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Microsource Minute

Volatilization: Part 1

 

One undesirable fate of surface applied nitrogen is volatilization, which can cause as much as 25-50% N loss. This is not a newly discovered risk though, as volatilization has been studied in the fertilizer industry for decades.  The negative impact on grower economics and the environment have been echoed over the years, along with best practices to mitigate losses such as incorporation or using an AAPFCO approved urease inhibitor.  But to take the next step towards nutrient efficiency, it may help to better understand the two mechanisms of ammonia volatilization and just how quickly it can occur.

Urea Hydrolysis Mechanism

Urea in the presence of the urease enzyme rapidly hydrolyzes to form Ammonium (NH4+).  If the nitrogen is not incorporated from rain, tillage, or injection the ammonium N can quickly volatilize to the atmosphere as Ammonia Gas (NH3).

The urease enzyme is found everywhere across the soil environment on living and dead plant tissue surfaces.  Higher amounts will be present in no-till fields where there are increased surface residues.

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High pH Mechanism

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Equilibrium concentrations of Ammonia (NH3) and Ammonium (NH4+) are determined by local pH.  Soils with pH from 5.0 – 7.0 will have higher amounts of the (NH4+) cations; as the pH increases and approaches 7.5, there is a rapid increase in ammonia gas (NH3) production.  NH3 gas lost to the atmosphere is considerable when pH ≥ 7.5.

Consequently, the reaction from adding urea to the soil surface ultimately forms Ammonium (NH4+), Carbon Dioxide (CO2) and a Hydroxyl Group (OH-).  The hydroxyl group shifts the pH higher, particularly around the reaction site, creating a micro environment of very high pH.  Regardless of soil pH, when urea is surface applied, it creates an Ammonium (NH4+) and high pH micro site around the urea granule and it deprotonates to NH3 gas within hours.

Note

  • Volatilization is enhanced by warm temps, moist soil, wind, and no-till (increased urease activity and more residue delaying soil contact)
  • Risk correlates to other nitrogen sources beyond urea. Since UAN is 50% Urea nitrogen, that percentage undergoes hydrolysis and is susceptible to loss if surface applied. The ammonium-nitrate portion forms an ion pair so the ammonium (NH4+) in UAN will not convert to ammonia.
  • Volatilization can be reduced with incorporation, soil texture (higher CEC soils have more capacity to hold the NH4+ cations) and urease inhibitors

Author: Ethan Enochs

Source: Purdue University, Agronomy e-Learning’s Nutrient Management course (2017). Figure adapted by J. McGrath and B. Erickson from Grains Research & Development Corporation (GRDC). 17 Sep 2009. Nitrogen Volatilisation From Northern Cropping Soils

 
 
Ethan Enochs