DIVISION S-8-NUTRIENT MANAGEMENT& SOIL & PLANT ANALYSIS

Soil Acidification from Long-Term Use of Anhydrous Ammonia and UreaO. T. Bouman, D. Curtin*, C. A. Campbell, V. O. Biederbeck, and H. Ukrainetz

ABSTRACTAcidity generated by N fertilizers depends on factors such as the

composition of the fertilizer, climatic and soil conditions, and thecrops grown. Our objective was to quantify the acidifying effects ofurea and anhydrous NHj when used as fertilizers for cereal productionin Saskatchewan, Canada. The fertilizers were injected annually (at10-cm depth) into a medium-textured, moderately acid (pH =5.5)Typic Haploboroll, at rates of 0, 45, 90, and 180 kg N ha*' for 9 yr.Soil acidity increased as N application rate increased, with anhydrousNH.i causing greater acidification than urea. Although pH valuesas low as 4.3 were recorded in soil treated with anhydrous NHa,KCl-exchangeable acidity remained low. The major effect of acidifica-tion was a depletion of exchangeable Ca and Mg. The solubility ofMn (but not Al) increased substantially as pH decreased, with solutionconcentrations of almost 30 mg Mn L~' being recorded 6 d afterinjection of NHj. Acidity generated by anhydrous NHj compared wellwith values predicted assuming that all of the applied NH3 was oxidizedto NO3 (with the production of 1 mol H+ mol~' of N) and that theseprotons were partly neutralized by OH~ released when NOj wastaken up and assimilated by plants. Acidification due to export ofbases in grain was insignificant because wheat (Triticum aestivum L.)and barley (Hordeum vulgare L.) remove only a slight excess of cationsover anions. Urea failed to realize its full acidification potential becauseof an apparent loss of urea-N from the soil by NH, volatilization.

FERTILIZER N REQUIREMENTS of cropping systemshave increased on the Canadian prairies due to declin-

ing indigenous fertility and because crop rotations arebeing extended (Campbell et al., 1986). Minimizingfertilizer-induced acidification is an important aspect offertilizer management because correction of soil aciditycan be costly where lime is not available locally, as isusually the case on the prairies.

Urea is the main source of fertilizer N in Saskatchewan(Saskatchewan Agriculture and Food, 1992), with anhy-drous NHj being popular where there is a local supply.Urea and anhydrous NH3 are less acidifying than otherN fertilizers such as (NH4)2SO4 and monoammoniumphosphate (Adams, 1984). Fertilization with anhydrousNH3 and urea produces acidity through the followingreactions (Adams, 1984):(NH2)2CO + 4O2 -* 2NO3~ + 2H+ + CO2 + H2O [1]

NH3 + 2O2 — NO3- + H+ + H20 [2]

O.T. Bouman, D. Curtin, C.A. Campbell, and V.O. Biederbeck, Agricul-ture and Agri-Food Canada Research Station, P.O. Box 1030, SwiftCurrent, SK, Canada S9H 3X2; H. Ukrainetz, Agriculture Canada Re-search Station, Saskatoon, SK, Canada S7N 0X2. Received 21 Sept. 1993.*Corresponding author.

Published in Soil Sci. Soc. Am. J. 59:1488-1494 (1995).

According to these reactions, each mole of urea-N oranhydrous NH3-N oxidized to NOs" produces 1 mol ofH+. Uptake of NOs~ by plants leads to the release of anequivalent amount of OH" into the rhizosphere, resultingin neutralization of acidity produced by nitrification. Theacidity can only be fully neutralized if the NO3~ producedis completely recovered by plants and assimilated toorganic N (Bolan et al., 1991). Leaching of NOs~ fromthe root zone causes permanent acidification by uncou-pling the proton balancing system. Other N transforma-tion processes may also influence acidification. For exam-ple, denitrification reactions consume protons and tendto reverse the acidifying effect of nitrification (Bolan etal., 1991). The process of NH3 volatilization from urea(and anhydrous NH3) is neutral with respect to acidity(no net proton generation), but soil acidification potentialdecreases in proportion to the amount of urea- orNH3-N lost.

Acidification is accelerated when the harvested cropremoves an excess of basic cations (Ca + Mg + K +Na) over anions (Cl + SO4 + H2PO4 + NO3) (Pierreand Banwart, 1973). Thus, another process by whichfertilization may cause acidification is by increasing theexport of basic cations relative to unfertilized soil (Bolanetal., 1991).

The degree of acidification resulting from the use ofurea or anhydrous NH3 may be dependent on climaticconditions (which partly determine the extent of NO3~leaching and denitrification) and on the crops grown.Adams (1984) stated that the degree of soil acidity "thatdevelops from an NH4 fertilizer is not a constant value,but it is an integrated value reflecting soil characteristics,cropping systems, and environmental variables." In theabsence of a tested soil acidification model, field measure-ments offer the only reliable approach to predicting howmuch acidity will develop under a given set of soil-environment-cropping conditions. No long-term fieldstudies appear to have been carried out on the northernGreat Plains in which anhydrous NH3 and urea werecompared as to their effects on soil acidity. Our objectivewas to quantify the effects of long-term fertilization withanhydrous NH3 and urea on soil acidity, exchangeablecations, and soil solution composition.

MATERIALS AND METHODSThe experiment was conducted on a moderately acid (pH

= 5.5) Elstow loam at the Experimental Farm of Agricultureand Agri-Food Canada, Scott, SK, from 1983 to 1992. The

Abbreviations: AAS, atomic absorption spectroscopy; CEC, cation-exchange capacity; BS, base saturation; DOC, dissolved organic carbon.

1488

BOUMAN ET AL.: SOIL ACIDIFICATION FROM NITROGEN FERTILIZATION 1489

soil, which has clay content of 300 g kg"1, is well structured,with a moderate, medium, subangular primary structure thatbreaks down to a moderate, subangular blocky secondarystructure. Moisture content at field capacity (—0.03 MPa) is240 g kg"1 and at wilting point (-1.5 MPa) it is 100 g kg'1.Climatic conditions at this site are characterized by short, warmsummers and long, cold winters. Mean annual precipitation is355 mm, and potential evapotranspiration 635 mm, for a meanmoisture deficit of 280 mm (Campbell et al., 1990).

Before laying out the treatments, the experimental site wastilled with a heavy-duty cultivator with sweeps. Tillage depthwas 7.5 to 10 cm. Two N sources, anhydrous NH3 and urea,were applied at rates of 0, 45, 90, and 180 kg N ha"1 yr~ ' .A randomized block design with a split-plot treatment arrange-ment was used with N rate as main plots (3.7 m wide and30.5 m long) and N source as subplots. Each treatment wasreplicated four times. Each year, except for 1989 and 1992,one-half of each plot was seeded to hard red spring wheatand one-half to barley. In 1989 and 1992, canola (Brassicacampestris L.) replaced the cereals to break disease cycles.In spring of each year, anhydrous NH3 and urea were injectedat the 10-cm soil depth in rows spaced 30 cm apart. AnhydrousNH3 was contained in a pressurized cylinder and applied tothe soil by a conventional NH3 applicator fitted with bandingknife shanks (Dycketal., 1993). Because of plot and equipmentsize, the N fertilizers were injected in the same vicinity eachyear. The crops were seeded = 1 wk after N application.Generally, the plots were tilled once between N applicationand seeding to control weeds (in two of the years, no tillagewas required). To avoid disturbance of the N fertilizer band,the tillage depth was 7.5 cm or less. Tillage was carried outin the same direction as the N fertilizer band. Apart frompreseeding tillage, no further cultivation was carried out. In-crop weed control was achieved using herbicides. Monoam-monium phosphate fertilizer (11-24-0), at a rate of 19 kg Pha~', was placed with the cereal seeds; in the case of canola,it was banded 2.5 cm below and 2.5 cm to the side of theseed row at the time of seeding. Grain yields and grain Ncontents were determined each year (Campbell et al., 1994a).

Soil samples were taken three times during the final yearof the experiment (3 d before and 6 and 26 d after fertilization)to assess the long- and short-term effects of fertilization. Long-term effects on chemical properties were examined using sam-ples (0-7.5-, 7.5-15-, and 15-30-cm depths) taken on 5 May1992, 3 d before N fertilization. Within each plot, six randomsamples were taken from each depth increment and mixed togive a composite sample. Soil cores were also taken from the30- to 150-cm layer (in 30-cm depth increments) to measureleaching of NO3~ and NH4

+. On 14 May and 3 June (i.e., 6and 26 d after the 10th N application), samples were taken(0-7.5, 7.5-15, and 15-30 cm) to assess the short-term effectsof fertilization on soil chemistry. Post-fertilization samplingwas done using a 6-cm-diam. corer. Three cores were takenfrom each of three locations along the fertilizer band (the bandwas still clearly visible at the time of sampling) and combinedwith a similar number of cores taken from the adjacent in-terband area. Separate samples were also taken for bulk densitydetermination using a coring cylinder with an internal diameterof 5 cm (Campbell et al., 1994b).

Soil samples were air dried and ground to pass through a2-mm sieve before chemical analyses were performed. Chemi-cal properties measured included pH (1:2 soil/0.01 M CaCksuspension) and exchangeable cation composition. Exchange-able cations were extracted using 1 M KC1. Calcium, Mg,and Mn were determined by AAS and acidic cations (Al andH) by titration (Thomas, 1982). Effective CEC was estimatedby summing KCl-extractable cations. Use of KC1 to extract

cations made it impossible to measure exchangeable K. How-ever, tests showed that K amounted to only 3 % of effectiveCEC. Potassium chloride was selected because the major basiccations (i.e., Ca and Mg) and Al + H could be determinedin a single extract. Preliminary tests also showed that thesoils contained only trace amounts of exchangeable Na. Basesaturation was estimated by expressing exchangeable bases asa percentage of effective CEC. Kjeldahl N was determined bythe method of Bremner (1960). Nitrate- and NH4

+-N wereextracted in 0.5 M NaHC03 and determined as described byHamm et al. (1970). Recovery of applied N was computedfrom the amounts of N exported in grain and the increases insoil N relative to the check (zero-N) treatment.

Saturated-paste extracts were obtained as follows: duplicatesoil samples were wetted to saturation with distilled water,equilibrated overnight at room temperature, and transferredto Buchner funnels for vacuum extraction of the solution. Afterfiltration, the solutions were analyzed for pH, Ca, Mg, Na,K, Mn, Al, NH,, NO3, Cl, and DOC. Calcium, Mg, K, Na,Mn, and Al were determined by AAS. Ammonium andNO3~ were determined by the method of Hamm et al. (1970),Cl by the mercuric thiocyanate method (O'Brien, 1962), andSO4 by the procedure of Hamm et al. (1973). A DohrmannDC-180 automated carbon analyzer (Rosemount AnalyticalDivision, Dohrmann, Santa Clara, CA) was used to measureDOC.

Analysis of variance and regression analysis were performedusing the CoStat software package (CoHort Software, 1990).Orthogonal contrasts (SAS Institute, 1988) were used for com-parison of selected treatment means.

RESULTS AND DISCUSSIONpH and Exchangeable Cations

Fertilized soils were more acid than unfertilized soil(Table 1). Protons produced by reaction of applied Ndisplaced exchangeable bases, Ca and Mg, and loweredthe effective CEC. Acidification increased with N appli-cation rate and was more pronounced with anhydrousNH3 than with urea. In Illinois, Khonje et al. (1989)observed a similar effect of N source after 9 yr ofapplication but, in Kansas, Darusman et al. (1991) foundno effect of N source after 20 yr. Although Schwab etal. (1989) reported that fertilizer-induced acidificationcaused a greater depletion of Mg than Ca, the relativeproportions of exchangeable Mg and Ca changed littlein our study (Table 1).

Soil pH (in CaCli) decreased by up to one unit dueto fertilization, and reached a low of 4.3 at the injectiondepth in soil treated with the highest rate of anhydrousNHs. The acidifying effects of the fertilizers were con-fined mostly to the top 15-cm soil layer; in the 15- to30-cm layer, significant acidification was only observedat the highest application rates. Despite the low pH valuesin fertilized soil, KCl-exchangeable acidity representedonly a small proportion of total exchangeable cations(Table 1). Because it was so low, we did not attemptto partition exchangeable acidity into its components,A13+ and H+. Base saturation values (Table 1) wereconspicuously high (generally >95%) compared withvalues reported for soils with similar pH in more humidregions (Thomas and Hargrove, 1984). However, thesevalues are consistent with those observed for a range of

1490 SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995

Table 1. Soil pH, exchangeable cations, and base saturation of soils that had received nine annual applications of urea or anhydrousNH3 at Scott, SK.

Nsource

CheckUrea

Anhydrous NH3

LSD (P < 0.05)NH3 vs. urea

CheckUrea

Anhydrous NH3

LSD (P < 0.05)NH3 vs. urea

CheckUrea

Anhydrous NH3

LSD (P < 0.05)NHj vs. urea

Nrate

kg ha-1

04590

1804590

180

04590

1804590

180

04590

1804590

180

KCl-exchangeable cations

PH

5.35.45.14.85.04.84.60.2**

5.24.84.84.54.74.54.30.2**

5.75.65.95.05.55.34.70.3*#

BSt

%

989997979796912

**

989798929692853

**

999999989999941

#*

CECt

0-7.5 cm12.211.711.810.711.211.310.40.4**

7.5-15 cm12.311.512.310.411.211.510.50.5**

15-30 cm12.413.212.412.213.611.811.61.2

NS

Ca

9.29.09.08.28.58.57.50.4**

9.48.99.67.78.68.57.20.5**

9.510.19.69.2

10.79.08.41.2NS

Mg

—— cmolc kg"1 —

2.72.52.52.22.42.32.00.2**

2.72.22.41.92.22.11.70.2**

2.83.02.82.72.92.72.50.3NS

Al + H

0.20.10.20.20.20.30.60.2**

0.10.30.20.60.30.61.20.4*

0.10.10.10.30.10.10.50.1*

Mn

0.090.090.080.120.110.150.270.08**

0.060.120.120.210.130.290.390.14**

0.020.010.010.040.020.030.190.02**

*, ** Significant at P < 0.05 and 0.01, respectively. NS = not significant.t BS and CEC are effective base saturation and cation-exchange capacity, respectively.

acid soils throughout western Saskatchewan (Curtin etal., 1984). Manganese was a minor exchangeable cation,but it increased considerably with N fertilization, particu-larly where high rates of anhydrous NH3 were applied(Table 1).

Low exchangeable Al (and high base saturation) rela-tive to soil pH suggests either that release of Al by acidweathering of soil minerals was inhibited or that Al, oncereleased, was immobilized, perhaps by complexation byorganic matter. This latter mechanism has been invokedby Richardson and Riecken (1977) to explain low ex-changeable Al in acid prairie soils in Iowa. Researchusing selective chemical extractants has shown that arelatively small proportion of organic functional groupsin Saskatchewan soils is complexed by Al (Curtin et al.,1984). Thus, these soils would probably have the capacityto immobilize additional Al by formation of Al-organiccomplexes. Regarding Al release by acidification, it ispertinent to note that soil mineral stability is influencednot only by soil pH, but also by the activities of solutessuch as Mg2+ and H4SiO4 (Kittrick, 1977). We speculatethat high concentrations of soluble Mg and H4SiO4 inour prairie soils, compared with leached soils in morehumid environments, would tend to stabilize clay miner-als, thereby inhibiting the release of Al. Climatic condi-tions (i.e., soil usually frozen for «5 mo each year anddry for much of summer) may also have limited Alrelease.

Soil Solution CompositionThere is increasing evidence that soil solution data

provide the most reliable means of identifying chemicalfactors limiting plant growth in acid soils (Curtin andSmillie, 1983; International Board for Soil Researchand Management, 1987). For simplicity, we used thesaturation extract as a proxy for the soil solution. Datafor anhydrous-NH3-treated soil showed that fertilizationdecreased solution pH to «4.5 (Table 2). Solution pHdid not change significantly between sampling dates, eventhough considerable acidity was apparently generated bynitrification in the days following N application. Thissuggests that the soil has an effective proton bufferingsystem at low pH. As expected, fertilization substantiallyincreased soluble salt concentration; salt concentrationdecreased with time after N application as solutes diffusedaway from the fertilizer band. Although low pH is knownto be a limiting factor for nitrification (Haynes, 1986),the results showed that nitrification of applied N occurredeven under strongly acid conditions (Table 2), indicatingpossible adaptation of nitrifying organisms to acidity.

The concentrations of Al and, especially, Mn increasedas pH decreased, with the increase in Mn being particu-larly strong a few days after fertilizer application (Table2). The phytotoxicity of these metals is dependent onthe ionic species present in solution. In the case of Al,for example, the monomeric species [A13+, A1OH2+,Al(OH)^] are believed to be highly toxic, whereas com-

BOUMAN ET AL.: SOIL ACIDIFICATION FROM NITROGEN FERTILIZATION 1491

Table 2. Effect of anhydrous NH, application rate on composition of saturation extracts of soil (7.5-15 cm) sampled three times duringthe 10th year of the experiment at Scott, SK.

N rate pH ECt

dS m-1

DOCt Mn Al

- mg L~' ————

Nil, Na K Mg

— mmolc

CaI -'L

NO, Cl so.

0 3d before fertilization

LSD (P < 0.05)

04590

180

5.55.14.94.60.3

0.30.40.40.70.3

769494

11016

0.2 0.10.8 0.23.3 0.55.5 0.53.9 0.3

0.10.10.10.4NS§

0.20.30.20.30.1

0.10.10.10.2NS

0.71.01.01.4NS

1.32.32.33.5NS

1.33.03.15.32.6

0.30.40.30.3NS

0.40.40.30.30.1

6 d after fertilization

LSD (P < 0.05)

04590

180

5.74.74.64.50.4

0.41.42.82.90.4

8384

10411820

0.2 0.23.7 0.1

12.6 0.329.1 0.74.3 0.3

0.00.14.37.33.1

0.20.40.60.50.1

0.10.30.90.90.2

1.03.25.85.01.0

1.98.2

12.511.82.1

2.211.326.226.24.2

0.30.20.30.2NS

0.40.30.30.30.1

26 d after fertilization

LSD (P < 0.05)LSD (P < 0.05)

(Time)

04590

180

5.55.04.74.60.4

NS

0.20.71.31.90.7

0.2

8791

12716647

17

0.4 0.11.6 0.29.3 0.3

18.7 0.76.8 0.2

2.6 NS

0.00.10.32.31.4

0.9

0.20.30.30.4NS

0.0

0.10.20.50.90.2

0.1

0.61.53.23.82.0

0.6

1.33.87.29.54.6

1.4

1.65.3

10.516.66.7

2.2

0.30.30.30.3NS

0.0

0.30.30.20.3NS

0.0

t EC = electrical conductivity.t DOC = dissolved organic carbon.§ NS = not significant (P < 0.05).

plexed forms (organic-Al, A1SO4+) may be nontoxic

(Noble et al., 1988; Parker et al., 1988). We usedSOILCHEM (Sposito and Coves, 1988) to compute thedistribution of Al and Mn species in solutions fromanhydrous-NHa-treated soil. It was assumed that DOC(Table 2) was in the form of fulvic acid (C content of50%, relative molecular mass 900, pKa 9.0).

The predominant Mn species in solution was Mn2+

(92-94% of total); the remaining Mn consisted largelyof Mn-fulvic acid complexes (4-7% of total Mn). Therewas a reasonably good relationship between pMn2+ (neg-ative logarithm of molar activity of Mn2+) and pH (Fig.1). The pMn2+ vs. pH relationship was similar to thatreported by Khanna and Mishra (1978) for saturatedpaste extracts of German soils. The solubility of Mn isnot determined solely by pH; exchange reactions withfertilizer cations may increase the concentration of Mn(and also Ca, Mg, and K) (Table 2). However, ourresults suggest that a reasonable estimate of Mn solubilitycan be obtained from the pH of the saturation extract.

Aluminum has a strong tendency to form organiccomplexes and calculations using SOILCHEM suggestedthat virtually all of the Al in solution was in complexeswith fulvic acid. Although the total concentration ofAl was increased by fertilization, parallel increases indissolved organic matter (Table 2) tended to enhance Alcomplexation and ensured that the concentration of toxicmonomeric Al species was very low even in the mostacid systems. Concentrations of A13+ estimated by SOIL-CHEM, which were <10~9 M, were probably too lowto have adverse effects on plant growth (Foy, 1984).The results imply that Mn is likely to be the main toxicantin this soil. However, it should be noted that the natureof the dissolved organic matter and the stability constantsfor Al-fulvic acid complexes are uncertain (Sposito andCoves, 1988). Thus, the possibility of Al toxicity cannotbe totally discounted. Taken together, the exchange and

solution data (Tables 1 and 2) indicate that acid weather-ing has released relatively little Al. The reason for thelow solubility of Al needs to be determined as it maybe a key to understanding how crops respond to aciditygenerated by N fertilizers.

Detailed discussion of crop responses to the two fertil-izer N sources throughout the 10 yr of the experimentis presented elsewhere (Campbell et al., 1994a). Briefly,cereals, especially barley, responded positively to fertil-izer N in years when weather conditions were favorable.Because of greater acidification resulting from use ofanhydrous NHs, cereal response to that N source wasless than to urea in the later years of the experiment,when the soil had been substantially acidified. Canola

A 3 days before• 6 days after• 26 days after

Fig. 1. Relationship between the negative logarithm of molar activityof Mn2+ (pMn2*) and pH of saturation extracts of soils (7.5-15cm) sampled 3 d before and 6 and 26 d after application of anhydrousNH3 in the 10th year of the experiment at Scott, SK. The brokenline was drawn in accordance with the equation of Khanna andMishra (1978) for German soils.

1492 SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995

Table 3. Nitrogen mass balance (cumulative for 9-yr period) comparing fertilizer N applied (90 and ISO kg ha~' yr~recovered in grain and soil at Scott, SK.

rates) to N

Total applied NN recoveryt

Grain NSoil (0-30 cm) organic NSoil (0-30 cm) NH,Soil (0-30 cm) NO,Deep core (30-150 cm) NR,Deep core (30-150 cm) NO3

Total recoveredBalance (recovered N - applied N)

90

810

1140 (208)t1

2964

140348 (556)$

-462 (-254)

Urea

180

———————————— k g N1620

1870 (362)

134214

223479 (841)

-1141 (-779)

Anhydrous

90

ha-'810

1190 (243)

1726

3304469 (712)

-341 (-98)

NH3

180

1620

1250 (324)

5828

3523787 (1111)

-833 (-509)

Contrast,urea vs.

anhydrousNH3

(P value)

0.220.730.090.050.280.06

0.10t N recovery was estimated as the difference between fertilized and check treatments.t Values in parentheses show N recovery when increases in soil organic N (which were not significant, P - 0.05) were included in the N balance calculation.

was sensitive to soil acidity and its yield was depressedby fertilization, especially by anhydrous NHs.

Nitrogen RecoveryThe cumulative N balance for the 1983 to 1992 period

(Table 3) suggests that substantial losses of N haveoccurred from the soil-plant system over the years atthe two high rates of fertilization. Only 30 to 43 % ofurea-N and 49 to 58% of anhydrous NHa-N could beaccounted for in terms of N exported in grain and Nremaining in the soil (Table 3). A relatively small propor-tion (8-15 %) of applied N was removed in grain. Nitratepresent in the deep cores (30-150 cm) represented thelargest single component of the N recovered in the soil-plant system. Although not significant at P = 0.05, therewas a tendency for soil organic N to increase withfertilization (Campbell et al., 1994a). If the fertilizer-induced increases in soil organic N (shown in parenthesesin Table 3) are taken at face value and included in theN balance, N recovery increases to 52 to 88%. SoilNO^ profiles (not shown) suggested that at least part ofthe missing N has been leached beyond the 150-cm depth.Although the experimental site is located in a regioncharacterized by net annual moisture deficit, this doesnot preclude the possibility of NOs" leaching beyond theroot zone in years with above-average precipitation, orwhen rainfall distribution is such that large amountscome in short periods. Campbell et al. (1984, 1993)have conclusively shown this to be the case under evenmore arid conditions at Swift Current, SK.

Proton BalanceFertilizer-derived protons that are not neutralized by

the H+ balancing system of the plant react with the soilto reduce the effective CEC (displacing basic cations)and increase exchangeable acidity. The displaced basesmay then be leached as companion cations to NOf. Aproton balance for fertilized soils was constructed byestimating the changes in exchangeable bases (Ca andMg) and acidity (Al + H) relative to the zero-N treat-ment. It was assumed that acidification was confined tothe 0- to 30-cm depth and that no significant amount of

H+ was lost from the soil by leaching. Amounts ofacidity were expressed on an area basis (i.e., kmolH+ ha"1) using bulk density values to convert fromgravimetric units. For soils that received nine annualapplications of anhydrous NHs at the 90 and 180 kg ha"1

rates, the increases in soil acidity were equivalent to 49and 103 kmol H+ ha'1, respectively (Table 4). Themaximum possible proton yield (i.e., 1 mol H+ mol"'N) is 58 and 116 kmol H+ ha'1 at the 90 and 180 kgha~' rates. Based on the amounts of N exported in grain(Table 3), OH" extruded by plants in exchange forNOf may have neutralized =9 kmol H+ ha"1. (Thesmall amounts of N in shoots/straw were returned to thesoil and thus do not need to be specifically accountedfor in terms of the proton balance.) Since the surplusof cations over anions in cereal (wheat and barley) grainis small (Pierre and Banwart, 1973), acidification dueto excess cation removal in grain was estimated to besmall. For anhydrous-NH3-treated soils, actual protonproduction compared very well with predicted values(Table 4). However, in urea-treated soil, proton genera-tion was much less than expected.

Table 4. Proton budget for plots receiving urea and anhydrousNH3 at rates of 90 and 180 kg N ha~' yr-' for 9 yr (i.e., totalof 810 and 1620 kg N ha"') at Scott, SK.

Protonsource

Potentialacidification! (a)

Protons neutralized byplant uptake of NOj| (b)

Excess base exports (c)Predicted acidification

(a + c - b)Measured acidification

Urea810

57.8

8.10.2

49.911.5

1620

—— kmolH*

115.6

13.30.4

102.750.5

Anhydrousammonia

810hi~l

57.8

8.50.3

49.648.9

1620

115.6

8.90.3

107.0103.1

t Potential acidification was estimated assuming that all fertilizer N wasoxidized to NO3, with production of 1 mol H* mol-' N.

t Estimated from the grain-N data in Table 3.§ Excess base (EB) export was calculated using an EB/N ratio of 0.03 for

barley grain (Pierre and Banwart, 1973). Calculations using the data ofEnsminger and Olentine (1978) suggested that the base removal in wheatgrain was negligible (EB/N « 0.01). Since yield of canola was depressedby fertilization (Campbell et al., 1994a), a small (negligible) decrease inbase export resulted from fertilization of that crop.

BOUMAN ET AL.: SOIL ACIDIFICATION FROM NITROGEN FERTILIZATION 1493

The close correspondence between measured and pre-dicted acidification implies that NH3 volatilization fromanhydrous-NH3-treated soil must have been negligible,as would be expected where NH3 is injected into a moistsoil with moderately high CEC (Harapiak et al., 1985).In early spring when fertilizer N was applied, the soilsurface was usually dry but the soil was moist at thedepth of placement. The acidic, moist condition of thissoil would have facilitated rapid adsorption of the anhy-drous NH3 (Fenn and Hossner, 1985). Less soil acidifica-tion, coupled with lower recovery of urea-N than anhy-drous NHs-N, suggests that some urea-N was lost fromthe soil by nonacidifying processes. The most likelymechanism of urea-N loss was NH3 volatilization. Incor-poration of urea at a depth of 10 cm was expected tominimize volatile losses. However, soil drying followingurea incorporation, particularly in the area disturbed bythe banding knife, and later by preseeding tillage, wouldhave favored NH3 volatilization. Movement of water asthe soil dries may transport urea to the soil surface whereloss of NH3 resulting from hydrolysis of urea is mostlikely to occur (Harapiak et al., 1985). The enzymatichydrolysis of urea to NH3 takes several days to reachcompletion, depending on moisture and temperature con-ditions (Fenn and Hossner, 1985; Harapiak et al., 1985),thus facilitating the gradual loss of NH3.

Our results suggest that the degree of permanent soilacidification by anhydrous NH3 is determined largely byleaching losses of NHj-derived NOf. Minimizing theleaching of NOf by matching fertilizer rates to croprequirements (i.e., avoiding overfertilization) may bethe key to preventing permanent acidification. On thesemiarid Canadian prairies, NOf leaching occurs mostlyduring fallow periods (Campbell et al., 1994b). Thus,minimizing the extent of summer fallow may help curtailfertilizer-induced acidification. Because cereal grainshave only a small excess of cations over anions (Pierreand Banwart, 1973), fertilizer-induced increases in grainexport will not cause significant soil acidification.

SUMMARY AND CONCLUSIONSApplication of high rates of anhydrous NH3 and urea

for 9 yr caused extensive soil acidification. The amountsof acidity generated by anhydrous NH3 compared wellwith values predicted assuming that all of the applied Nwas oxidized to NOf (with the production of 1 mol H+

mor1 N applied), and that assimilation of NOf intoorganic N by plants resulted in release of OH" intothe soil in amounts equivalent to N exported in grain.Although urea should, in theory, cause the same degreeof acidification as anhydrous NH3, our results showedthat urea induced significantly less acidification. Thiswas attributed to an apparent loss of urea-N from thesoil due to NH3 volatilization, a nonacidifying process.

Leaching of NOf, which uncouples the proton balanc-ing system, was the dominant process causing permanentacidification when soils cropped to cereals were fertilizedwith urea or anhydrous NH3. When NOf leaching lossesare minimized by matching the fertilizer application rateto the N requirement of the crop, little acidification

should occur. In our study, nine annual applications ofurea and anhydrous NH3 at a "practical" rate (45 kg Nha"1) caused little soil acidification.

Although the highest rate of anhydrous NH3 causedintense acidity, with pH values as low as 4.3, exchange-able Al and the concentration of Al in soil solutionremained low. In contrast, the solubility of Mn wasstrongly pH dependent, with solution Mn concentrationsof almost 30 mg L"1 being recorded for soil treatedwith the 180 kg N ha"1 rate of anhydrous NH3. It wasconcluded that Mn, rather than Al, is likely to be themajor toxicant when soils such as the one used in thisstudy are acidified by excessive fertilization.

ACKNOWLEDGMENTSFunding for this research was provided by the Parkland

Agriculture Research Initiative. Technical assistance of JonGeissler, Darrell Hahn, Rod Ljunggren, and Larry Sprouleis gratefully acknowledged. We thank Dr. A.R. Mermut,University of Saskatchewan, for determining Al in our soilsolution samples.

1494 SOIL SCI. SOC. AM. J., VOL. 59, SEPTEMBER-OCTOBER 1995