Understanding Nitrogen Inhibitors

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Last year, Defra introduced new regulations regarding ammonia mitigation for urea fertilisers, which also apply to liquid UAN fertilisers. According to these new rules, starting in 2024, any fertiliser that contains urea, including UAN (with the exception of urea solutions used for foliar applications), must incorporate a urease inhibitor if it is applied after April 1st each year. This requirement is intended to slow the release of ammonia and reduce volatilisation losses. These guidelines are outlined in the Red Tractor Assurance Standard, which allows members to:
Use untreated or unprotected urea fertilisers only from January 15th to March 31st each year. Use urease inhibitor treated or protected urea fertilisers for the remainder of the year.

Note that these standards currently apply only in England. Although adding an inhibitor to nitrogen becomes more expensive, it can help improve fertiliser efficiency, as more of the nitrogen remains in the soil for plant uptake rather than being released into the atmosphere.

To understand nitrogen inhibitors, it’s important to know how the nitrogen cycle in the soil works and what happens when nitrogen fertilisers are applied:

As shown in the diagram, plants can uptake nitrogen in either ammonium or nitrate form. Ammonium is temporarily held onto soil particles through electrical charges whereas nitrate is free to move via the water in soils. Through the processes of nitrification, ammonium is converted to nitrite by Nitrosomonas bacteria and then to nitrate by Nitrobacter bacteria. Fertilisers that contain ammonium and/or nitrate nitrogen therefore have a reasonably direct path to plant roots.

However, urea fertilisers have an extra step involved before arriving at the ammonium stage for plant roots to receive nitrogen:

When urea or urea ammonium nitrate (UAN) is applied to the soil it is converted to ammonia gas by a process called hydrolysis. Ureolytic bacteria are the workers which are responsible for producing urease enzymes and together with nickel, the hydrolysis process is performed. However, this process can occur rapidly under certain conditions, such as higher temperatures, lighter soils, and alkaline soils (pH > 7.5). This can then lead to significant nitrogen losses by as much as 58% through ammonia volatilisation.

Urease inhibitors slow the hydrolysis process down by reducing the activity of urease enzymes. At the moment, most of the urea inhibitors in the UK market work on this process to thereby reduce ammonia emissions and qualify for Defra’s emissions target. These common inhibitors are known as NBPT (N-(n-butyl) thiophosphoric triamide) and are found in products such as Sustain, Limus, Nitroshield, N-Dual Protect and Amiplus. Limus goes an extra step further with two urease inhibitors NBPT and NPPT which blocks several types of urease enzymes and claiming to reduce ammonium emissions by 98%.

As the names suggests, nitrification inhibitors are involved in the process of ammonium converting to nitrate and therefore are not confined to urea fertilisers. As shown in the first diagram, another process in soil that can lead to nitrogen losses is the conversion of ammonium to more leachable forms of nitrogen, such as nitrates and nitrites. This process also produces emissions in the form of nitrous oxide, another greenhouse gas. The inhibitors in this category are DCD (dicyandiamide), DMPP (dimethylpyrazole) and nitrapyrin which work by stalling the nitrosomanas bacteria to convert ammonia to nitrite. Didin and N-Dual Protect are products containing DCD with N-Dual providing the benefit of both urease and nitrification inhibitors. Nitapyrin is found in the product Instict, which claims inhibition for up to 8-12 weeks depending on the soil temperature.

Urease inhibitors tick the box for Defra’s ammonia emissions target reduction of 16% by 2030. They reduce ammonia emissions for 1-2 weeks, improve nitrogen use efficiency and are particularly useful when soil temperatures are higher. Nitrification inhibitors on the other hand reduce nitrous oxide emissions which are not currently on Defra’s targets for agriculture but will be an important tool for doing farm carbon budgets. They are highly versatile as they effectively complement any fertiliser that contains ammonium, including slurry, digestate, sludge, urea, UAN, and ammonium nitrate. While nitrification inhibitors don’t reduce ammonia losses, they are useful for where there is a risk of leaching, such as early in the season and will last for several weeks.

Due to the nitrogen cycle being reliant on soil biology, nitrogen fertilisers and their associated inhibitors are going to work better in aerobic conditions and where good carbon levels are
maintained. Although the urease inhibitors are short lived, the continuous long-term use is not clear and many of these products are inorganic. A five year study of inhibitors has shown to
have no impact on soil bacterial and fungal populations or structures, however this study was only tested on grassland. In an arable study, a no-till soil showed an increase in the activity of
urease enzymes compared to a ploughed soil. Origin Enhanced Nitrogen (OEN) a protected urea, is the highlight in terms of soil health as it is an organic compound which has shown to
provide a 10% improvement in soil bacteria and no impact on earthworms. Research and trials on nitrogen inhibitors generally indicates better results when the soil is in the right balance with air, moisture and organic matter. This is where carbon sources like Citadel and Nurture N used in a regular program can help promote favourable soil health to achieve the best results from these inhibitors and fertilisers. At Aiva we are currently researching and trialling natural products which may provide reduced emissions from nitrogen fertilisers as well as promoting soil health. We will keep you informed of any progress we make.