on the environment occurs through the leaching of nitrogen into groundwater. Most members of the public do not understand how or why this is important but those that do are justifiably concerned. However it must be remembered that nitrogen leaching into the water table and streams does not just occur under pastoral agriculture. It also results from District Council treated sewage effluent disposal systems, along with various food processing sites – such as meat works, cheese factories, milk processing plants, breweries, etc.

There is a lot of publicity given to well-intentioned efforts by individuals, farmers, businesses and various governmental and NGO entities, to reduce nitrogen leaching. Some of these efforts are credible but many of them not. A recent conversation with a senior person in Fonterra indicated that Fonterra is completely aware that much of the riparian planting of indigenous species on farms has little or no effect on reducing nitrogen and phosphorus leaching into groundwater and streams. But such planting gives people a nice warm feeling because it looks as if they were doing something useful.

Recently, Miscanthus New Zealand Limited (MNZ) was concerned to note that the Rotorua Land Use Directory, in data publicising a $1.5 million land use fund which is aimed at reducing nitrogen leaching, said that they consider any leaching of less than 18 kg of nitrogen per hectare per year to be low. A recognised benchmark for low leaching from indigenous forest and pine forest with just normal rainfall is more like 2 - 3 kg of nitrogen per hectare per year. But even that is not really low when compared with what can be achieved with Miscanthus stands.

Fonterra has done some excellent research with a 2 ha Miscanthus stand that is owned by MNZ and located on land near Darfield. At that site, high nitrogen dairy factory washings water is being spray irrigated onto both pasture and the Miscanthus stand, with lysimeter studies measuring the amount of leaching taking place. The research shows that once the Miscanthus is established properly, the leaching of nitrogen from the Miscanthus stand is less than 0.5 kg per hectare per year. And that is with 70 kg of nitrogen per hectare per year being applied. International research experience also demonstrates low leaching from Miscanthus stands. But Overseer – the computer program on which many government officials rely - is not even aware of the value of Miscanthus in reducing leaching. However, some years ago in Taupo, the Lake Taupo Protection Trust accepted growing Miscanthus as a low nitrogen land use.

Peter Brown, managing director of MNZ said that he is keen for this research data to be accessed by more people so they can understand the opportunities presented by use of Miscanthus stands as a receptor crop for high nutrient effluent application. The published paper on this work can be accessed by contacting either Brown at MNZ or Fonterra’s Environmental Science department in Palmerston North.

Brown commented “There is a huge opportunity here for farmers, local authorities and others who have to dispose of effluent, to produce a commercially useful crop while at the same time significantly reducing the amount of nitrogen leaching into groundwater. In addition to the Fonterra work, we know of a proposal for use of Miscanthus as a component in a land-based wastewater treatment system. This was drafted by Tony Rhodes of PGG Wrightson. It was done independently of MNZ and has been submitted to the Tararua District Council for consideration. The same approach could be taken with any other District Council in New Zealand that wishes to avoid having all the nutrients in their treated sewage effluent ending up in the groundwater or the local river.”

The number of commercial end uses for the annually harvested Miscanthus product is already large and is expanding all the time. So this is a unique opportunity for local authorities, industries and individuals that have a high nutrient waste disposal issue to address. By planting Miscanthus stands to receive their effluent disposal they can make a positive change on behalf of the New Zealand environment while at the same time creating a commercial useful product. When the greenhouse gas benefits of growing Miscanthus are also counted, even more people will realise that this is a crop with a very bright future.


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