Ammonia volatilization from urea David E. Kissel University of Georgia

Overview Evolution of Urea consumption Definitions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Urea Ammonium nitrate Nitrogen solutions Total compound N / / % Millions of tonnes USA Evolution of N Fertilizers Consumption Anhydrous ammonia

Overview Evolution of Urea consumption Definitions 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 Evolution of Urea consumption Definitions 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)

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 (CRH) is the RH at which urea dissolves

Effect of Temperature on CRH and Water Solubility of Urea

Urea RH > CRH (80%) Crop residue Urea dissolution and diffusion Dry Soil

Urea Urea dissolution and diffusion Urea Crop residue Urea 1/2 inch Wet Soil

Urea Crop residue Urea 2 inches

Urea Urea Diffusion into Soil Urea Diffusion Depth Determined by soil moisture

Urea Diffusion into Soil (after 10 days)

Overview Evolution of Urea consumption Definitions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

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

Urea Crop residue Urease inhibitor Urea

Overview Evolution of Urea consumption Definitions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

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

Effect of soil pH on NH 3 as % of Total N

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

Increase in soil pH depends on soil H + buffering capacity Higher clay and OM contents lead to higher soil H + buffering capacity

Atmosphere H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ NH 4 + Urea NH 4 + Urea ½ inch pH 6789 NH 4 + CEC H+H+ Urea Urea hydrolyzes on the soil surface

H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ H+H+ NH 4 + Urea NH 4 + Urea 2 inches pH 67 NH 4 + Urea CEC Urea hydrolyzes below the soil surface

Soil + Effect of Temperature on NH 3 Volatilization NH 4 + Urea H+H+ ½ inch Temperature

Effect of Temperature on Urea Hydrolysis

Soil + Effect of Temperature on NH 3 Volatilization NH 4 + Urea H+H+ ½ inch Temperature

Effect of Temperature on % of NH 3 pH = 8.5

Soil + Effect of Temperature on NH 3 Volatilization NH 4 + Urea H+H+ ½ inch Temperature

Effect of Temperature on Conversion of Aqueous NH 3 to Gaseous NH 3

Soil + Effect of Temperature on NH 3 Volatilization NH 4 + Urea H+H+ ½ inch Temperature

Soil Wind speed + Effect of Wind Speed on NH 3 Volatilization NH 4 + Urea H+H+ Atmosphere ½ inch

Ammonia Loss (% of Applied N) UAN UREA Nitamin 51mm Replication II UAN UREA Nitamin 51mm Replication I Days After Application UAN UREA Nitamin 51mm Replication III

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

Overview Evolution of Urea consumption Definitions Urea Dissolution and Diffusion Urea Hydrolysis Ammonia Volatilization Research Results Summary

Urea Applied to Pasture - Fall 2004 Days after application NH 3 loss (% of applied N) Soil water content (g g -1 ) Rainfall (mm) Nitamin® UAN Urea Rainfall Soil water content Temp = 47 o F

Days after application NH 3 loss (% of applied N) Nitamin® UAN Urea Soil water content (g g -1 ) Rainfall (mm) Rainfall Soil water content Temp = 63 o F Urea Applied to Pasture – Spring 2005

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

Days after application NH 3 loss (% of applied N) Nitamin® UAN Urea Rainfall Soil water content Soil water content (g g -1 ) Rainfall (mm) Temp = 62 o F Urea Applied to Pasture - Fall 2005

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

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

Temp = 69 o F Urea Applied to Pasture – Spring 2006

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

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

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.

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

Urea Applied to Pine Plantations March 14 – April 12 (59 o F) Rain

Urea Applied to Pine Plantations 66 o F Rain

Laboratory Results (77 o F, 90% RH)

Organic Layer NH 4 + CEC Urea Pine needle photo

Urea NH 4 + Organic Layer NH 4 + CEC Pine needle photo

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?