Better Crops/Vol. 93 (2009, No. 4) 

- loss. loss. 3 volatilization losses from 3 volatilization in the days follow 3 - loss than dry fertilizers and other reports losses 3 3 Fertilizers are most commonly broadcast on sources. emission, transport, major and deposition from

Placement When broadcast on the soil surface, both liquid or dry urea urea dry or liquid both surface, soil the on broadcast When Field factors favoring NH surface-applied urea: – No rain or irrigation after application (or light mist) – Crop residue on the soil surface – High temperatures – High soil pH – Low clay and organic matter (low CEC) – Initially moist soil followed by drying Figure 1. that indicate opposite results. These apparent contradictions apparent These results. opposite indicate that are likely due to the specific field conditions influencing the movement of urea into the soil, where it becomes protected from volatilization loss. the soil surface, soil applied the as on a fertilizer surface band, urea-containing or Leaving applied as band. a subsurface rainfall/irrigation) or tillage (by incorporation without surface increases the risk of NH ing application. Since urea moves freely with until it or urea-containing sources can be susceptible to NH There are some reports where liquid urea sources are more susceptible to NH ------loss 3

. Whether 3 . This article

emissions in North North in emissions 3 O 2 loss occurs following occurs loss

3

(Figure 1) + H = 9.3) 2 a No. 1, 2009) discussed the the discussed 2009) 1, No.

3 loss 3 + CO CO + 2 4 ) O (pK 4 2 -containing fertilizers such as am + 4 All fertilizers containing Better Crops Better 2NH volatilization is largely governed by the by governed largely is volatilization (NH + H 3 3 ➝ ➝

+ NH O loss. 2 3 gas. Ammonium is commonly used commonly is carbonate Ammonium gas. 3 ➝

– from animal production facilities. While livestock livestock While facilities. production animal from + 2H 3 3 Ammonium Carbonate of nutrient in a particular cropping situation requires a requires situation cropping particular a in nutrient of etting the most value from all nutrient inputs is neces is inputs nutrient all from value most the etting source right the Selecting efficiency. maximize to sary CO CO +2H ) are theoretically subject to volatile loss. However, 2 2

+ OH + ) gas when left on the soil surface. Ammonia losses from fertilizer can represent ) ) 4 + 3 4 2

When urea is applied to soil, it reacts chemically with wa with chemically reacts it soil, to applied is urea When Nitrogen Source A variety of soil chemical properties interact with envi A previous article ( article previous A 4

Abbreviations and notes: N = nitrogen; CEC = cation exchange capacity. exchange cation = CEC nitrogen; = N notes: and Abbreviations

Urea NH (NH raising pH) (reaction consumes acidity, Hydrolysis Reactions of Urea (NH to release NH release to for smelling salts because it readily releases NH reactions these undergoes urea solution, a in or alone applied when applied to soil: ter (hydrolysis) and the urease enzyme to produce ammonium produce to enzyme urease the and (hydrolysis) ter decompose quickly can that compound unstable an – carbonate acidic solution when they dissolve in the soil (pH between 4.5 between (pH soil the in dissolve they when solution acidic and 5.5). In most circumstances, these N forms do not have significant NH alkalinity (pH) of the zone surrounding the fertilizer particle or droplet. Many NH monium nitrate or initially form a slightly (NH properties of each specific fertilizer and its reactionscontacting the after soil can result in large differences in N loss. NH for potential The reviews some of the major factors that contribute to NH from N fertilizer. ronmental conditions at the site of the fertilizer application to determine the extent of NH total emissions. Where significant NH significant Where emissions. total fertilization, it is possible that the crops may be under-fertil ized due to this unintentional N loss. loss of NH of loss NH to contributor largest the are operations to significantly contribute also fertilizer N from losses America, to the atmosphere. Regulatory agencies are increasing their awareness of the role of N gases in the atmosphere and their potentially undesirable consequences. consideration of economic, environmental, and social objec tives. One of the objectives is to keep all nutrient losses to a losses volatile to subject be can fertilizers N Some minimum. ecosystem and productivity, human health. The major factors controlling NH from fertilizersthis article. in reviewed are G Nitrogen fertilizer is an essential component on most farms. Although urea-based fertil urea-based Although farms. most on component essential an is fertilizer Nitrogen izers are the most common global N source, they are susceptible to loss as ammonia (NH a significant economic loss for farmers and can have a negative effect on air quality, By Robert Mikkelsen By Robert Ammonia Emissions from Emissions Ammonia Agricultural Fertilizer Operations: Figure 2. The effect of (A) urea alone or (B) urea with NBPT urease inhibitor on soil pH over 4 days following application. The soil pH increased from 4.6 to over 9 surrounding the granule within 1 day following fertilization with untreated urea. Each image is approximately 1 in. (27 mm) square of soil. Data from Stephan Blossfeld and Agrotain International. period of urea hydrolysis Estimates of NH3 volatilization from urea fertilizer vary widely due to the many conditions where this popular and increased pH that NH loss is most likely. fertilizer is used. Published reports of volatile N loss 3 from surface-applied urea range from near zero to close Many environmental fac- to 100% in extreme conditions. When urea or urea- tors can influence the rate containing N sources remain on the soil surface for a of urea hydrolysis, such prolonged period of time, typical losses between 10 and as the urea concentration, 40% may be expected. In your environment, NH losses urease enzyme activity, 3 temperature, moisture, Figure 3. Chemical model of the may be greater or less than this. Always use appropri- urease enzyme responsible ate fertilizer management practices such as right source, and the presence of crop residues. During the for hydrolysis. rate, timing, and placement to keep NH3 losses to a minimum. warm growing season, it Source: Protein Data Bank (PDB 1E9Z) Supramolec- is common for most of ular Assembly and Acid Resistance of Helicobacter Pylori Urease. hydrolyzes to NH +, apply urea immediately prior to rainfall applied urea to be hydro- Ha, N.-C., Oh, S.-T., Sung, J.Y., Cha, K.-A., Hyung 4 lyzed within a week. Lee, M., and Oh, B.-H. 2001. Nat. Struct. Biol. 8: or irrigation if possible to allow it to move with water beneath 480. the soil surface. Ammonia loss occurs Broadcasting urea onto the canopy of crops (such as a pas- even from acid soils since urea hydrolysis causes the pH to rise at the site of the fertilizer placement and transform NH ture or a forest) may result in significant NH losses. Surface 3 3 to a gas. The example in Figure 2 shows the soil pH rising banding the added N in concentrated zones can reduce NH3 losses compared with spraying the urea N across the field in from 4.6 to over 9 following urea application. A greater pH this situation. In forested soils, NH loss following urea ap- buffering capacity of soil is generally related to less volatile 3 NH loss, and soils that are high in clay and organic matter plication can be greater from bare soil than from soil with a 3 complete forest floor due to the pH buffering capacity of the tend to have greater pH buffering. humus layer. The bacterial process of nitrification subsequently oxidizes NH + to NO –, releasing acidity in the process and dropping Ammonia loss is a concern in no-till crop production where 4 3 the soil pH below the initial soil pH before urea was added. N is commonly applied to the soil surface. No-till practices + – As NH4 is oxidized to NO3 , the risk of volatile loss decreases. may result in a layer of crop residue that can increase the risk – However, once in the NO3 form, other pathways of N loss of NH3 losses, compared with bare soil. Volatile losses can be significant in these circumstances since i) urease activity is become more likely (such as denitrification or leaching). generally high in crop residues, ii) crop residues form a barrier which can prevent urea from reaching the mineral soil, and iii) a vegetative mulch may keep the soil more moist – all of

which can increase NH3 loss. When subsurface application is not feasible, application of urea in a surface band is superior to broadcast application

for minimizing NH3 loss. This occurs as the capacity of the soil to hydrolyze urea is exceeded within this localized band, giving additional time for downward movement of urea into the

soil where it is protected from NH3 loss. Therefore, the most effective way to conserve urea is to get urea into the soil and not on top of the soil for a prolonged period. + Soil pH The conversion of NH4 to NH3 gas is governed by pH. A variety of reactions occur following N fertilization that will influence the microsite pH. During urea hydrolysis, the pH surrounding the granule initially rises (> pH 8) as am- Moisture is the major mechanism for moving surface-applied urea into

Better Crops/Vol. 93 (2009, No. 4) Crops/Vol. Better monium bicarbonate is formed (Figure 2). It is during this the soil. 10 Better Crops/Vol. 93 (2009, No. 4)

11 Photo courtesy of Agrium Advanced Technology Advanced Agrium of courtesy Photo . . ------loss. 3 ) to urea urea to ) 2 loss. (Figure 2) 3 losses from fertilizer in these 3 loss from both acid and calcare and acid both from loss 3 Fertilizer must sometimes be ap on controlled release fertilizer release on controlled can effectively amount the reduce BC Ammonia volatilization from applied N fertilizers repre A number of compounds have been screened to identify coat as used successfully been have materials of variety A Urea can be reacted with a variety of strong acids to CaCl as (such salts calcium soluble of addition The sents an economic loss of a valuable resource and a potentialavaluable resource anda ofeconomic lossan sents aware andmanagement careful With quality. air for concern ness of the conditions conducive for loss, be properlyN managedfertilizer to can minimize the potential for volatile loss. based Director, Regional America North Western IPNI is Mikkelsen Dr. California; e-mail: [email protected]. at Merced, ous soils by depressing pH as well as reacting with the carbonate carbonate the with reacting as well as pH depressing by soils ous con Urea fertilizer urea hydrolysis. during formed molecules urease inhibit partially to compounds copper and boron taining is phosphate and a of activity monoammonium coating acidic NH minimize help to available commercially also an effective way to block or delay urea hydrolysis. The prod The hydrolysis. urea delay or block to way effective an uct most commonly used, especially in North America, N-(n-Butyl)-thiophosphoric is triamide (NBPT) sold under the with combined is product This Agrotain. of name commercial days several of period a for enzyme urease the inhibit to urea to two weeks depending on the application rate NBPT is especially beneficial for gaining time for rainfall or irrigation to move surface-applied urea into the soil where it is not susceptible to volatile loss. ings for controlled-release fertilizers to limit the solubility of urea. Coatings can effectively reduce the amount of soluble sig and time one any at environment soil the to exposed urea circumstances. nificantly reduce N losses in many maintain a low pH in the vicinity of urea- the fertilizer and granule acid or urea-sulfuric of products Commercial solution. phosphoric acid are available for specialized purposes. NH reduce to shown been has Coatings and minimize NH soil environment the to exposed of soluble urea Fertilizer Modifications plied when conditions are not optimal. Several approaches have been used to reduce NH coatings, fertilizer inhibitors, urease including circumstances, salts. acidification, or the addition of calcium

+ - - - - 4

- 3 loss 3 , high 3 concen 3 loss may 3 -based 4 losses ranged losses ranged from irrigation and irrigation from 3 3 -based fertilizers are loss. + volatile loss. 3 4 3 is added to irrigation water, water, irrigation to added is from surface-applied urea 3 3 is released during respiration respiration during released is 2 from the soil solution. Soils + 4 form. Therefore, NH + 4 loss, this factor often interacts with interacts often factor this loss, 3 losses are expected when the relative Urease enzymes are produced by al 3 Soil cation exchange sites are a major When urea or NH Volatile losses of NH of losses Volatile Ammonia losses generally increase with losses can be significant. 3 Losses of NH . emission factor for all N fertilizer applications applications factor for all N fertilizer emission gas over the NH 3 is depleted during the daytime, causing pH to rise as as rise to pH causing daytime, the during depleted is 3 2 and reduce volatile losses. Since sandy soils generally +

When anhydrous or aqua NH aqua or anhydrous When Flooded Soils Flooded Urea Hydrolysis Temperature Windspeed Soil Properties Soil Moisture 4 age NH N fertilizer. to be 2.4% of added from 0.9 lb to 6.2 lb N/A (averaging 3.2% of applied N) 3.2% of applied lb to 6.2 lb N/A (averaging from 0.9 using NH of cropping situations from a variety aver suggested a state-wide The researchers fertilizers. An extensive set of field studies was conducted in the in was conducted studies set of field An extensive NH large-scale to estimate of California Valley Central NH Total applied fertilizer. losses from nitrate) since only half of the fertilizer is present as urea. significant volatile losses can occur as water is applied to the to applied is water as occur can losses volatile significant field through sprinklers or irrigation furrows.ammonium Less and NH urea of mixture 1:1 (a UAN using when occurs and the water pH decreases again. Placement of urea below the soil surface largely eliminates NH urea is hydrolyzed to ammonium carbonate. Photosynthesis by by Photosynthesis carbonate. ammonium to hydrolyzed is urea water the of pH the increase also will water in algae and plants CO when CO nighttime, During 9. as high contribute to the likelihood of loss. When broadcast into flood into broadcast When loss. of likelihood the to contribute fertilizer a than volatilization to susceptible more is urea water, such as ammonium sulfate since the pH is likely to rise as flood water may be large. A high concentration of NH water pH, warm temperature, and elevated wind speed all most all plants, animals, and microorganisms. Its absence is almost never a limiting factor for converting urea into NH (Figure 3) spikes may occur during hot times of the day. However, since However, day. the of times hot during occur may spikes NH favors also drying soil temperature and windspeed. trations. Higher temperatures also shift the equilibrium to favor NH daily and year, the of part warmer the during higher slightly be rising temperatures due to effects on both chemical and bio hydrolysis the speed temperatures Higher reactions. chemical NH greater and pH soil higher a in resulting urea, of and drying soils are often related, both of these factors tend to tend factors these of both related, often are soils drying and aggravate the potential for volatile NH changes and NH conditions windy Since conditions. windy in increase generally mechanism for removing NH with a greater CEC generally have the ability to retain more NH pH soil of magnitude the capacity, buffer and CEC lower have applied urea into the soil and essential in making the nutrients nutrients the making in essential and soil the into urea applied uptake from the soil solution. available for plant quite dry. Higher NH quite dry. urea. of humidity critical the than greater is air the of humidity surface- moving for mechanism major the is moisture However, added to dry soil, the dissolution is slow and biological and chemical reactions do not rapidly occur. also becomes very slow and approaches zero when the Ureasoil is hydrolysis