Inhibiting nitrification
Dr Ross Monaghan, a prinicpal scientist with Bioeconomy Science Institute (formerly AgResearch) at Invermay, is concerned about nutrients from the soil affecting waterways and causing water pollution. This includes excessive levels of nitrogen in groundwater and in surface water (where eutrophication may occur in the latter).
Animal urine patches
Animal urine patches are patches of nutrient-rich grass (dark green) caused by urine deposits of cows (and sheep to a lesser extent). These patches contain concentrated amounts of nitrogen.
Nitrogen – a nutrient
One of the nutrients Ross is researching is nitrogen. Nitrogen is important for all life and encourages the fast and healthy growth of plants. Animals also need nitrogen for healthy growth. They get their nitrogen from the plants they eat.
Agricultural pastures and crops need nitrogen from the soil to be healthy. Soils deficient in nitrogen are replenished with nitrogen derived from clover fixation, fertilisers or effluent.
Too much nitrogen
The problem arises when there is more nitrogen in the soil than can be used by the available vegetation or converted by bacteria to atmospheric nitrogen (N2). The excess nitrogen compounds are leached from the soil (through rain) and/or are transported by surface run-off into waterways. Once in the waterways, the nitrogen compounds can get into groundwater or into streams, lakes and estuaries. Excessive nitrogen (nitrate NO3-) in drinking water can be harmful for animals and humans – particularly babies. Excessive nitrate in water can also cause an abundance of aquatic plant or algal growth. These plants can choke waterways, killing other plants and organisms. As the plants (or algae) die, they use up oxygen from the water, also killing other organisms that need oxygen to live. This process is called eutrophication.
Nutrient leaching
Dr Ross Monaghan describes the problems caused by nutrients leaching into water systems. He also explains why winter is a high-risk period for nitrogen leaching.
Ross’s research involves exploring ways to manage the risk of nitrogen leaching from the soil into waterways. One aspect of his research involved the use of nitrification inhibitors to slow down the production of nitrate that has the potential to be harmful.
Nitrification inhibitors
In the nitrogen cycle, nitrogen from the atmosphere enters the soil-plant system via fixation by legumes. Special bacteria attached to the roots of the legume plants process this nitrogen to form ammonium (NH4+). Other bacteria in the soil are capable of converting ammonium to unstable nitrite (NO2-) and then to more stable nitrate (NO3-) that can be readily taken up by plants. This is known as nitrification.
Nitrification
Nitrification is one of the steps in the nitrogen cycle where ammonium is converted to nitrate. First, ammonium is converted to nitrite by specialised bacteria. Other bacterial species are responsible for the conversion of nitrite to nitrate. Nitrate is easily taken up by plants, but it is also susceptible to loss because it doesn’t stick to soil and is quite mobile. It is easily leached from the soil through rain.
Nitrification inhibitors prevent nitrification from taking place by supressing the action of the nitrifying bacteria. They inhibit or block the enzymes in the bacteria from converting ammonium to nitrite and nitrate. Consequently, much less mobile nitrogen (nitrate) is produced. Ammonium has a tendency to stick to soil and is less mobile than nitrate so is less likely to leach into waterways.
Nitrification inhibitors are available globally and are viewed as a mitigation tool to prevent nitrogen losses and reduce greenhouse gases. Ross and others at AgResearch investigated a commonly used inhibitor called DCD (dicyandiamide) on New Zealand pastures. Ross wanted to know how this product worked in New Zealand’s various weather conditions.
Their research showed that rainfall, temperature and microbial activity all have an effect. The inhibitors worked more efficiently in the cooler South Island where there is often less rainfall and less microbial action because of lower temperatures. Warmer weather, higher rainfall and microbial activity tended to break down inhibitors more quickly, so they needed to be applied more frequently.
DCD suspended
DCD use in Aotearoa was voluntarily suspended in 2013 when very small traces of residue were unexpectedly detected in milk. Although the residue posed no food safety risk, the Ministry for Primary Industries noted that international dairy customers expect New Zealand products to be residue-free. Ongoing research led by AgResearch scientists has identified a new inhibitor that shows similar efficacy to DCD, but without the same risks. The inhibitor is undergoing longer-term field trials.
Ross continues to specialise in nitrogen cycling and working on ways to reduce nutrient and sediment losses to waterways.
Nature of science – economic trade-offs
One aim of science research is to produce marketable products of economic, social or environmental value. Although DCD is globally reported to be one of the most effective tools for reducing N2O emissions and for effectively mitigating nitrogen losses, significant trade implications for the New Zealand dairy sector were an important factor in its suspension.
Related content and activity ideas
Learn more about the nitrogen cycle in this article and interactive.
Take a closer look at dairy farming and the nitrogen cycle with this article and interactive.
This article looks at the environmental impacts of farming.
This article looks at ways in which dairy farmers look after the land and waterways.
Students may enjoy experimenting with components of the nitrogen cycle in the student activity Nitrification and denitrification.
Nutrient pollution has two practical activities to help students understand how nutrients are washed off the land and into water systems.
Useful links
DCD use was voluntarily suspended in January 2013. DCD does not present a food safety concern, but residues found in milk may present a trade issue. Find out more in this news story.
Research continues to identify novel nitrification inhibitors. Find out more on the NZAGRC website.
DairyNZ has information on some of the things that can be done on the farm to improve water use and/or maintain waterbodies and wetlands.
