A prickly culprit

It would seem that agricultural activities are not the only things polluting our waterways with nitrogen. It has recently been discovered that gorse, that prickly green plant with the yellow flowers, is responsible for leaching as much nitrogen into our waterways as some dairy farms. Nitrates (compounds that contain nitrogen and oxygen) are major nutrients that damage water quality – previously we had thought that the main sources of nitrates were from agricultural fertilisers and cow poop being washed into waterways either directly or from soil leaching and groundwater movement.

Dr Guna Magesan, a senior scientist with Crown Research Institute Scion (formerly the Forestry Research Institute), recently completed a two-year study on nitrogen leaching from older gorse stands in Rotorua lakes catchments. He found that, in the areas studied, gorse leaches 40 to 60 kilograms of nitrogen per hectare per year through the soil into underground waterways and on into the region's lakes. On a per hectare basis, this is similar to the levels of nitrogen leaching from dairy farms.

In contrast, a stand of pine trees (Pinus radiata) in the control area produced less than 1 kilogram of nitrogen per hectare annually over the same period.

Dr Magesan’s research covered two catchments in the Rotorua lakes region. "Our study concentrated on nitrogen, which is an important growth stimulant and contributes significantly to algal bloom and weed growth in water bodies, particularly lakes. This, in turn, can cause environmental hazards, for example, making the water harmful for drinking, unsafe for recreation and uninhabitable for aquatic life.

Dr Magesan says that water eutrophication is a particular problem in the Rotorua area, but that the focus has been on agriculture and other sources as the cause, with very little research on gorse and other leguminous weeds, such as broom.

Gorse is widespread in New Zealand, it is highly invasive and fast growing, and has already taken over some 900,000 hectares nationwide. Each plant can live for more than 40 years, and seeds can remain viable in the soil for up to 30 years.

"My hope is that these results will motivate interested parties such as the Ministry of Agriculture and Forestry, the Ministry for the Environment and the Department of Conservation to take another look at how we can reduce the detrimental effect of such weeds. Areas for research could include not only eradication methods, but also alternative uses for gorse, such as new materials or bioenergy."

Environment Bay of Plenty commissioned and funded Dr Magesan's study in the Rotorua area. Dr Magesan hopes to now extend his research to other areas of New Zealand to see where else gorse is contributing to eutrophication.