How a Computer Model Reveals Ammonia's Great Escape from Flooded Soils
Imagine a farmer carefully applying nitrogen fertilizer to a flooded rice field, only to have nearly half of it mysteriously disappear into thin air.
This isn't magic—it's the phenomenon of ammonia volatilization, an invisible process that represents one of agriculture's most significant challenges. When nitrogen fertilizers are applied to flooded soils like rice paddies, a substantial portion can escape into the atmosphere as gaseous ammonia, resulting in billions of dollars in lost agricultural productivity worldwide and contributing to environmental pollution.
The pioneering computer model developed by Jayaweera and Mikkelsen was revolutionary because it translated the complex chemistry of ammonia loss into predictable mathematical terms 4 . At its core, the model identified five primary factors that control how much ammonia escapes from flooded soils.
Higher NH₄-N levels increase volatilization potential.
Alkaline conditions favor conversion to gaseous NH₃.
Warmer conditions accelerate chemical reactions.
Deeper water creates longer diffusion paths.
Flooded soil systems, such as rice paddies, create the perfect storm for ammonia loss. The chemical dynamics in these environments are unique because the floodwater acts as both a medium for chemical reactions and a barrier through which gases must escape.
The model operates on two key components: the chemical aspects dealing with the equilibrium between ammonium ions (NH₄⁺) and aqueous ammonia (NH₃) in floodwater, and the volatilization aspects based on the two-film theory that describes how ammonia gas transfers across the water-air interface 4 .
NH₄⁺ + OH⁻ ⇌ NH₃ + H₂O
The critical chemical equilibrium that controls ammonia volatilization
While computer models provide theoretical predictions, scientists need ways to validate them with real-world measurements. One crucial experiment detailed by Pioneer research illustrates how researchers track ammonia's escape using dositubes—relatively inexpensive passive gas detection devices that can be deployed directly in farm fields 1 .
Install dositube chambers in flooded fields immediately after fertilizer application.
Measure for 14-28 days to capture both immediate and delayed losses.
Collect and analyze dositubes at regular intervals to quantify ammonia escape.
The data gathered from such experiments reveals striking patterns of loss. The timing of these losses follows a predictable pattern, with the highest flux occurring within the first few days after application.
| Fertilizer Type | Application Rate (kg N/ha) | Cumulative Loss (%) | Source |
|---|---|---|---|
| Traditional Urea | 80 | 14.8% | |
| Traditional Urea | 160 | 18.1% | |
| Traditional Urea | 240 | 24.2% | |
| Loss Control Urea (LCU) | 80 | 10.1% | |
| Loss Control Urea (LCU) | 160 | 12.7% | |
| Loss Control Urea (LCU) | 240 | 17.5% |
Increases with higher pH
Shifts equilibrium toward gaseous NH₃Increases with higher temperature
Accelerates chemical conversionDecreases with greater depth
Creates longer diffusion pathIncreases with higher wind speed
Removes accumulated NH₃ fasterRecent scientific advances have produced innovative fertilizers designed specifically to combat ammonia loss. One promising development is Loss Control Urea (LCU), which contains only 6% loss control agent (LCA) mixed with 94% traditional urea .
The secret to LCU's effectiveness lies in hydrogen bonding between the LCA and nitrogen sources in the presence of water, significantly enhancing adsorption capacity for ammonium nitrogen .
The LCA consists of silica aluminate minerals like montmorillonite that create a micro/nano network surface with enhanced adsorption properties .
Another approach to nitrogen conservation involves using nitrification inhibitors (NIs), which slow the bacterial conversion of ammonium to nitrate in soils. While effective at reducing direct nitrous oxide (N₂O) emissions—showing a 48% reduction in some studies—these inhibitors come with a significant trade-off 2 .
In direct N₂O emissions with nitrification inhibitors
When accounting for this increased ammonia volatilization and the subsequent indirect N₂O emissions when this ammonia is re-deposited elsewhere, the overall effectiveness of nitrification inhibitors decreases significantly. The mitigation effect drops from 48.0% to between 28.2% and 39.7% when these indirect pathways are considered 2 .
| Strategy | Mechanism | Advantages | Limitations |
|---|---|---|---|
| Loss Control Urea (LCU) | Hydrogen bonding with NH₄⁺ reduces volatilization | Reduces both volatilization and leaching | Slightly higher cost than conventional urea |
| Nitrification Inhibitors | Suppresses bacterial oxidation of NH₄⁺ to NO₃⁻ | Effectively reduces N₂O emissions | Increases NH₃ volatilization, especially in high-pH soils |
| Deep Fertilizer Placement | Places fertilizer below soil surface | Reduces direct exposure at water-air interface | Requires specialized application equipment |
| Non-urea Fertilizers | Uses ammonium salts instead of urea | Can reduce volatilization by up to 75% | May contribute to soil acidification over time |
Understanding and combating ammonia volatilization requires specialized tools and methods.
Passive gas detection devices used to measure ammonia concentrations in the air above flooded soils 1 .
Modern kits that can determine soil ammonia nitrogen content quickly using microplate readers 3 .
Traditional method for determining ammonia nitrogen using toxic reagents like phenol 3 .
Instruments that revolutionize soil analysis by enabling high-throughput testing of multiple samples 3 .
Solutions like 1 mol·L⁻¹ KCl and 2 mol·L⁻¹ NaCl used to extract ammonia nitrogen from soil samples 3 .
The journey from theoretical computer models to practical agricultural solutions demonstrates the power of interdisciplinary science.
What began as mathematical equations describing chemical equilibria and gas transfer mechanisms has evolved into innovative technologies that help feed the world more sustainably. The computer model developed by Jayaweera and Mikkelsen provided the critical theoretical foundation that enabled researchers to understand, predict, and ultimately reduce nitrogen losses from agricultural systems 4 .
Today, this work continues as scientists develop increasingly sophisticated solutions—from engineered clays that trap ammonium through hydrogen bonds to precision application methods that account for local conditions. As climate change and population growth place additional pressures on global food systems, such research becomes ever more vital.