Enhanced efficiency fertilisers (including nitrification inhibitors and urease inhibitors and slow release fertilisers have been developed to increase the efficiency of fertiliser use by crops. Currently Nitrogen use efficiency is fairly low from what is applied to what the crop takes up, as such research has been targeted to try and increase the efficiency percentage and minimise the risk of losses of nitrogen (either through nitrate leaching or nitrous oxide emissions).
Nitrification inhibitors are compounds which delay the bacterial oxidation of NH4+ (ammonium) by depressing the activities of nitrifiers in the soil.
Urease inhibitors are compounds that delay the hydrolysis of urea.
Slow release fertilisers show the rate of nutrient release through coating or chemical modification of the fertiliser itself.
What do they do?
Science has studied these compounds intensively and findings indicate that they can be effective in increasing nitrogen use efficiency and have other benefits such as reducing labour and fuel costs and reducing the incidence of nitrate leaching. The information below comes from a review paper which looks at different experiments on these compounds and tries to draw some conclusions as to their effectiveness in the field.
The IPCC 3rd Assessment report confirmed that management of Nitrogen Fertiliser by the use of Nitrification inhibitors, slow release fertiliser and organic manure could tentatively cut nitrous oxide emissions from nitrogen fertiliser use by 30% on a global scale. The next (4th ) Assessment report looking at nutrient management including the technologies described above in more detail and concluded that the mean mitigation potential of nitrous oxide through nutrient management was 0.07tonnes of carbon dioxide equivalent per hectare per year.
What has the research looked at
Nitrification inhibitors have been the most widely studied as a mitigation option for nitrous oxide emission from agricultural soils. There have been some studies that have looked at the use of polymer coated, and sulphur coated fertilisers and a few using urease inhibitors.
Despite all these published field experiments, it is difficult to draw general conclusions because the performance varies depending on soil and climatic conditions and field management strategies. The authors of the paper looked at different studies and categorised results using different land use and type of fertiliser applied. This study also only included data from field experiments (rather than ones in a lab that were grown in pots).
What did they find out?
On average nitrification inhibitors significantly reduced nitrous oxide emissions compared with conventional fertilisers. The effect of nitrification inhibitors on nitrous oxide emissions also varied with land use type, with grassland having the best average reduction in N2O emissions of 60%. The coated fertiliser also significantly reduced N2O emissions compared with conventional fertilisers. When the study looked at urease inhibitors, the effect on emissions was not significantly different from the control treatments.
How do nitrification inhibitors work?
Both nitrification and denitrification are important pathways for nitrous oxide production in soil. They work by inhibiting ammonium monooxygenase, thereby blocking the first reaction or ammonium to nitrite. By minimising the rate of nitrification until the primary crop is in its log phase of growth. Nitrification inhibitors can give the crop a better opportunity to absorb nitrate and increase nitrogen use efficiency. By suppressing nitrification, inhibitors potentially reduce subsequent denitrification and nitrate leaching, thus reducing N2O emissions. The effectiveness of nitrification inhibitors was found to be more consistent compared with coated fertilisers.
What affected the field experiments?
Environmental factors such as temperature, alter the effectiveness of nitrification inhibitors in the field. They also differ in terms of how water soluble they are and their volatility. One product used in America found that using it:
Increased crop yield by 7%
Improved soil nitrogen retention by 28%
Reduced nitrate leaching by 16%
Reduced nitrous oxide and methane emissions by 51% (compared with conventional N fertiliser)
Practical considerations with Nitrification Inhibitors
In general higher nitrous oxide emissions were observed from grassland than the other land uses in the studies. Nitrification inhibitors were more effective in reducing nitrous oxide emissions from grassland compared with their effect on other crop types.
Another advantage of nitrification inhibitors is that they can be used with both chemical and organic fertilisers, whereas coatings can only be used with bagged Nitrogen. Nitrification Inhibitors are effective in reducing nitrous oxide emissions from chemical and organic fertilisers and the consistent effect indicates that they are a potent mitigation option for future emissions.
Coated fertilisers (PCF)
These work by releasing nutrients by diffusion through a semi-permeable polymer membrane and the release rate can be controlled by varying the composition and thickness of the coating. PCFs can be effective in increasing nitrogen use efficiency and can substitute for split applications thus reducing the requirement for multiple field operations and in turn reducing labour and fuel costs.
When nitrogen release from PCFs is well synchronised with plant demand, PCFs have the potential to reduce nitrogen losses to the environment, such as nitrate leaching and nitrous oxide emissions. In contrast, nitrogen use efficiency can be reduced significantly and environmental losses increased when nitrogen released from PCF doesn’t match plant demand. The effect of PCFs on nitrous oxide mitigation showed varying results depending on land use, crop and soil type.
These slow the conversion of urea to ammonium and hence reduce the concentration of ammonium present in the soil solution (and the potential for that ammonium to be volatilised as ammonia). Together with uptake by plants, a lower concentration of ammonium in the soil can result in less nitrogen potentially undergoing subsequent nitrification and denitrification. One of the drawbacks of urease inhibitors is that they only delay the hydrolysis of the urea and the urea will eventually be hydrolysed and become ammonia. These compounds have not been as widely tested as the other two groups of inhibitors, and as such more studies are needed.
Take up by the industry
Although these products have been well researched, there is limited uptake of them in the field. At the moment, the uptake of the nitrification inhibitor technology is relatively slow, but that will change in the next 10 or 20 years as policies are developed that try to manage the nitrogen losses that are occurring and improve nitrogen use in the field. As farmers taking account of application timing, source of nitrogen being applied, application method, soil texture, and tillage are all factors that should be evaluated to determine how efficiently Nitrogen is being used in the system.