Ammonia volatisation
Urease inhibitors
Urease enzymes Urease enzymes are produced by plants and microbes in the soil. They have an active site that can bind urea. Once urease and urea are bound, the urease enzyme transforms urea into ammonia and carbon dioxide. If the urea has not been washed into the soil, this results in ammonia volatilisation. When ammonia and carbon dioxide leave the active site, the site is free to once again convert (hydrolyse) another urea molecule and the process can start over again.
How to reduce ammonia losses There are three main ways nitrogen losses from ammonia volatilisation can be minimised: 1. Rainfall or irrigation (> 10 mm) : Suffcient rainfall or irrigation will help wash the urea into the soil and buffer the pH spike, hence minimising volatilisation. However, considering the last couple of dry springs, ensuring a suffcient rain event shortly after application can be a challenge, whilst irrigation isn’t feasible for the majority of UK crops. 2. Incorporation (tilling > 10 cm) into the soil: Incorporation also helps minimise volatilisation. However, for winter crops, where sowing and fertilising are conducted in different seasons, tilling is not practicable. 3. Urease inhibitors: A reliable and pragmatic way to consistently reduce ammonia losses is to use urea-containing fertilisers with a urease inhibitor. How urease inhibitors work Urease inhibitors (eg. NBPT) temporarily inhibit urease enzymes from converting urea into ammonia - until the urea has been suffciently washed into the soil. They do this by binding to the urease enzyme, preventing urea from binding to the active site, delaying the hydrolysis process and hence minimising volatilisation.
Urea binds to the urease enzyme
Active Site
Urea
Urease enzyme
The urease enzyme releases the ammonia and carbon dioxide
The urease enzyme transforms urea into ammonia and carbon dioxide
(NH 3
)
(CO 2
)
Ammonia (NH3) volatilisation
Ammonia (NH3)
Carbon dioxide (CO2)
NBPT blocks the urease enzyme, slows down urea hydrolysis and hence ammonia volatisation.
Standard urease inhibitor
Volatilisation Urea granules are hygroscopic meaning they absorb moisture from the air and can begin to move into the soil even in the absence of rainfall. Once moisture is present, the urea is no longer stable. It’s at this point the urease enzymes start the hydrolysis process, converting urea into ammonium. As ammonium is alkaline, this conversion process temporarily raises the pH of the soil around the urea granule. If the pH spike is not buffered by rainfall, it results in ammonia volatilisation.
Urea granule
pH 8.5
Standard urease inhibitor
Urea
Urea
Urease enzyme
Urease enzyme
Different urease enzymes require different urease inhibitors
pH 6.0
The challenge with standard urease inhibitors Soils differ in their urease enzyme composition and urease activity. A broad range of organisms in soil (bacteria, fungi and plants) all produce slightly different urease enzymes. These different urease enzymes require different urease inhibitors, meaning some will remain active despite the use of a standard urease inhibitor.
Ammonia loss is a localised reaction
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