Efficacy of nitrogen and urease inhibitors in Scotland – an evidence review

Scotland’s Climate Change Plan makes a policy commitment to reduce greenhouse gas (GHG) emissions from nitrogen fertiliser through improved understanding, efficient application and better soil condition.

This report considers the potential for nitrogen and urease inhibitors to support emission reductions in Scotland, considering Scottish circumstances and conditions, such as soils, crops, rainfall and temperature.

These inhibitors are particularly important for those who are modelling both GHG emissions and air quality. However, while some studies provide consistent messages concerning the evidence of their effectiveness and their impacts on the wider environment, others are contradictory.

Main findings

The evidence indicates that:

  • There is generally a positive potential impact of inhibitors on GHG and ammonia emissions under Scottish conditions, especially for nitrification inhibitors.
  • There are no significant concerns over the efficacy of inhibitors in Scotland. Low uptake relates to the niche market; inhibitors are primarily supplied for agronomic benefit with relatively marginal economic gains in most circumstances.
  • While the efficacy of inhibitors has been confirmed by the review, there remain uncertainties over the magnitude of emissions reductions. There are also questions relating to the environmental risk, trade-offs with potential emission/pollution switching, industry knowledge and practical implementation.
  • The persistence of the effects for both nitrification and urease inhibitors are likely to be impacted by a warmer climate, although any impact is likely to be minimal. Emissions from unabated fertilisers are expected to increase as climate change progresses. Under these conditions, the role of inhibitors as a tool in mitigating emissions becomes increasingly important.

The evidence for environmental risks includes:

  • There is little evidence exploring the impacts of N inhibitors on soil health and on impacts to non-target and nitrifying organisms.
  • Use of nitrification inhibitors can lead to increases in ammonia emissions. However, alongside this, there are benefits for other environmental indicators (particularly GHG emissions and nitrate leaching). The potential increase in ammonia emissions can be mitigated by use of nitrification and urease inhibitors together.
  • Some research highlights the risk of DCD (dicyandiamide – a nitrification inhibitor) leaching into surface and ground waters. This can have adverse effects on aquatic systems.
  • There are concerns regarding animal consumption (directly or via traces found on grass/hay) as DCD has been found in dairy products in New Zealand. This led to DCD being banned in New Zealand.
  • Increased risk of ammonia release from use of nitrification inhibitors will have adverse impacts on ecosystem biodiversity through deposition and increased N loading to sensitive sites.

The main practical/commercial considerations are:

  • Nitrification and urease inhibitors are not widely used due to poor cost effectiveness under conventional economic analysis at farm gate (i.e. not considering externalities of environmental or societal costs).
  • In the agriculture industry, there remains significant misunderstanding over the roles and practical application of inhibited fertilisers.
  • Investment in nitrification inhibitors will not be driven by market pull. Stakeholders feel N inhibitors are not currently attractive prospects for increased investment.
  • Urease inhibitors are more commercially viable (compared with nitrification inhibitors) and have potential economic benefits due to the potentially high emissions of ammonia losing significant N content. Interest and awareness of urease inhibitors is greater.
  • Price sensitivity: farmers in the UK are very sensitive to fertiliser price and will seek the most cost-effective source of N. A perception of little or no economic value in inhibited fertilisers will discourage adoption.