Some compounds added to urea or urea-containing fertilizers can reduce the rate of the first “hydrolysis” step, and slow the rate of
ammonia production. Under certain conditions, this can help
reduce ammonia loss
to the atmosphere.
Urease Enzymes and Nitrogen Loss from Urea
Urea is the most widely used form of N fertilizer, and can be formulated as dry granules, prills, or as a fluid alone or mixed with
ammonium nitrate (UAN). Urea is also present in animal manures. All these forms of urea have the disadvantage of undergoing considerable losses as
ammonia gas if not incorporated into soil
soon after application.
Once dissolved in water, urea is converted to ammonium bicarbonate within
a few days following application by the naturally occurring enzyme,
urease. Urease is produced
by many soil microorganisms and plants, and is present
in nearly all soils.
When urea is hydrolyzed by urease,
much of the resulting ammonium is held on
soil
cation exchange sites. During the conversion, the pH temporarily rises and ammonia gas is produced. The loss of ammonia, termed volatilization, can be from nil to over 50%.
(NH2)2CO + 2H2O => 2 NH4HCO3 => 2 NH3 + H2O + CO2
Ammonium carbonate Ammonia gas
The factors conducive to N loss as ammonia from urea are: surface application, less than10 mm (0.4 in.) of rainfall and/or irrigation in the first few days after application, presence of
crop residues, open crop canopies,
high temperatures, high soil pH and low cation exchange capacity soils. Moving the applied urea below the soil surface with tillage or through rainfall
and irrigation also effectively minimizes ammonia
loss from urea.
Urease inhibitors are used to temporarily reduce
the activity of the enzyme and slow the rate at which urea is hydrolyzed. There are
many compounds that can inhibit
urease, but only a
few that are non-toxic, effective at low concentrations, chemically
stable and able to be mixed with or coated onto urea-containing fertilizers.
The most widely used urease inhibitor is N-(n-Butyl) triphosphoric triamide (NBTPT), which converts to active NBPT (N-(n-Butyl)
phosphoric triamide). Other widely studied urease inhibitors include phenylphosphorodiamidate (PPD/PPDA) and hydroquinone.
Ammonium thiosulfate and some metals can also inhibit urea hydrolysis. There are many other organic compounds, especially structural analogues of urea, capable of inhibiting urease.
Management Practices
Urease inhibitors are potentially useful tools for controlling or reducing gaseous losses of ammonia following fertilization with urea. They can restrict urea hydrolysis for up to 7 to 14 days, after which rain, irrigation,
or
soil mixing would be required to further restrict ammonia losses.
Because the magnitude of ammonia
loss varies with soil type, climate and crop cover, the reduction due to the use of a
urease inhibitor can also be variable. Research suggests NBTPT-treated urea use can reduce ammonia loss
by
50% to 90% when compared to untreated urea.
The potential boost in crop yield from the preserved N will depend on the nutrient demand
of the crop, the indigenous soil N supply, and other management practices.
Urease inhibitors provide farmers with an additional tool to keep
applied N in the root
zone, which can have agronomic and environmental benefits.
This article originally appeared as #25 Nutrient Source Specifics, a series published by the International Plant Nutrition Institute.
This article originally appeared as #25 Nutrient Source Specifics, a series published by the International Plant Nutrition Institute.
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