New Zealand has had a reputation for being ‘clean and green’ – a country of environmental beauty. Compared to many countries in the world, this is true. However, it’s becoming apparent that New Zealand has an environmental pollution problem. An OECD (Organisation for Economic Co-operation and Development) report (2007) describes deterioration in water and air quality in New Zealand – due, in part, to agriculture.

Water quality

Water quality describes the condition of water and includes its chemical, physical and biological characteristics. Standards developed for water quality relate to the suitability of water for a particular purpose, for example, the water quality needed for healthy ecosystems (lakes, estuaries and rivers), human recreation (such as swimming) and drinking. Standards are being developed in New Zealand to rate the quality of water in our waterways.

Water pollution

Concerns have been raised about the effect of intensification of farming on water quality. Farmers add fertilisers to improve pasture growth to ensure animals have enough to eat. Fertilisers contain chemical compounds that add to the nitrogenphosphorus and potassium content of the soil. Effluent from farm animals also adds nutrients to the soil. Excess fertiliser not taken up by plants can be leached from the soil or carried away in run-off. The nutrients may end up in waterways and groundwater.

Eutrophication

Nutrients are carried into lakes, rivers and estuaries. This enrichment – particularly of nitrogen and phosphorus – causes excessive growth of aquatic plants. These plants take up room, making less room for fish. As the nutrients are used up, the plants die off. As they decay, they take up oxygen from the water, leaving less oxygen for other organisms and causing them to die off. This is known as eutrophication. Excessive plant growth also limits sunlight from penetrating the water, causing the death of benthic organisms dependent on sunlight (such as seagrass in estuaries). Animals that depend on these plants for food and shelter need to leave the area or die.

Toxic water

Excessive nitrogen in its nitrate form can become harmful when it gets into groundwater. When consumed, the water has been known to be toxic to babies and pregnant animals.

Erosion and water

Erosion from land clearance and other farming practices can add fine sediment to water, spoiling the aesthetic appeal of streams and lakes and making it unfit for swimming or drinking.

Contaminated soil

Soil is considered contaminated when the levels of hazardous substances are higher than that of ‘normal’ levels and there is likely to be a risk to the health of people, animals and the environment. Scientists set normal (or background) levels for specific areas of land at a standard deemed to be safe. Agricultural soils can be contaminated, for example, by the overuse or wrong use of fertilisers and pesticides.

Greenhouse gases

Gases that trap heat in the atmosphere are known as greenhouse gases (the atmosphere warms up like it does in a greenhouse). The gases allow sunlight to enter the atmosphere freely. When sunlight strikes the Earth’s surface, some of it is reflected back towards space as infrared radiation (heat). Greenhouse gases absorb this infrared radiation and trap the heat in the atmosphere. This stops us from being too cold and gives us a pleasant atmosphere in which to live.

Global warming

The concern among some scientists today is that more of these greenhouse gases are being produced than previously. In time, this causes an overall warming of the atmosphere known as global warming. The main contributors of greenhouse gases are water vapourcarbon dioxide, methane, nitrous oxideozone and fluorinated gases from industrial processes.

Agricultural emissions

Since 1990, New Zealand’s agricultural greenhouse gas emissions have grown by about 1% each year. In 2010, just under half of the total greenhouse gases were produced by agricultural activities (47.1%). Intensified farming has led to added emissions of nitrous oxide and methane in particular. Although there is more carbon dioxide in the atmosphere than other greenhouse gases (apart from water vapour), methane is 23 times more powerful than carbon dioxide at trapping heat, and nitrous oxide is 310 times more powerful!

Nitrous oxide is formed during the nitrogen cycle. Excessive nitrogen compounds in the soil – added through fertilisers or effluent – produce more nitrous oxide. Methane is a byproduct of animal rumination (released in sheep and cow burps) and organic decay.

Balancing food production with sustainability

There is no doubt that agricultural intensification can come at a cost to the environment. However, we need to balance this cost with the importance of food production. As the Earth’s population grows, so does the need for farmers and others to produce more food.

Using scientific research – such as nitrification inhibitors and denitrification beds – can help maintain water quality. Farm management practices such as fencing off streams are also effective. Other solutions may lie in regulations (such as the Resource Management Act 1991 to control nitrogen fertiliser use), market-based initiatives (such as pollution permits) and incentives to farmers to reduce their nitrous oxide emissions and nutrient losses.

Nature of science

The environmental concerns associated with farming are often the subject of media reports and emotive debate. It is the role of scientists to provide reliable information and data concerning farm practices and potential environmental impacts. Local and national governments use this information to develop policies and regulations.

Published 30 July 2013