Inhibiting nitrification
Dr Ross Monaghan from 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 off other plants and organisms. As the plants (or algae) die, they use up oxygen from the water, also killing off other organisms that need oxygen to live. This process is called eutrophication.
Nutrient leaching
Dr Ross Monaghan from AgResearch at Invermay in Otago 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 this involves interfering with the nitrogen cycle 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. Other bacteria in the soil are capable of converting ammonium (NH4) 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 inhibitors
Dr Ross Monaghan from AgResearch at Invermay in Otago explains his work in managing the risk of nitrogen loss to water systems through the use of nitrification inhibitors. Nitrification inhibitors prevent part of the process of the nitrogen cycle from occurring – limiting the production of nitrates that can be lost to the environment.
Jargon alert
In the nitrogen cycle, gaseous nitrogen enters the soil-plant system via fixation. In this process, N2 is converted to ammonium. Through some other steps, ammonium is converted to nitrate. Nitrate is mobile and gets into waterways through run-offNitrogen inhibitors are chemicals that prevent the oxidation from ammonium to nitrate taking place, limiting nitrate loss.
Point of interest
DCD-based nitrification inhibitors were voluntarily withdrawn from the market in early 2013 after traces were found during routine milk testing. Because DCD is not considered to be a food safety risk, there is no international standard for allowable amounts found in food products. Without this standard, some countries use a default limit of zero. DCD traces in milk are seen as a trade risk rather than a health risk. Ongoing research led by AgResearch scientists has identified a new inhibitor that shows similar efficacy to DCD, but without the same risks. The new inhibitor is undergoing longer-term field trials.
Ross is researching a product that prevents the oxidation from ammonium to nitrate, preventing nitrification from taking place. The product – referred to as a nitrification inhibitor – inhibits or blocks 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 can be sprayed on or applied as a granular product. However, it seems that the inhibitors work better in some places than others. Research is showing rainfall, temperature and microbial activity all have an effect on the inhibitors. The inhibitors work 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 tend to break down inhibitors more quickly so they need to be applied more frequently.
The use of the nitrification inhibitor DCD (dicyandiamide) 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 notes 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.
Urine patches
Ross and the scientists at AgResearch have found that urine patches left by cows (and sheep to a lesser extent) contain concentrated amounts of nitrogen. Ross has targeted urine patches in his research – applying nitrogen inhibitors to these areas in particular.
Nature of science
Intensified farming has impacted negatively on the natural way in which nutrients like nitrogen cycle through a given ecosystem. Imaginative thinking, often of the ‘outside of the square’ variety, has enabled scientists to develop ways of mitigating the heavy nitrogen load that often accompanies intensive farming.
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.
