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DCD-based nitrification inhibitors were voluntarily withdrawn from the market in early 2013 after traces were found during routine milk testing.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.

Transcript

DR ROSS MONAGHAN

What we’ve been trying to do is manage that risk of nitrogen loss in drainage – we use the term ‘nitrogen leaching’ – and one of the ways is by adding a chemical to our pastures that is either sprayed on as a liquid or applied as a granular product that diffuses through the soil. It’s targeting those urine patches to keep the nitrogen there in the immobile ammonium form.

Nitrification inhibitors are products that prevent the oxidation of ammonium to nitrite. It appears as though that’s a fairly harmless interference in the nitrogen cycle. All it’s doing is keeping the nitrogen in the ammonium form, which the plants can use, and preventing the accumulation of the nitrate, which carries more environmental risk because it can be lost in drainage or as a gaseous form.

It’s early days in the development of this technology, and what we’ve found is that the inhibitors work well in some places and less well in others, so there’s a lot of research going on to try and allow us to understand what are the main driving factors or influencing variables that will maximise the efficiency and make sure we get good value for money where we apply it.

Some of the key regional factors that are influencing the effectiveness of the inhibitor are rainfall. Too much rain or too much drainage really washes that inhibitor out of the soil, which means, on the West Coast, it’s hard to keep it in the topsoil where we need it long enough for it to be effective. Another regional factor is temperature, and what we’ve found is that, where it’s very warm and moist, you have a lot of microbial activity in the soil, and those microbes break down the nitrification inhibitor relatively quickly.

So the good news in the south here is that, because it’s relatively cool, not too wet, the inhibitor doesn’t break down or leach out of the soil as quickly as in warmer wetter parts of the country. So for the southern South Island at least, it appears as a reasonably cost-effective way of trying to minimise that nitrate leaching risk.

At the moment, the uptake of the nitrification inhibitor technology is relatively slow, but that might change in the next 10 or 20 years as the regional councils develop policy that really tries to manage the nitrogen losses that are occurring, and that’s against a backdrop of this on-going intensification of agriculture and land use that is really driven by economic drivers and opportunity.

Acknowledgements: Dr Ross Monaghan, Dr Selai Letica and Belinda Glass, AgResearch, Invermay Professor Louis Schipper