Soil Water and Ammonia Volatilization Relationshipswith Surface-Applied Nitrogen Fertilizer Solutions

T. Al-Kanani,* A. F. MacKenzie, and N. N. Barthakur

ABSTRACTSoil water content is an important factor influencing NH3 vola-

tilization from N fertilizers. Information on the relationship betweenwater evaporation and NH3 volatilization from urea (U) or urea-ammonium nitrate (UAN) solutions, and on the kinetics of bothprocesses is lacking. We examined NH3 volatilization from surface-applied solutions of U and UAN in samples of St. Bernard (loamy,mixed, nonacid, frigid Typic Hapludoll) and Ste. Sophie (sandy,mixed, nonacid, frigid Typic Haplorthod) soils exposed to waterpotentials ranging from < —1.5 to —0.01 MPa. An air-train systemwas used. Ammonia volatilization increased as soil water contentincreased. Volatilization differences between moist (> -0.038 MPa)and air-dry (< —1.5 MPa) samples were reduced as the clay contentof the soil increased. However, the effect of clay content on NH,volatilization became more pronounced as soil approached air dry-ness. Water evaporation was consistent with first-order kinetics,whereas NH, volatilzation followed two first-order kinetics. The in-itial rate constants (A,) for NH3 volatilization (0.338-0.348 d~') fromU solution were consistently higher than those for water evaporation(0.099-0.104 d'1) at -0.01 MPa water potential and 70% relativehumidity. Differences could be attributed to different energy require-ments. Ammonia volatilization vs. water evaporation followed a log-arithmic relationship.

SURFACE APPLICATIONS of U-fertilizer solutions loseN through NH3 volatilization (Fenn and Hossner,

1985; Al-Kanani et al., 1990). Losses of NH3 fromagricultural soils may reach 55% of applied U-N (Al-Kanani et al., 1990). The extent of NH3 volatilizationis determined by soil pH, texture, temperature, mois-ture, exchangeable cations, fertilizer source, and rateof application (Fenn and Hossner, 1985; Ferguson andKissel, 1986). Of these factors, soil water content andwater evaporation from soils have received little at-tention. In addition, very little information is availableon water-NH3 loss relationships from surface-appliedU and UAN solutions. Such information would im-prove N-fertilizer management in areas where liquidN fertilizers are used.

Fenn and Escarzaga (1976) showed that the effect ofsoil water content on NH3 volatilization from NH4solutions was minimal when soil water content ex-ceeded the critical value of 8% (w/w). Bouwmeester etal. (1985) found that NH3 loss from granular U appliedto calcareous soils was related to initial moisture con-tent, and increased by 8% for every 10% increase ininitial soil moisture.

Water and NH3 losses from soils may be related(Wahhab et al, 1957; Chao and Kroontje, 1964; FennDep. of Renewable Resources, Faculty of Agricultural and Envi-ronmental Sciences, McGill Univ., 21111 Lakeshore Rd. Ste. Annede Bellevue, Quebec H9X ICO Canada. Supported by a grant fromthe Natural Sciences and Engineering Research Council of Canadaunder the University-Industry Cooperative Research and Devel-opment Program and Nitrochem, Inc. Received 8 Feb. 1991. Cor-responding author.

Published in Soil Sci. Soc. Am. J. 55:1761-1766 (1991).

and Escarzaga, 1977). Rates of water and NH3 lossfrom NH4OH solutions applied to soils showed a pow-er relationship that was independent of soil texture(Chao and Kroontje, 1964). Chao and Kroontje (1964)concluded that NH3 volatilization obeyed a differentkinetic equation than that for water loss. This conclu-sion was in contrast to observations by Wahhab et al.(1957), who reported a constant ratio between NH3volatilization from (NH4)2SO4 and moisture loss. Ad-ditionally, Fenn and Escarzaga (1977) found a negativelinear relationship between NH3 and water-vapor loss-es from a sandy soil, whereas Ferguson and Kissel(1986) observed that the surface application of U crys-tals to a dry or rapidly drying soil resutled in negligiblelosses of NH3.

In view of these conflicting results useful informa-tion may be provided by kinetic modeling. Further-more, such information would be useful in thedetermination of soil conditions for optimum N re-tention. Thermodynamic-based models have beenused successfully to describe NH3 volatilization fromsurfaces of bulk soil under somewhat restrictive con-ditions (Parton et al., 1981; Rachhpal-Singh and Nye,1984, 1986; Sadeghi et al., 1988). These models, how-ever, require a greater number of variables than mostkinetic models. Moreover, one would expect NH3 lossto be diffusion controlled, as considerable soil volumeswere used in these studies. A shallower depth of soilcould minimize the effect of diffusion into the soil andsimulate conditions near the soil surface, where thehydrolysis of U is increased and NH3 volatilization ishigher.

The objectives of this study were to: (i) evaluate theeffect of initial soil moisture content and water evap-oration on NH3 loss from surface-applied U and UANsolutions, and (ii) assess the kinetics of NH3 volatili-zation from U and UAN solutions and water evapo-ration from shallow-depth soil samples.

MATERIALS AND METHODSAmmonia and Water Losses

Soil samples were collected from cultivated maize (Zea maysL.) fields from areas mapped as St. Bernard and Ste. Sophie ineastern Canada. Samples were air dried, ground to pass a 2-mm sieve, thoroughly mixed, and stored at room temperaturein moisture-proof containers. Selected physical and chemicalproperties of these soils are presented in Table 1.

A constant volume (2.8 X 10~5 m3) of air-dried soil, weigh-ing 45 to 47 g, was placed to a depth of 1.0 cm in a 6.0-cm(i.d.) by 8.0-cm-long screw-top jar. Except for air-dried (<— 1.5 MPa) samples, deionized, distilled water was applieduniformly at appropriate amounts relative to final moisturetension, and soils were incubated for 4 d at 23.1 ± 0.4 °C.A constant flow of NH3-free, water-saturated air was con-tinuously passed through the jars. No water loss was detectedduring incubation.

At the end of incubation, dropwise surface applications of400/tL of each N solution were initiated. The designationsof 100-0 and 50-50 for U and UAN indicate percentages of

1761

1762 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

Table 1. Selected physical and chemical properties of St. Bernardand Ste. Sophie soils.

PropertyTextureClay contentt (kg-')Organic-matter content}: (g kg'1)Cation-exchange capacity§ (mmolc kg*1)Total-N content!! (g kg*1)pH#Extractable K contenttt (mg kg"1)Extractable P contenttt (mg kg"')

Soil

St. BernardSandy clay loam

21938.3

134.72.76.5

40.09.0

Ste. Sophiesand8725.373.9

1.35.9

108.032.0

t Hydrometer (McKeague, 1976, p. 4-33).j Walkley-Black (Nelson and Sommers, 1982).§BaCl2 (Hendershot and Duquette, 1986).11 Kjeldahl (Bremner and Mulvaney, 1982).# Water (1:5 soil/water).tt Mehlich III (Mehlich, 1984).

total N (10% by weight of N solution) derived from U andammonium nitrate (AN), respectively. The rate of N appliedwas 41.54 mg jar1, which was equivalent to 147 kg N ha*'(surface-area basis). The final moisture contents of soil(water and 400 f*L N solution) were equivalent to 0.256,0.129, and 0.039 kg kg-', these moisture contents repre-sented soil water potentials of -0.01, -0.038, and —1.4MPa, respectively, fpr Ste. Sophie and -0.01, -0.035, and— 1.4 MPa, respectively, for St. Bernard soils. Soil waterpotential of air-dried samples with 400 fiL N solution was< —1.5 MPa. Jars including control samples (no N added)were covered immediately following N-solution applicationsand connected to an air train, which provided a constant airflow of 0.1 L s~'. This air flow was equivalent to 30 volumeexchanges min-'. A Gilmont flow meter (Gilmont Instru-ment, Great Neck, NY) was used to measure and adjust theair-flow rate through individual jars. This air flow was se-lected to induce maximum loss of NH3 (Vlek and Stumpe,1978). Incoming air was passed through a 1 M H2SO4 so-lution followed by a series of two distilled-water humidifiersto maintain a constant air relative humidity of 70%. Vola-tilized NH3 was trapped in a 2% H3BO3 solution. TrappedNH3 was determined by titrating with 0.05 M H2SO4 usingan automatic titrator (Mettler DL20 CompactTitrator, Met-tler Inst. AG, Greifensee, Switzerland). Incubations wereperformed at an average temperature of 23.1 °C with max-imum deviations of ± 0.4 °C. Soil water was not replenishedduring the incubation period. Incubations were performedin triplicate, and daily measurements of NH3 loss were madefor 10 d.

Two treatments at initial —0.01 MPa water potential wereexamined to evaluate relationships between water evapo-ration and NH3 volatilization. In Treatment A, the soil wasallowed to dry at constant relative humidity of 70%. In Treat-ment B, water content was kept constant by weighing dailyand replenishing water with a syringe through the rubberpart of the jar's lid.

Following incubation, soils in the jars were shaken for 3 hwith 175 mL of 2 M KC1 solution containing 875 Mg phen-ylmercuric acetate. Filtrates were analyzed for U-N (Mul-vaney and Bremner, 1979), NH;-, NO]-, and NOj-N(Keeney and Nelson, 1982) using a Technicon Autoanalyzer(Technicon Industrial Systems, Tarrytown, NY). Recovery ofadded N was calculated from the NH3-N and soil contentsof U-, NHJ-, NO;-, and NOj-N after deducting amountspresent in control samples to which no N solution wasapplied.

Changes in soil water content were measured gravimet-rically at 24-h intervals. Analysis of variance was done onthe basis of a randomized complete block design with a fac-

torial arrangement of treatments. Means were separated onthe basis of least significant differences (LSD).

Kinetic ModelsA general expression for kinetic models can be written as

db/dt =•• k(l - b)n [1]where k is the apparent rate coefficient, b is the fractionalloss of material, n is the order of the reaction, and t is time.Equation [2] can be derived by integrating Eq. [1] for any nand can be expressed as

W = A(t/tl/2) [2]where f(b) is a function of the fractional loss of material.The exact mathematical form of f(b) depends on n and onthe initial conditions of the reaction. The constant A can becalculated from the actual mathematical expression of thekinetic model. The time for 50% reaction (half-life) is rep-resented by tl/2. For a first-order reaction, n = 1, and Eq.[1], after integration and application of initial conditionsb — 0 at t = 0, can be reformulated as

W = ln[l/(l - b}} - to •= 0.693(^1/2) [3]Equation [3] was used to evaluate reaction-rate parameters.

Because fluxes are induced by thermodynamic forces, theprocess of gas escape from a soil surface can be written as

J = L dC/dz [4]where J is the flux, dC/dz is thermodynamic force, which isthe concentration gradient of the escaping gas across the soilsurface-atmospheric boundsiry layer. The coefficient L is afunction of wind velocity (v), temperature (T), soil surfaceroughness (s), and adsorption (a) and desorption rate con-stants (d). In our laboratory experiments, these factors werecontrolled and were assumed to remain constant. Volatili-zation could then occur with negligible influence of diffusiondue to a shallow depth of soil sample.

RESULTS AND DISCUSSIONEffect of Soil Water on Ammonia Volatilization

Ammonia losses at each moisture content followeda similar pattern for both soils, but were higher in Ste.Sophie than in St. Bernard soil. Therefore, data ob-tained with one or both sioils will be presented. Am-monia losses from U solution were significantly relatedto initial soil water content (Fig. 1). This observationis in agreement with the results of Ernst and Massey(1960) and Hargrove (1983). Hargrove (1988) reportedthat, for maximum NH3 volatilization, the soil mois-ture content must be at or near field capacity. In bothsoils, cumulative loss of applied N as NH3 ranged be-tween 2.8 and 55% from U solutions (Table 2). Forthese U solutions, cumulative NH3 volatilized fromTreatment A (initial water potential of —0.01 MPa,then allowed to dry) was 62 and 95% higher than theair-dried (< —1.5 MPa) treatment for St. Bernard andSte. Sophie soils, respectively. These observations sug-gest that the drying process and evaporation of waterhave pronounced effects on NH3 volatilization fromsoils with initial water potential of —0.01 MPa. Fer-guson and Kissel (1986) indicated that the drying pro-cess and water evaporation can cause rapid movementof U towards the soil surface, leading to high NH3volatilization.

Ammonia volatilization from U solutions at initial

AL-KANANI ET AL.: SOIL WATER AND AMMONIA VOLATILIZATION RELATIONSHIPS 1763

I 46

'40

1SO 5 1O 1B 2O SB

initial water content (% by weight)Fig. I. Relationship between initial soil water content and cumulative NH, losses (total of 10 d) from urea solutions surface applied to

St. Bernard soil.

soil water potentials > — 1.4 MPa was consistentlyhigher in Ste. Sophie than in St. Bernard soil (data notshown). For St. Bernard soil, a relatively high claycontent and subsequently increased cation-exchangecapacity, in comparison with Ste. Sophie soil, wouldreduce NH3 volatilization, presumably through NHJadsorption. If the rate of NHJ ion formation as a resultof U hydrolysis exceeds the capacity for NHJ adsorp-tion on clay surfaces, however, a high concentrationof NHj may remain in soil solution. Such high con-centrations of NHJ may enhance NH3 volatilization.

Effects of Time of Incubation on AmmoniaVolatilization and Nitrogen Recovery

Loss of NH3 was detected at Day I and most of theNH3 was lost within the first 5 d after N application(Fig. 2). The cumulative loss of NH3 with time wascurvilinear for —0.035 MPa and higher water poten-tials, compared with a linear relationship observed for—1.4 MPa and air-dried samples (< — 1.5 MPa). Cu-mulative losses of NH3 from U in Treatments A andB were similar except at Days 2 and 3 of incubation,

where differences were significant (LSD [0.05] = 1.1).For Treatments A and B, cumulative NH3 losses fromU were relatively constant after Day 6 (Fig. 2). Therewas no change in these curves when the incubationperiod was extended to 20 d (data not shown). ForTreatments A, B, and —0.035-MPa water potential,the attainment of a near-equilibrium state of NH3 lossafter Day 6 may indicate either completion of U hy-drolysis or a significant reduction in hydrolyzable U.It is worth mentioning here, however, that <3.7% ofadded U remained nonhydrolyzed in Treatments A,B, and -0.035 MPa after 6 d of incubation.

Urea hydrolysis at soil water potentials of < — 1.4MPa was minimal, as noted by significant amounts ofU recovered in air-dried samples (Table 2). This isprobably due to the reduction in urease activity at highsoil water tension. Nitrogen recoveries were similarfor both soils, except for air-dried samples. Low fer-tilizer-N recoveries from St. Bernard and Ste. Sophiesoils may indicate greater N immobilization in thesesamples. Nitrogen immobilization could, at least part-ly, be attributed to the relatively high initial C/N ratiosobserved in these soils.

Table 2. Mean recovery of surface-applied N solutions (41.5 mg N) after 10 d in response to soil water potential.

FertilizerSoil solutionf

St. Bernard UUUANUAN

LSD (0.05) for N solutionLSD (0.05) N solution at dry vs. wetSte. Sophie U

UUANUAN

LSD (0.05) for N solutionLSD (0.05) N solution at dry vs. wet

Soil waterpotential

MPa

<-1.5-0.01

<-1.5-0.01

<-1.5-0.01

<-1.5-0.01

NH3-Nvolatilized

18.147.56.6

19.03.6

NS*2.8

54.70.3

16.79.0

11.8

Urea-N

51.40.4

26.40.26.75.6

57.20.1

34.70.11.8

11.8

N extracted from

NH4-N

— % of added N

19.129.833.242.24.71.83.3

26.821.246.24.23.0

soil

(NO3 + NO2)-N

0.20.4

18.320.0

1.0NS0.10.3

12.219.39.0NS

Total

88.878.184.581.6NS2.4

63.481.968.482.3NS4.1

t U = 100% urea solution, UAN = equal proportions of urea and ammonium nitrate.t NS = not significant.

1764 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

ISO

fca?

•40

0<-L5MPaD -1.4 MPa• -0.035MPaO treatment AT

' A treatment B J

30

SO

2 1O

a „

-o.oiMPa - - -..'35—--*^ ,.•ft^;?a7.*2036««

'''

•3 1O

Time (days after fertilizer application)Fig. 2. Cumulative NH3 losses from urea solutions surface applied to St. Bernard soil at various initial water potentials. (Treatment designations:

<-l.5MPa = air dried, -l.4MPa = 0.039 kg moisture kg"' soil, -0.035 MPa = O.I29 kg moisture kg"' soil, -O.Ol MPa [TreatmentsA and B] = 0.256 kg moisture kg"' soil).

1.3

OJB

a,L CMS

0.4

o.a

D St. Bernard• Ste.Sophie

i

i 0.5

D St.Bernard• Ste.Sophie ____-..—D

10

Time (days after fertilizer application)Fig. 3. The relationship between time of incubation and (a) water loss and (b) NH3 volatilization from St. Bernard and Ste. Sophie soils.

AL-KANANI ET AL.: SOIL WATER AND AMMONIA VOLATILIZATION RELATIONSHIPS 1765

Kinetic EffectPlots of water-vapor loss vs. time (Eq. [3]) showed

a significant linear relationship, as demonstrated byhigh values of the coefficient of determination (R2) andlow standard error of estimate (Fig. 3a; Table 3). Thislinear behavior strongly indicates first-order kineticsfor water evaporation from soils. Karathanasis andEvangelou (1987) also found dehydration kinetics forAl- and Ca-saturated soil to be mostly a first-orderreaction represented by Eq. [3]. However, the volatil-ization of NH3 throughout the entire course of reactiontime did not follow a single first-order relationship.These findings suggest that water evaporation andNH3 volatilization are governed by different kineticlaws, and may partly be attributed to the difference intheir partial vapor pressures in soils during incubation.Therefore, several kinetic models were fitted to NH3-volatilization data, including phase boundary, Avra-mi-Erofe'ev equations, and diffusion models. None ofthese models satisfactorily explain our NH3-volatili-zation data.

In spite of the complexity of NH3 volatilization fromsurface-applied N fertilizers, our results showed theprocess to be controlled by two first-order kinetics (Fig.3b). There was a break in the curve near Day 4 (Fig.3b), prior to which values of the initial rate constant(ki) were 4.9 to 9.7 times greater than the subsequentrate constant (&2) (Table 3). The two semilogarithmictime-course relationship for NH3 volatilization indi-cates a complex kinetic reaction. The reaction-rate-Table 3. Rate constants for NH3 volatilization (initial, *„ and sub-

sequent, *j) and water evaporation (A) from wet (Treatment A)soils following urea application.

NH, volatilization Water evaporationSoil SE* SE IP SE

d-' d'1

St. BernardSte. Sophie

0.338 0.991 0.28 0.035 0.99 0.410.348 0.997 0.10 0.071 0.992 0.40

d-0.099 0.998 0.130.104 0.995 0.21

t Coefficient of determination. Values were significant at P < 0.01.£ Standard error of estimate.

constant values ranged from 0.0348 to 0.348 d'1 forNH3 volatilization and 0.099 to 0.104 d-1 for waterevaporation (Table 3). Ammonia volatilizationshowed a logarithmic relationship with water evapo-ration from soils (Fig. 4). The rate for water evapo-ration was found to be consistently lower than theinitial rate constant, kt, for NH3 volatilization, indi-cating different energy requirements. Chao andKropntje (1964) reported that the rate of water evap-oration and the rate of NH3 volatilization from soilssaturated with NH4OH solution were not similar.

Because of the intimate relationship between U hy-drolysis and NH3 volatilization (Tomar et al., 1985),the first-order kinetics of NH3 volatilization can be en-visaged to follow from the established first-order ki-netics of U hydrolysis in soils (Sankhayan and Shukla,1976; Vlek and Carter, 1983). Vlek and Stumpe (1978)also reported that NH3 volatilization from NH4OH or(NH4)2SO4 solutions followed first-order kinetics. Inboth soils, the difference between the two rate constantsof NH3 volatilization was probably due to two factors:(i) reduced hydrolyzable U in solution due to its de-pletion by hydrolysis, and (ii) NH4 sorption.

Equation [4] provides an insight in interpreting theobserved differences between NH3 volatilization andwater evaporation from soils. Equation [4] is similarto the mass-transfer equation for NH3 volatilizationdeveloped by Koelliker and Miner (1973), except thatthe partial pressure of NH3 in the air at t = 0 wasassumed to be negligible in our experiment. The con-centration gradient dC/dz is expected to be high forNH3 volatilization across the soil surface-atmosphericboundary layer, as only trace amounts of NH3 mayexist above the soil surface inside individual jars atany given time. For water evaporation, the concen-tration gradient is relatively low due to the presenceof water vapor in the system. Therefore, the thermo-dynamic force influencing NH3 loss is expected to bemuch higher than that for water-vapor flux from thesame soil surface, assuming other physical parametersremain constant. This could, at least partly, explainthe consistent difference in kinetic behavior between

•1§40

§

'30

.1

3

D St. Bernard• Ste.Sopnie

so 7O 803O 4O 5O BO

Cumulative water evaporation (%)Fig. 4. Relationship between NH3 volatilization and water evaporation from wet (Treatment A) St. Bernard and Ste. Sophie soils.

1766 SOIL SCI. SOC. AM. J., VOL. 55, NOVEMBER-DECEMBER 1991

water-vapor and NH3 losses from surface-applied fer-tilizers observed in this research and previous re-search. If the thermodynamic forces that areresponsible for NH3 and water-vapor losses from soilsare different, then the possibility of coupling betweenthe individual fluxes also exists.

CONCLUSIONSNitrogen losses from U and UAN solutions through

NH3 volatilization were significantly influenced by theinitial water content of the soil. Maximum NH3 lossoccurred at initial soil water potential of —0.01 MPa,regardless of drying or water replenishment. The dataalso suggested that increased clay content of the soilreduced NH3 loss, possibly due to NHJ adsorption. Atair-dry soil moisture (< —1.5 MPa), the hydrolysis ofU was minimal. This was either due to low ureaseactivity at a high soil water potential and/or fewerencounters between the substrate and the urease mol-ecules due to the limited movement of U into the air-dried sample. The rate of NH3 volatilization followedtwo first-order kinetics, regardless of soil texture. Al-though NH3-volatilization and water-evaporation pro-cesses were not similar, the plot of NH3 volatilizationvs. water evaporation reveals a significant logarithmicrelationship between these quantities. In areas wherelarge amounts of N solutions are used, such a rela-tionship may become of practical importance.