Ammonia Volatilization from Urea, Mechanisms, Magnitude, and Management

Ammonia Volatilization from Urea, Mechanisms, Magnitude, and Management David E. Kissel University of Georgia

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

World Evolution of N Fertilizer Consumption Urea 51% Ammonium nitrate Nitrogen solutions 1973 / 74 2004 / 05 Ammonium phosphate Total compound N 0 10 20 30 40 50 Millions of tonnes www.fertilizer.org

USA Evolution of N Fertilizer Consumption Urea 21% Ammonium nitrate Nitrogen solutions 1973 / 74 2004 / 05 Anhydrous ammonia Total compound N 0 1 2 3 4 5 Millions of tonnes www.fertilizer.org

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Definitions Dissolution: urea absorbs water and converts from solid to liquid Hydrolysis: urea converts to ammonium (NH 4 ) Diffusion: movement due to motion of molecules Urea has no charge, diffuses easily NH 4 has charge, diffuses slowly

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Q. Does any ammonia volatilize from UAN solutions before they are applied or during the application? A. None or very little, depending on if a small amount of ammonia is added to protect against corrosion of mild steel.

Composition of UAN 28% N 14 % N from Ammonium Nitrate 7% ammonium N 7% nitrate N 14% N from Urea 30% water small amounts of inhibitors to inhibit corrosion of mild steel

Inhibitors to protect against corrosion of mild steel 0.5 % ammonia (raises ph to about 7.5) Ammonium phosphates at 0.2 % P 2 O 5 Others

NH4 NH4 4 = NH NH3 3 H H NH 4 = H (ion) (gas)

Urea Hydrolysis in UAN? DOES NOT OCCUR Therefore no ammonia is formed from the urea portion of the UAN solution from this process. Therefore no ammonia loss from the urea portion of UAN solution before soil application.

Summary of ammonia loss from UAN Urea in UAN does not hydrolyze in the fertilizer tank. The NH 4 from the ammonium nitrate portion of the UAN cannot be lost as. The amount of added to some UAN to inhibit corrosion is very small, around 10 lb per ton. A small portion of this may be lost during application.

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Urea Dissolution Urea has to be dissolved before it can be hydrolyzed

Urea Dissolution Urea has to be dissolved before it can be hydrolyzed Urea can absorb water from the atmosphere and from the soil/crop residue

Urea Dissolution Urea has to be dissolved before it can be hydrolyzed Urea can absorb water from the atmosphere and from the soil/crop residue Absorption of water from the atmosphere depends on relative humidity (RH)

Critical Relative Humidity for Urea (%) Effect of Temperature on CRH and Water Solubility of Urea 90 16 CRH 85 14 80 12 75 10 70 8 65 Solubility 6 60 4 20 40 60 80 100 120 Air Temperature ( o F) Urea Solubility (lb/gallon of water)

Urea dissolution and diffusion RH > CRH (80%) www.agrium.com Urea Urea Urea Crop residue Dry Soil

Urea dissolution and diffusion www.agrium.com Urea Urea Urea Crop residue 1/2 inch Urea Urea Urea Wet Soil

Urea Diffusion into Soil (after 10 days) Urea Concentration in Soil (ppm) 0 0 100 200 300 400 500 600 700 Wetter Soil Depth (mm) 10 20 30 40 Drier 50

Urea Urea Urea Crop residue 2 inches Urea Urea Urea

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Urea Hydrolysis Urea 2 H 2 NH 4 Urease enzyme Urease is derived from crop residues and soil microorganisms

Urease inhibitor Urea Urea Urea Crop residue Urea Urea Urea

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Volatilization Process Atmosphere Hydrolysis Urea 2 H 2 NH 4 Clay OM NH 4 NH 4 NH 4 Soil air Soil solution (gas) NH 4 (aq) Equilibrium NH (aq) 3 H

Effect of soil ph on as % of Total N (as % of Total Ammoniacal N) 30 25 20 15 10 5 0 68 o F 5 6 7 8 9 Soil ph

Volatilization Process Atmosphere Hydrolysis Urea Clay OM H NH 4 NH 4 NH 4 NH NH 3 4 Soil air Soil solution (gas) NH 4 (aq) Equilibrium NH (aq) 3 H

Increase in soil ph depends on soil H buffering capacity Loss (kg N ha -1 ) 12 8 4 Low Buffering Capacity High Buffering Capacity Higher clay and OM contents lead to higher soil H buffering capacity Soil ph 0 9 8 7 Low Buffering Capacity High Buffering Capacity Ferguson et al. (1984) 6 0 2 4 6 8 10 12 Days Since Urea Application

Atmosphere Urea Urea Urea Urea NH 4 4 NH 3 4 NH 4 H 4 NH NH 4 4 H 4 H H H H H H CEC Urea hydrolyzes on the soil surface ½ inch ph 6 7 8 9

Urea hydrolyzes below the soil surface Urea Urea Urea Urea Urea Urea Urea Urea NH NH 4 4 NH 4 4 NH NH NH H 4 NH 4 4 H 4 H H H H H H CEC ph 6 7 2 inches

Effect of Temperature on Volatilization Temperature Soil ½ inch Urea H NH 4

Effect of Temperature on Urea Hydrolysis 6 Relative Urease Activity 5 4 3 2 1 0 30 40 50 60 70 80 90 100 110 Temperature ( o F) Gould et al. (1973)

Effect of Temperature on Volatilization Temperature Soil ½ inch Urea H NH 4

Effect of Temperature on % of (as % of Total Ammoniacal N) 35 30 25 ph = 8.5 20 15 10 5 0 30 40 50 60 70 80 90 100 110 Temperature ( o F)

Effect of Temperature on Volatilization Temperature Soil ½ inch Urea H NH 4

Effect of Temperature on Conversion of Aqueous to Gaseous Relative Driving Force for Conversion of Aqueous to Gaseous 5 4 3 2 1 30 40 50 60 70 80 90 100 110 Temperature ( o F)

Effect of Temperature on Volatilization Temperature Soil ½ inch Urea H NH 4

Effect of Wind Speed on Volatilization Atmosphere Wind speed Soil Urea NH 4 H ½ inch

Ammonia Loss (% of Applied N) Wind Speed (m s -1 ) 20 UAN UREA Nitamin 15 Replication I 10 5 51mm 0 20 UAN UREA Nitamin Replication II 5 4 High Low 15 3 10 51mm 2 5 1 0 20 15 10 5 UAN UREA Nitamin 51mm Replication III 0 00:00:00 08:00:00 16:00:00 00:00:00 Time 0 0 20 40 60 80 100 120 Days After Application

Optimum Conditions for Volatilization Small diffusion depth into soil RH > CRH High temperature High wind speed Low ph buffering capacity

Overview Urea Consumption Definitions UAN Solution Reactions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Rainfall (mm) Soil water content (g g -1 ) loss (% of applied N) Urea Applied to Pasture - Fall 2004 50 40 30 20 10 Nitamin UAN Urea Temp = 47 o F 50 40 30 20 10 Rainfall Soil water content 0.4 0.3 0.2 0.1 0 20 40 60 80 100 120 Days after application

Urea Applied to Pasture Spring 2005 loss (% of applied N) Rainfall (mm) Soil water content (g g -1 ) 50 40 30 Nitamin UAN Urea Temp = 63 o F 20 10 50 40 30 Rainfall Soil water content 0.4 0.3 20 0.2 10 0.1 0 20 40 60 Days after application

Atmosphere Urea Urea Urea Urea NH 4 Urea NH 4 Urea NH 4 Clay OM NH (aq) 4 (aq) NH H 3 Equilibrium

Urea Applied to Pasture - Fall 2005 loss (% of applied N) Rainfall (mm) Soil water content (g g -1 ) 50 40 30 Nitamin UAN Urea Temp = 62 o F 20 10 50 40 30 Rainfall Soil water content 0.4 0.3 20 0.2 10 0.1 0 20 40 60 Days after application

Time of RH > CRH (h) Time of RH above CRH in the first 30 d 25 20 15 10 5 0 0 5 10 15 20 25 30 Days after application Total: RH > CRH =19 63 days %

Atmosphere RH > CRH (80%) NH NH NH NH 4 4 4 NH 4 4 Urea Urea Urea NH 4 NH 4 Fescue thatch Soil water content (0.07 g g -1 ) Permanent wilting point (0.04 g g -1 )

Urea Applied to Pasture Spring 2006 loss (% of applied N) Rainfall (mm) Soil water content (g g -1 ) 50 40 30 Nitamin UAN Urea Temp = 69 o F 20 10 50 40 30 Rainfall Soil water content 0.4 0.3 20 0.2 10 0.1 0 20 40 60 80 Days after application

Atmosphere NH 4 CEC Urea Urea Urea NH 4 Urea Urea Urea NH 4 NH NH NH (aq) 4 (aq) 3 H 3 Equilibrium Soil water content (0.15 g g -1 ) Field capacity (0.18 g g -1 )

Ammonia volatilization losses under field conditions Fertilizer Urea UAN Summary of Pasture Results Nitamin Fall 2004 --------------- Ammonia loss (% of applied N) ----------------- 19 a* 6 b 6 b Spring 2005 12 a 13 a 14 a Fall 2005 Spring 2006 46 a 33 b 34 b 24 a 18 a 18 a Within a column, values followed by the same letter are not significantly different according to Fisher s LSD at p=0.05

Summary for Pastures/No-Till Dry Soil, Dry Residue, RH < CRH: OK Dry Soil, Dry Residue, RH > CRH: not OK Wet Soil, Wet Residue: not OK Apply on dry soil, dry residue, expected low RH, and hope for 1 inch of rain whenever it rains. Apply on wet soil if 1 inch rain/irrigation is expected.

Summary for Clean-Till Crops Dry Soil: OK Wet Soil: not OK Apply on dry soil and hope for 1 inch of rain whenever it rains. Apply on wet soil if 1 inch of rain/irrigation is expected.

Knifed of disked in better than Dribbled Dribbled better than Broadcast Summary for Pastures/Crops Knifed or disked in Dribbled Broadcast

Urea Applied to Pine Plantations Loss (% of Applied N) 60 40 20 0 May 2 - May 31, 2002 40 mm 0 mm 20 mm Rain 66 o F 0 5 10 15 20 25 30 35 Days After Application

Laboratory Results (77 o F, 90% RH) Cumulative Loss (% of Applied N) 20 15 10 5 0 20 mm before urea 20 mm right after urea 20 mm 1 day after urea 20 mm 3 days after urea 5 mm rain added to all treatments 0 5 10 15 20 25 30 Days After Urea Application

Pine needle photo Urea www.microscopyu.com NH 4 NH 4 CEC CEC Organic Layer

Pine needle photo Urea Urea NH 4 NH 4 NH4 www.microscopyu.com NH 4 NH 4 CEC CEC Organic Layer

Summary for Pine Plantations Dry Forest Floor, RH < CRH: OK Dry Forest Floor, RH > CRH: not OK Wet Forest Floor: not OK Apply on dry forest floor, with expected low RH, and hope for 1 inch of rain whenever it rains. Apply on wet forest floor if 1 inch rain is expected.

Questions?

Atmosphere NH 3 Urea Urea Urea NH NH NH NH 4 4 4 NH 4 4 NH 4 NH 4 Fescue thatch or OM NH 4 NH 4 CEC CEC

Loss (% of Applied N) Laboratory Results (77 o F, 90% RH) volatilization in the lab 30 Urea 20 10 UAN a b b 0 Nitamin 0 10 20 30 Days after application