Nitrification inhibitors slow the conversion of ammonium (NH4+), deposited into the soil in the form of urine, to nitrate (NO3), which leaches into waterways. Nitrous oxide (N2O) is released into the atmosphere as part of this process. DCD (dicyandiamide) is a nitrification inhibitor that has proven to be effective in reducing nitrate leaching while boosting pasture growth.
The NZAGRC has assessed DCD and a similar product, DMPP. Both were equally effective in reducing nitrous oxide emissions from urine patches in grazed pasture, with emissions reductions of about 60% under a range of conditions.
However, DCD was taken off the New Zealand market after the discovery of residues in milk. Future options to meet international trade requirements are being considered.
But nitrification inhibitors are not cost-effective if the only motivation for using them is to reduce greenhouse gas emissions. At an application cost between $100 and $250 per hectare, and given that DCD is effective only over a five-month period during winter, farmers would be spending more than $200 for every tonne of CO2-equivalent emissions avoided.
In some dairy catchments, however, nitrification inhibitors may be an important way to meet water quality requirements; emissions reductions would be a beneficial side effect. A technologically similar option, but based on current evidence without any residues, is the use of urease inhibitors.
These restrict the conversion of urea and urine to ammonium in the first place and are particularly effective in restricting emissions from applied N fertilisers. As they break down very quickly in the soil they are less effective in reducing emissions from urine patches where emissions can occur over a substantial period.
The cost and benefits for New Zealand-specific climate and soil types, and pastoral farming systems, are not well documented as yet.
Read more about other New Zealand nitrous oxide emissions mitigation research