denitrification in four california soils: effect of soil profile characteristics1

6
DIVISION S-3—SOIL MICROBIOLOGY AND BIOCHEMISTRY Denitrification in Four California Soils: Effect of Soil Profile Characteristics 1 J. W. GILLIAM, S. DASBERG, L. J. LUND, AND D. D. FocHT 2 ABSTRACT The effects of soil profile characteristics upon rate and products of denitrification were investigated using four soils in laboratory col- umns under steady-state water flow conditions. A general relationship existed between soil texture and amount of denitrification. Soils with heavy textured subsoils readily reduced added NOj~. However, the soil with the lowest clay content had restricted water flow in the surface horizon and reduced the greatest amount of NOs~. The relationship between soil texture and denitrification is a result of the relationship between water flow and denitrification. It was concluded that any soil condition which impedes water flow will be positively related to denitrification and that spatial variability in denitrification is likely to be as great as observed variability in water movement. The ratio of Nj to N 2 O found during denitrification was extremely variable with measured values from 100:1 to 1:4. The low con- centrations of NjO measured during denitrification occurred in soils where the denitrification was occurring deep in the soil profile, but there was no indication that this low concentration was a result of further reduction of N 2 O as N 2 O diffused through the soil profile. Our data indicate that it is currently not possible to accurately predict relative amounts of N 2 and N 2 O which will be produced during most denitrification in soils, and that estimates of that N 2 O produced from agricultural lands have a large uncertainty factor. Additional .Index Words: nitrous oxide production, diffusion of nitrogen gases. Q UANTITATIVE PREDICTION of the amount of denitrifica- tion which may occur in most fertilized fields and the ratio of N 2 to N 2 O which will result from this denitrification is currently not possible. The need for quantitative es- timates of denitrification and NOs~ movement to ground and surface waters has been apparent for several years. The possibility of N 2 O evolution during denitrification con- tributing to depletion of ozone in the stratosphere has only recently received attention (Crutzen, 1974; McElroy, 1976). Previous work in California (Pratt et al., 1972; Adriano et al., 1972; Lund et al., 1974) and the midwest (Gentzsch et al., 1974) has demonstrated a relationship among soil profile characteristics and NOs~ concentration within and below root zones. In general, these studies have shown lower NOs" concentrations below fine-textured soils or soils with textural discontinuities than below coarse-tex- tured soils. This is assumed to be a result of differences in denitrification in these fields. Despite the success in California within particular study areas of relating Contribution of Dep. of Soil Sci. & Agric. Eng., Univ. of California, Riverside, CA 92502. The research was supported by grant GI 34733x of the National Science Foundation. Received 30 Mar. 1977. Approved 30 Aug. 1977. 2 Visiting Research Soil Scientists, Assistant Professor and Associate Professor, respectively. Senior author was on leave from North Carolina State Univ., Raleigh, North Carolina, and second author was on leave from the Inst. of Soils and Water, Agric. Res. Organization, The Volcani Center, Bet Dagan, Israel. concentrations below rooting depth to particle size dis- tribution of the profile control sections, it has been difficult to extrapolate this information to other areas to predict NOa" concentrations there (Lund et al., 1975). The proportion of N 2 O evolution during denitrification in soil systems has been reported to be from 0 to 100% of the total gases produced (CAST, 1976). Although much of the variations found' between soils in gas ratios produced during denitrification can be attributed to differences in pH and aeration (Focht, 1974), most ratios have been de- termined in sealed systems which may differ significantly from situations where gases are allowed to diffuse freely to the atmosphere. Recent work of Rolston et al. (1976) indicated that it is possible to obtain a direct estimate of denitrification in open systems by measuring I5 N in N 2 O and N 2 following application of 15 N-labeled NOa" and calculating losses by diffusion of gases from the soil system. This laboratory research was initiated to obtain a better understanding of the effects of profile characteristics upon denitrification in irrigated soils and to determine the magnitude of the N 2 O evolved during denitrification in different soil profiles. Four soils were selected to represent a range of profile characteristics: (i) uniform coarse texture, (ii) uniform fine texture, (iii) coarse-loamy over fine silty, and (iv) fine-loamy over coarse-loamy. MATERIALS AND METHODS The four soils selected to represent distinct profile characteris- tics were a Hanford, Sorrento, Milham, and Zamora and are described in Table 1. The Hanford soil, a coarse-loamy, mixed thermic Typic Xerothent, was selected to represent a uniform, coarse-textured profile. The Sorrento, a fine-silty, mixed thermic Typic Camborthid, is a soil with a uniform fine-textured profile. The Zamora is a fine-silty, mixed thermic Mollic Haploxeralf, with coarse over fine-textured material, and the Milham is a coarse-loamy, mixed thermic Calcic Haploxeroll, with fine-loamy over coarse-textured material. The field soils were bulk-sampled in six layers: 0-25 cm, representing the plow layer, and five subsequent 15-cm layers. Table 1—Characteristics of experimental soils. Soil Hanford Milham Sorrento Zamora Layer cm 0-25 25-100 0-25 25-55 55-100 0-25 25-100 0-25 25-55 55-100 Texture fsl fsl 1 1 si sicl sicl fsl 1 sicl Clay % 12.6 11.3 20.9 14.5 8.6 35.5 33.6 12.7 23.4 33.3 PH 6.2 7.6 7.9 8.1 8.1 7.4 7.5 6.3 6.9 7.3 Organic matter % 0.77 0.31 0.72 0.20 0.11 2.19 1.28 1.52 1.47 1.40 Bulk density g/cm" 1.6 1.6 1.5 1.5 1.6 1.3 1.3 1.4 1.4 1.3 61

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DIVISION S-3—SOIL MICROBIOLOGY AND BIOCHEMISTRY

Denitrification in Four California Soils: Effect of Soil Profile Characteristics1

J. W. GILLIAM, S. DASBERG, L. J. LUND, AND D. D. FocHT2

ABSTRACTThe effects of soil profile characteristics upon rate and products of

denitrification were investigated using four soils in laboratory col-umns under steady-state water flow conditions. A general relationshipexisted between soil texture and amount of denitrification. Soils withheavy textured subsoils readily reduced added NOj~. However, thesoil with the lowest clay content had restricted water flow in thesurface horizon and reduced the greatest amount of NOs~. Therelationship between soil texture and denitrification is a result of therelationship between water flow and denitrification. It was concludedthat any soil condition which impedes water flow will be positivelyrelated to denitrification and that spatial variability in denitrificationis likely to be as great as observed variability in water movement.

The ratio of Nj to N2O found during denitrification was extremelyvariable with measured values from 100:1 to 1:4. The low con-centrations of NjO measured during denitrification occurred in soilswhere the denitrification was occurring deep in the soil profile, butthere was no indication that this low concentration was a result offurther reduction of N2O as N2O diffused through the soil profile. Ourdata indicate that it is currently not possible to accurately predictrelative amounts of N2 and N2O which will be produced during mostdenitrification in soils, and that estimates of that N2O produced fromagricultural lands have a large uncertainty factor.

Additional .Index Words: nitrous oxide production, diffusion ofnitrogen gases.

QUANTITATIVE PREDICTION of the amount of denitrifica-tion which may occur in most fertilized fields and the

ratio of N2 to N2O which will result from this denitrificationis currently not possible. The need for quantitative es-timates of denitrification and NOs~ movement to groundand surface waters has been apparent for several years. Thepossibility of N2O evolution during denitrification con-tributing to depletion of ozone in the stratosphere has onlyrecently received attention (Crutzen, 1974; McElroy,1976).

Previous work in California (Pratt et al., 1972; Adrianoet al., 1972; Lund et al., 1974) and the midwest (Gentzschet al., 1974) has demonstrated a relationship among soilprofile characteristics and NOs~ concentration within andbelow root zones. In general, these studies have shownlower NOs" concentrations below fine-textured soils orsoils with textural discontinuities than below coarse-tex-tured soils. This is assumed to be a result of differences indenitrification in these fields. Despite the success inCalifornia within particular study areas of relating

Contribution of Dep. of Soil Sci. & Agric. Eng., Univ. of California,Riverside, CA 92502. The research was supported by grant GI 34733x ofthe National Science Foundation. Received 30 Mar. 1977. Approved 30Aug. 1977.2Visiting Research Soil Scientists, Assistant Professor and AssociateProfessor, respectively. Senior author was on leave from North CarolinaState Univ., Raleigh, North Carolina, and second author was on leavefrom the Inst. of Soils and Water, Agric. Res. Organization, The VolcaniCenter, Bet Dagan, Israel.

concentrations below rooting depth to particle size dis-tribution of the profile control sections, it has been difficultto extrapolate this information to other areas to predictNOa" concentrations there (Lund et al., 1975).

The proportion of N2O evolution during denitrification insoil systems has been reported to be from 0 to 100% of thetotal gases produced (CAST, 1976). Although much of thevariations found' between soils in gas ratios producedduring denitrification can be attributed to differences in pHand aeration (Focht, 1974), most ratios have been de-termined in sealed systems which may differ significantlyfrom situations where gases are allowed to diffuse freely tothe atmosphere. Recent work of Rolston et al. (1976)indicated that it is possible to obtain a direct estimate ofdenitrification in open systems by measuring I5N in N2Oand N2 following application of 15N-labeled NOa" andcalculating losses by diffusion of gases from the soilsystem.

This laboratory research was initiated to obtain a betterunderstanding of the effects of profile characteristics upondenitrification in irrigated soils and to determine themagnitude of the N2O evolved during denitrification indifferent soil profiles. Four soils were selected to representa range of profile characteristics: (i) uniform coarse texture,(ii) uniform fine texture, (iii) coarse-loamy over fine silty,and (iv) fine-loamy over coarse-loamy.

MATERIALS AND METHODSThe four soils selected to represent distinct profile characteris-

tics were a Hanford, Sorrento, Milham, and Zamora and aredescribed in Table 1. The Hanford soil, a coarse-loamy, mixedthermic Typic Xerothent, was selected to represent a uniform,coarse-textured profile. The Sorrento, a fine-silty, mixed thermicTypic Camborthid, is a soil with a uniform fine-textured profile.The Zamora is a fine-silty, mixed thermic Mollic Haploxeralf,with coarse over fine-textured material, and the Milham is acoarse-loamy, mixed thermic Calcic Haploxeroll, with fine-loamyover coarse-textured material.

The field soils were bulk-sampled in six layers: 0-25 cm,representing the plow layer, and five subsequent 15-cm layers.

Table 1—Characteristics of experimental soils.

Soil

Hanford

Milham

Sorrento

Zamora

Layercm

0-2525-100

0-2525-5555-100

0-2525-1000-25

25-5555-100

Texture

fslfsl11si

siclsiclfsl1

sicl

Clay%

12.611.320.914.58.6

35.533.612.723.433.3

PH

6.27.67.98.18.17.47.56.36.97.3

Organicmatter

%0.770.310.720.200.112.191.281.521.471.40

Bulkdensityg/cm"

1.61.61.51.51.61.31.31.41.41.3

61

62 SOIL SCI. SOC. AM. J . , VOL. 42, 1978

After air drying, grinding to pass a 2-mm sieve, and mixing, thesoils were filled into 12-cm I.D. 100-cm PVC columns, accordingto the six layers. Two columns of each soil were packed to theirfield bulk densities which had been determined from undisturbedcores.

The columns were equipped with 1 -cm diam tensiometer cupsat depths of 5, 15, 30, 45, 60, 75, and 90 cm. These wereconnected to mercury manometers to monitor soil water potentialand to serve as soil solution samplers. Gas sampling ports,consisting of glass capillaries filled on inside with glass wool andclosed with rubber septums, were sealed in the column walls atdepths of 5, 10, 15, 20, 25, 30, 37.5, 45, 52.5, 60, 67.5, 75,82.5, and 90 cm. Additional ports were drilled in the column atdepths of 10, 20, 30, 45, 60, 75, and 90 cm, for the insertion of Ptelectrodes for redox potential measurements. At the bottom of thecolumn three 2-cm diam tensiometer cups were sealed andconnected by means of a suction flask to a controlled vacuum.After construction and before soil addition; each column wasfilled with water and carefully checked for leaks.

The columns were wetted slowly from below with a solutioncontaining 1.13 meq/liter NaHCO3, 0.13 meq/liter KHCO3, 1.6meq/liter CaQ2 and 1.48 meq/liter MgSO4. This solution waschosen to approximate the average chemical composition of theirrigation water in the San Joaguin Valley. After the soil wascompletely saturated, a constant vacuum of 400 mbars wasapplied to the bottom tensiometers. The above-described solutionwas applied to the top at a rate of 4 mm/day, by means of an 8-channel syringe pump activated every 30 min. After steady-statewater flow had developed as determined by constant soil waterpotential profiles and effluent outflow, a NO3~ pulse was appliedto each column. Before applying the tagged NO3~, soil solutionsamples were taken to determine soluble organic C and initialNO3~ contents.

One gram of KNO3, 99% enriched with I5N, was added to eachcolumn resulting in a rate of 147 mg NO3~-I5N per column(equivalent to 122 kg NO3"-'4N/ha), in 10 ml of water. Afteraddition of NO3~, gas samples were taken periodically with gas-tight syringes and analyzed as soon as possible (not longer than 3hours) by combined gas chromatography-mass spectrometry on aFinnigan Model 3100C. 15N2O, I5N2) and O2 concentrations weredetermined from prepared standards using the mass spectrometer,for the specific mass of the molecular ion, as the detector. APorapak Q column was used for separation of N2O, and amolecular sieve 5A column was used for separation of N2 and O2in separate analyses. Occasional samples were analyzed forI4N15NO and 14NI5N, and it was observed that very little NO3~containing I4N was being denitrified. Solution samples were takenweekly from the column, and NO3~ in the effluent solution wasdetermined. The isotopic composition of the NO3~-N in theeffluent was determined on a composite of 3-week periodsamples. At the completion of the experiment, when the effluentwas virtually free of NO3~, soil samples were taken from severaldepths and total I5N enrichment was determined.

RESULTSColumn Conditions

The steady-state soil moisture tension profiles in the foursoils, resulting from the same water application rate (4 mm/day) and the same suction at the bottom of the columns, aregiven in Fig. 1. The duplicate columns of all soils gavenearly identical tension profiles. The differences in watertransmission properties of the four soil profiles are evidentfrom this picture. The Milham soil shows the highest soilmoisture tension. The loamy upper part of the profile doesnot seem to interfere with water movement. The Hanfordsoil, although coarser textured than the Milham in the upperpart, shows the development of a saturated layer in the top

SOIL MOISTURE TENSION (cm woter)50 100 ISO 200 250 300 350 40O

too>-Fig. 1—Soil moisture tension profiles in the four soils.

soil. This soil is known for its low infiltration rate(Christensen et al., 1967) even though it is very sandythrough the profile. The Sorrento, although fine texturedthroughout the profile, has a relatively high organic mattercontent (Table 1) and a developed structure, resulting inrelatively high soil moisture tensions. The Zamora soil hasa fine-textured subsoil, which interferes with water move-ment in the lower profile resulting in low soil moisturetensions in the lower horizons as compared to the othersoils studied.

The oxygen concentrations measured from soil airsamples throughout the experimental period (Table 2) areconsistent with the data of Fig. 1. The Milham soil showedhigh Oa contents in the whole profile. The Zamora had lowO2 contents in the fine-textured subsoil. The fine-texturedSorrento profile had decreased O2 contents beginning at 25-cm depth. The Hanford soil showed the greatest variation inO2 contents, resulting in averages of intermediate values.Although the top two tensiometers showed saturation in theHanford soil, we were able to extract soil gas samples fromthese layers, the analyses of which seemed to indicate acontinuous gas phase in the profile. Soil samples taken afterthe completion of the experiment showed air-filled porosi-ties of 11% at 0-25-cm depth, increasing to 18% at thebottom of the profile.

Table 2—Oxygen concentrations in the soil profiles.

Depth

HanfordMilhamSorrentoZamora

10

16.619.418.720.0

25

12.816.811.019.0

45

— % 0,1 -15.317.37.3

17.8

67.5

14.517.210.98.8

90

14.718.510.810.5

t Average for weekly samples in two columns.

GILLIAM ET AL.I DENITRIFICATION IN FOUR CALIFORNIA SOILS 63

Table 3—Summary of denitrification results.

Hanford Sorrento Zamora

Coii Coin Coil Coin Coii Coin"N in effluent

(% of added)"N in soil (% of added)Denitrification

(% of added)Depth of denitrifica-

tion, cmPeriod of denitrifica-

tion, day

21 22 42 6115

27 319

79 78 43 24 64 60

0-15 15-30 30-60 45-90 60-90 60-90

4-16 13-25 18-53 74-95 39-74 37-86

The redox potential measurements were less consistent,showing the inherent variability of this measurement in soilsystems (Meek and Grass, 1975). However, the trendswere consistent with the above mentioned data - Ehreadings above 300 mV in the Milham soil, below 300 mVin the lower parts of the Sorrento and Zamora soils, whilethe Hanford soil showed low Eh readings in the entireprofile.

Nitrate Movement and LossThe distributions with time of the added NO3~ between

the soil solution, effluent, and denitrification are shown inFig. 2-4. The NO3~ in the column was calculated fromconcentration data from samples at each tensiometer depthand the water contents at these depths. These data aresomewhat variable from week to week because of nitratepeaks being at the same depth as cups when sampled oneweek and between ceramic cups at the next sampling. Atthe end of the experiment, a 15N balance was made bysubtracting the I5N in column and 15N in the effluent fromthe 15N added. This gave an accurate estimation ofdenitrification "by difference" (Table 3).

The simplest case of I5N balance occurred in the Milhamsoil, since essentially all of the 15N applied was recoveredin the effluent. Nitrate appeared in the effluent at ap-proximately the same rate as it decreased in the column. Nosigns of denitrification were found in this soil in the form ofNaO or 15N2 (above ambient) in the soil atmosphere. Thissoil, contrary to the other three soils, showed a high nativeNC>3~. Before and after the 15N pulse passed through thecolumns, it contained approximately 35 mg NOs~-N.

Denitrification was initiated 3-5 days after addition of theNC>3~ in the Hanford soil columns as indicated by presenceof 50-150 ppm N2O in the soil atmosphere. Essentially allof the denitrification which occurred in this soil happenedin the top 25 cm of the soil during the first 3 weeks afterNC>3~ was added (Fig. 2). There was some variationbetween the two columns in the time of maximum de-nitrification although the total denitrification was nearlyidentical. In one soil column, approximately 75% of theadded NOa" was lost in 2 weeks and in the other columnonly 30% was lost in the same time period. Thesevariations are considered to be less than one would observein the same soil in the field where variations in soilstructure and permeability are much greater than in thesecarefully packed columns. Once the NOs" had movedbelow the top 25 cm of the column, NOs" analysisindicated that very little denitrification occurred. The 62

no

no

•o

80

TO

CO

go

40

30

20

10

COLUMN 1HANFORD

COLUMN O

0 0 2 0 3 0 4 0 9 0 6 0 7 0 8 0 9 0 0 O 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

DAYS DAYSFig. 2—Distribution of added N with time in the Hanford soil.

SORRENTO

110

100

9O

80

70

60

»

40

30

20

10

COLUMN t COLUMN H

O COLUMNA EFFLUENTO DENITRIFIED

0 1 0 2 0 3 0 4 0 9 0 6 0 7 0 8 0 9 0 0 1 0 2 0 3 0 4 0 9 0 6 0 7 0 8 0 9 0

DAYS DAYSFig. 3—Distribution of added N with time in the Sorrento soil.

contents of the subsoils was relatively high even though thediffusion rate through the near saturated topsoil must havebeen very low. This high C>2 content and low denitrificationrate in the lower part of the profile are presumably a resultof a low amount of available energy for microbes in thiszone.

The denitrification pattern in the uniformly fine-texturedSorrento soil was considerably different from either of theprevious soils discussed (Fig. 3). There was no indicationof denitrification in either soil column until the added NOs"reached a depth of 30 cm approximately 25 days afteraddition. In one column, denitrification did not begin until45 days when the added NOs" was concentrated in the 45 -60 cm zone of the soil. The differences between the twocolumns was not a result of differences in any of the

64 SOIL SCI. SOC. AM. }., VOL. 42, 1978

COLUMN 0

O 1 0 2 0 3 0 4 0 9 0 6 0 7 0 8 0 9 0 0 1 0 2 0 3 0 4 0 5 0 6 0 7 0 8 0 9 0

DAYS DAYSFig. 4—Distribution of added N with time in the Zamora soil.

measured parameters (moisture content, tension, Eh of bulksoil, and oxygen content of soil atmosphere). The dif-ferences were apparently in the microstructure of the soilcolumns because there was no difference in the apparentwater conductivity of the soil. This soil was the only onestudied where the total denitrification in the two columnswas appreciably different. However, the denitrificationpatterns in both columns were similar, showing no de-nitrification in the topsoil and a very significant amount ofdenitrification in the middle to lower horizons.

The Zamora soil, having an increasing clay content withdepth, presented still another denitrification pattern (Fig.4). There was no denitrification in this soil until the addednitrate reached a depth of approximately 60 cm 40 daysafter the nitrate was added. Approximately 63% of theadded NOa~ was lost from solution during the next 6 weeksas the NOa~ moved through the 60- to 90-cm zone. Thedepth of denitrification initiation coincided with a texturaldiscontinuity where the clay content increased abruptlyfrom 23 to 33%.

Concentrations and Losses of N2 and N2OThe concentrations of 15N2 and 15N2O at selected points

in each column was measured every 2 days during the first2 weeks after NOa~ addition and twice weekly later in theexperiment. No N2 or N2O from the added NO3~ was everfound in the Milham soil. Very little N2O (2-10 ppm) wasdetected in the Zamora and Sorrento soils. This wassomewhat surprising because the conditions for denitrifica-tion were somewhat marginal in the upper profile in thesetwo soils and essentially no denitrification occurred untilthe added NOa~ leached below 30 and 60 cm in theSorrento and Zamora soils, respectively. Since it is gener-ally accepted that environmental conditions which severelylimit the rate of denitrification are the same conditionswhich favor the production of N2O over N2 (CAST, 1976),it was expected that N2O would be found as the NOa~leached into soil horizons favoring a slow rate of de-

nitrification. However, no significant concentrations ofN2O were ever detected at any depth or time in these twosoils. In both soils, the presence of 100-300 ppm of 15N2 inthe soil atmosphere preceded by 1 to 3 weeks the detectionby soil solution samples of loss of NOa~. The I5N2concentration in both soils in the soil horizon wheremaximum denitrification was occurring was normally about1,000 ppm although values from 500 to 2,000 ppm wererecorded.

The Hanford soil had a near saturated topsoil at the waterapplication rate used and 15N2O was detected in 2-3 daysafter NOa" application. The conditions were favorable fordenitrification, and approximately 75% of the added NOa~was denitrified in a 2-week period after denitrificationbegan. In this soil, the N2O concentration was always 2-4times as high as the 15N2 concentration. During the periodof denitrification (as determined by disappearance ofNOa~) the concentration of N2O was measured to be from200-1,000 ppm at various points in the soil profile.Although there was no indication of further loss of NOa~once the NOa~ leached below 30 cm, the N2O con-centration in the bottom 50 cm of the profile stayedrelatively constant at 300-500 ppm for another month.

We also attempted to measure directly the amount ofN2O and 15N2 evolved from the column by sealing the topof the column for a short period of time (2-6 hours) andmeasuring the amount of gas evolved into the air spaceabove the soil during this period. Our calculations indicatedthat a steady-state denitrification rate of 10-25% of theadded NOs~ over a 2-week period would produce easilymeasurable quantities of N2O and/or 15N2 in the top if thegases were lost at a uniform rate. This attempt to quantita-tively determine the NO3~ lost by the measurement of lossof N2 and N2O was never successful. We could account forless than half of the NO3-N lost through the measurementof N2O and 15N2. We checked many possibilities forproblems in technique - duplicate gas analyses were madeby another laboratory, possibility of leaks carefullychecked, etc., and found no flaws in procedure. We usedHenry's law and the reported water solubilities of N2O andN2 to determine quantities of these gases lost in the leachateand present in the column solution. We also checked for thepresence of NO and NO2 in soil gases and were unable tofind measurable concentrations of these gases. There weresignificant amounts of 15N found in the organic N plusNH1f-N form at the end of the experiment in the Sorrentoand Zamora soils (Table 3). This could account for a part ofthe problem of accounting for the added N during theexperiment in the soils, but does not help for the Hanfordsoil. An attempt was made to calculate the amount of 15N2diffusing out of the top of the columns based on observedconcentration gradients in the columns according to:

I" dc ;-*- dt

where F = flux of 15N2 in cm3/day,A = area of column incm2, dc/dt is the concentration gradient measured over theperiod dt (in days), and D is the diffusion coefficient incm2/day calculated from the diffusion of N2 in air at 25°C(Do) and the air-filled porosity (Ea) according to D = Do

GILLIAM ET AL.: DENITRIFICATION IN FOUR CALIFORNIA SOILS 65

Ea4/3 (Millington, 1959). Using this technique to estimate prediction of denitrification in large fields will be verydenitrification, we considerably overestimated denitrifica- difficult even though one may understand all of the factorstion in the Sorrento soil and underestimated denitrification influencing denitrification rates.in the Zamora soil. Denitrification in unsaturated subsoils has rarely been

The inability to account for all of the N lost from shown before. Myers and McGarity (1972) observedNO3~form in commonly measured gases is not new. denitrification in solodized solonetz B horizons (45-60 cm),Nommik (1956) concluded that his data "seem to indicate but they added an energy source to the soil samples. Meekthat an intermediate had been accumulated in the soil which et al. (1969) observed decreases in nitrate concentrationswas neither nitrite nor ammonia." However, in Nommik's with depth in their field soils, but most of the loss occurredexperiments, all of the NOs'-N lost appeared finally as Na above 30 cm. However, in the soils studied here, it was+ N2O. Similar observations were also made by Garcia observed that large amounts of denitrification could occur(1973), and Mahendrappa and Smith (1967). In our at depths of 30-75 cm. This occurred using only C alreadyexperiment, all of the NOs" lost in the Hanford soil present at these depths or brought with the percolatingoccurred during the first 2-3 weeks of the experiment, but water. There apparently is a large difference in theseveral weeks later the N2O concentration in the soil availability of C in the subsoils of the western United Statesatmosphere was still 300-500 ppm. The data which we and those in humid areas when the topsoils contain similarobtained offer no evidence as to what an intermediate amounts of organic C. Gambrell et al. (1975) observed noproduct between NOa~ and N2O or N2 might be or even denitrification in Goldsboro subsoils from North Carolinaconclusive evidence that one exists; however, we join the where the organic contents in the surface horizon werelist of those unable to account for all added 15N in similar to those in this study.commonly measured forms. Verhoeven (1952) suggested As noted previously, all possible ratios of N2 to N2Othat the temporary N deficit observed by others (Elema et have been observed previously in laboratory studies. Someal., 1934, and Korsakova, 1927) might be due to the ability investigators have speculated that their measurement ofof N2O to form supersaturated solutions. Van derStaay and N2O was higher than would have escaped in the fieldFocht (1977) noted that high concentrations of N2O (up to because of further reduction of N2O as it passed through the5%), were evolved in vessels incubated with kaolinite, soil (Nommik, 1956) and others (Wijler and Delwiche,nitrate, and washed-cell suspensions, but that no N2O was 1954) have speculated that their measurements of N2O weredetected in reaction vessels containing montmorillonite. lower because of readsorption of N2O and further con-

version of N2 in their closed systems. Our data indicate thatDISCUSSION rat'° °^ ^2 to ^2^ produced during denitrification in

different soils is extremely variable. The N2/N2O ratio inThe amounts of denitrification observed in this ex- the Sorrento and Zamora soils was approximately 100:1

periment could easily be changed by imposing different and in the Hanford soil the ratio was approximately 1:4.water regimes or by significantly changing the column Denitrification in the soil which had a low production oftemperature. However, it is believed that the more im- N2O occurred deep in the soil profile, but there was noportant conclusions drawn from the work would have been indication that further reduction of N2O was occurring asthe same. A general relationship existed between soil N2O diffused through the soil column toward the surface astexture and amount of denitrification. For example, the very little N2O was ever detected at any depth. Our datasoils^with heavy textured subsoils readily reduced added indicate that it is very difficult to predict the relativeNO3~. This supports field observations made by Pratt et al. amounts of N2 and N2O produced during denitrification and(1972) and Lund et al. (1974) that NOj~ below the root that estimates of total N2O produced from agricultural landszone in irrigated soils with clayey subsoils is reduced as have a large uncertainty factor,compared to predicted concentrations with no denitrifica-tion. However, the Hanford soil which had lowest claycontent reduced large amounts of NOs~ because of therestricted water flow in the top horizon. The relationshipwhich has been observed between soil texture and de-nitrification is simply a result of a relationship betweenwater flow and denitrification. It is concluded that any soilcharacteristic which impedes water flow through the soilwill be positively correlated with denitrification. It is likelythat high rates of denitrification would be observed in manysandy loam soils in the irrigated western United Stateswhere water infiltration problems are not unusual (Chris-tensen et al., 1967). It is also likely that large variations inamounts of denitrification occurs in very small areas withina field. Spatial variability in soil water movement is knownto be large in many soils (Bigger et al., 1975). It ispostulated that spatial variability in denitrification in theirrigated soils of the western U.S. is at least as large asvariability in water movement. Thus, accurate quantitative

66 SOIL sci. soc. AM. j., VOL. 42, 1978