Directory UMM :Data Elmu:jurnal:S:Soil Biology And Chemistry:Vol32.Issue3.2000:

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Bioremediation of nitrate-contaminated shallow soils and waters

via water table management techniques: evolution and release of

nitrous oxide

Pierre-AndreÂ Jacinthe

a

, Warren A. Dick

a,

*, Larry C. Brown

b

a

School of Natural Resources, The Ohio State University, 1680 Madison Avenue, Wooster, OH 44691, USA b

Department of Food, Agricultural and Biological Engineering, The Ohio State University, Columbus, OH 43210, USA

Accepted 30 August 1999

Abstract

Nitrate (NO3±N) commonly accumulates in soils because of fertilizer additions or when crop demand is much less than the

rate of NO3±N production. Water table management (WTM) has been proposed to stimulate denitrifying bacteria, thus

removing the accumulated NO3±N by converting it to N2O (a greenhouse gas) and N2. We studied the emission of N2O and N2

as aected by water table depth. Undisturbed soil columns (30 cm dia by 90 cm long) from three soil series (Blount, somewhat poorly drained Aeric Ochraqualf; Clermont, poorly drained Typic Glossaqualf; and Huntington, well drained Fluventic Hapludoll) were treated with 2.11 g N (as KNO3) applied as a band 10 cm below the surface. Two dierent WTM schemes were

studied: static (WTM1) and dynamic (WTM2). We repeated WTM2 using15_{N and this treatment, applied to the Huntington}

soil only, was designated WTM3. In general, N2O concentrations in a soil column responded to ¯uctuations in water table

depth. Concentrations of N2O were usually higher in soils immediately below, as compared to above, the water table. The

Clermont columns departed from this general trend. Maintaining the water table at 50 cm depth resulted in N2O emission rates

(1.8±44 mg N2O±N mÿ 2

dÿ1

) comparable to those reported for cultivated ®elds. A water table only 10 cm below the surface caused N2O emission rates to increase considerably (60±560 mg N2O±N mÿ2dÿ1). Four days after imposition of a water table

10 cm below the soil surface, N2O comprised 95% of the N gas emitted (i.e. N2O mole fraction was 0.95). One week later,

however, the N2O mole fraction was 0.35 which was signi®cantly (PR0.05) lower than the mole fraction (0.68) measured prior

to raising the water table. These results suggest that when using WTM practices, the best option to maintain high NO3±N

removal rates and to reduce the proportion of N2O in the emitted gases is to maintain a high water table for a prolonged period

Keywords:Denitri®cation; Greenhouse gas; Nitrate bioremediation; Nitrate fertilizer; Nitrogen management

1. Introduction

Water table management (WTM) has been proposed as a way of removing excess nitrate (NO3±N) from soils and protecting subsurface waters from NO3±N pollution. Water table management involves creating saturated conditions in the upper portion of the pro®le

by raising the water table. As a result, oxygen is rapidly depleted in the soil pores, thus creating con-ditions favorable for denitri®cation. Denitri®cation is the biological process whereby NO3±N is used as an alternative electron acceptor by soil microorganisms and is converted into nitrous oxide (N2O) and dinitro-gen (N2) gases. Smith and Du (1988) and Parkin and Meisinger (1989) showed that much of the soil denitri-®cation potential resides in the surface layers. There-fore, by creating an anoxic environment in these layers, denitri®cation activity would be stimulated and

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residual soil NO3±N would be converted into N gases before it could be leached.

Formation and release of N2O into the atmosphere is a concern because of the ozone-depleting eect of this gas (Cicerone, 1987) and it is also an important greenhouse gas which may lead to proposed warming (Yung et al., 1976). Enhancing denitri®cation would increase gaseous N release from soil. If a major por-tion of the gases produced is N2O, then application of WTM techniques could increase atmospheric N2O loading and an attempt to solve the NO3±N pollution problem may, instead, lead to increased global warm-ing. However, N2O is not the only end product of denitri®cation and N2O may be further reduced to N2. If N2, instead of N2O, is the predominant gas formed during the application of WTM techniques, these tech-niques would represent environmentally sound ways of removing NO3±N from soil.

Application of WTM techniques creates a water-saturated subsoil overlain by unwater-saturated soil of vary-ing biochemical and physical properties. Denitri®cation gases, produced in the saturated soil, diuse upward to the surface due to concentration gradients. In their migration toward the soil surface, these gases must

move through soil horizons of dierent gas diusivity, porosity, redox potential, soil water content, organic matter content and N2O-reductase activity. Several of these factors aect the rate of gas migration and resi-dence time in the soil pro®le. The relative proportion of N2O and N2 escaping from the soil surface would, consequently, be aected.

Smith (1980) indicated that the ratio of N2O-to-N2 emitted during denitri®cation depends on the balance between the rate of N2O diusion from sites where it is produced and the rate of N2O reduction to N2. A low rate of N2O diusion rate favors N2O reduction. Letey et al. (1980) noted that when N2O diusion was restricted, this gas was converted into N2 which then escapes into the atmosphere. Similarly, Rolston (1981) suggested that if denitri®cation occurred deeper in the soil pro®le, the mole fraction of N2O [N2O/ (N2O+N2)] would be smaller than if the process took place near the soil surface. Other researchers (Cady and Bartholemew, 1960; Council for Agricultural Science and Technology, 1976), however, were of the opinion that N2O produced in soils via denitri®cation is almost always reduced to N2before it escapes to the atmosphere.

Table 1

Physical and chemical properties of soils

Parameter Soil depth (cm) Soil series

Blount Clermont Huntington

Location Union County, OH Jennings County, IN Pike County, OH

Taxonomy Aeric Ochraqualf Typic Glossaqualf Fluventic Hapludoll

Drainage class somewhat poorly drained poorly drained well drained

Bulk densitya_{(g cm}ÿ3₎ _0±10 _1.53 _1.56 _1.30

pH 0±5 5.5 5.1 7.2

5±10 6.1 5.2 7.3

10±15 4.9 4.9 7.5

15±20b 5.4 5.2 7.5

Organic C (g kgÿ1soil) 0±5 27.9 23.3 31.0

5±10 22.3 6.0 24.4

10±15 11.5 6.4 20.1

15±20b 11.8 8.7 21.8

Nitri®cation potentialc_{(mg NO}

3±N kgÿ1weekÿ1) 0±5 31.0 0.15 89.7

5±10 12.3 0.10 85.3

10±15 14.3 0.10 60.8

15±20b _16.7 _0.08 _53.3

Denitri®cation potentiald_{(mg N}

2O±N kgÿ1dÿ1) 0±5 0.93 0.88 5.78

5±10 1.44 0.05 2.14

10±15 0.53 0.02 1.25

15±20b 0.89 0.01 0.88

a

Bulk density was determined for the 0±10 cm soil depth.

b

Values below the 20 cm depth are reported in Jacinthe (unpubl. Ph.D., Ohio State University 1995).

c

Aerobic incubation (258C) of NH4-amended soil samples for 3 weeks. d

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We hypothesized that the overall eects of applying WTM strategies to stimulate NO3±N removal and to aect atmospheric N2O loading will depend on indi-vidual soil properties (physical and biochemical), and on the position of the water table within the soil pro-®le. Our overall objective was to provide information regarding the evolution, behavior and fate of the N gases resulting from subsurface denitri®cation in agri-cultural soils stimulated by water table management. Speci®cally, the composition of the denitri®cation gases (N2O and N2) emitted at the soil surface was determined.

2. Materials and methods

Undisturbed soil columns (30 cm dia by 90 cm long) collected from three sites in Ohio and Indiana and encased in 30 cm dia polyvinyl chloride (PVC) cylin-ders were used in our study. Procedures for soil col-umn collection were as described in K.M. Coltman (unpubl. M.Sc. thesis, Iowa State University, 1992) and Hutton et al. (1992). A description of the three soils used (Blount, Clermont and Huntington series) is provided in Table 1 and Jacinthe et al. (1999).

To simulate the presence of residual NO3±N in the soil pro®le below the surface, the top 10 cm layer of soil in each column was removed and 2.11 g KNO3±N (equivalent to 300 kg NO3±N haÿ1 on an area basis) was spread over the exposed section thereby creating a band of NO3±N at the topsoil±subsoil interface. Then the column was repacked with the excavated soil. The soil columns were saturated at dierent depths with deionized water to simulate a water table. The position of this water table within the soil columns varied during the experiment and was controlled by adjusting the height of a water-supplying bottle attached on the side of each column.

Two water table management (WTM) treatments were evaluated and are described in detail in Jacinthe et al. (1999). In brief, the water table management 1 (WTM1) treatment consisted of maintaining a static water table 50 cm below the soil surface during the ®rst 92 d and then the water table was raised to 10 cm below the surface for 18 d. The water table manage-ment 2 (WTM2) treatmanage-ment involved simulation of a changing or dynamic water table. In this treatment, the water table level was held at: 50 cm below the soil surface between d 1 to 5; 10 cm depth between d 9 to 14; 70 cm depth between d 45 to 49; 50 cm depth between d 51 to 92; and 10 cm depth between d 92 to 110. At all other times, position of the water table was variable. During recharge the water table was raised 10 cm dÿ1 (d 6 to 9), and at d 92 the water table was rapidly raised from 50 to 10 cm depth. In drainage mode, the water level was lowered 2 cm dÿ1 except at

d 110 when the columns were allowed to drain freely from the bottom. The dynamic water table manage-ment treatmanage-ment (WTM2) was repeated except that labeled K15NO3(13.1720.37 at% 15N) was applied at the same rate (2.11 g N columnÿ1) as for the columns receiving unlabeled N. This treatment was designed WTM3 and was applied to the Huntington soil only.

In the WTM1 and WTM2 treatments, triplicate col-umns of the Huntington and Clermont soil series were used. In all other cases, duplicate columns were used.

2.1. Soil atmosphere sampler

The PVC columns had holes drilled into them at 5, 20, 40, 60 and 80 cm depth to install soil atmosphere sampling devices. Soil atmosphere samplers were con-structed so that air samples could be obtained in both unsaturated and saturated conditions. Silicone tubing was found to be ideal for this purpose since this ma-terial, while being nonwater penetrable, is permeable to N2O (Jacinthe and Dick, 1996). Soil atmosphere samplers were made of 15 cm long pieces of silicone tubing (Cole±Parmer Instrument, Chicago, IL) with an outer diameter of 1.75 cm o.d. and a wall thickness of 23 mm. The inner end of the tube was sealed with sili-cone caulking (Dow Corning, Midland, MI), while the other end was inserted into a plastic reducer connected to a 1.2 cm o.d. threaded plastic ®tting protruding out-side the PVC cylinder. This plastic ®tting was tightly screwed to the PVC wall and was ®tted with a septum. During sampling, an hypodermic needle was inserted through the septum and an air sample was withdrawn. The air sample was transferred to an evacuated, crimp-sealed glass vial ®tted with a gray butyl rubber septum. In general, gas samples were analyzed for N2O content within 1 week of their collection.

2.2. Nitrous oxide emission from the surface of soil column

Flux of N2O from the soil column surface was mon-itored using a closed chamber consisting of two parts (Jacinthe and Dick, 1997). The chamber's lower sec-tion was made of a 30 cm long by 15 cm dia PVC pipe. One end of this PVC pipe was beveled to facili-tate its insertion into the soil and the other end was ®tted with a PVC coupling to accommodate the top portion of the chamber. The bottom section of the chamber was inserted 15 cm depth into the soil col-umn. The top portion of the chamber consisted of a PVC endcap ®tted with a gas sampling port. The chambers remained in place throughout the exper-iment.

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when the water table was near surface and high N2O ¯uxes were expected. The rate of N2O emission was determined by linear regression of N2O concentration inside the chamber against time. After each ¯ux measurement, the endcap was removed to expose the soil column surface to ambient atmosphere.

Gas samples were collected every 3 d during the ®rst 2 weeks of the study. As the experiment progressed sampling frequency was reduced to weekly, biweekly, and then once a month. Sampling frequency was modi-®ed somewhat with 15N-treated columns to minimize analytical costs. After 110 d, the soil columns were drained and N2O emission was monitored for an ad-ditional 20 d in the WTM1 and WTM2 columns. The WTM3 columns, however, were destructively sampled 1 d after drainage.

To assess the relative proportion of N2and N2O in the gaseous products emitted from the soil columns at d 92, 96 and 105, the acetylene (C2H2) inhibition method was used to measure total denitri®cation (Yoshinari et al., 1977; Ryden et al., 1979). The ®rst day (d 92) was selected because it represents conditions prior to raising of the water table and after a pro-longed period of a static water table level. The second day (d 96) was selected to represent results immedi-ately after raising of the water table and when active denitri®cation in the surface layer of soil would be expected. Finally, the last measurement (d 105) was selected to observe any changes caused by depletion of NO3 substrate concentrations due to denitri®cation. Acetylene was supplied to the soil columns using tech-niques similar to those described in Fustec et al. (1991) and Grundmann and Chalamet (1987). First, the rate of N2O emission was determined as described above without C2H2addition. Then, two hypodermic needles were inserted through the septum of the gas chamber; one serving as an exit port and the second, connected to a C2H2 supplying line. Acetylene was allowed to ¯ow from a tank at a rate 120 mL minÿ1for 6±8 min. After such time, the C2H2 partial pressure inside the chamber headspace was 17.121.8% (v/v). After 120 min of exposure to this concentration of C2H2, the rate of N2O emission was monitored over a 90 min period. The mole fraction of N2O, or the [N2O]/ [N2+N2O] ratio was computed as the ratio of the rate of N2O emission without C2H2to the rate with C2H2.

2.3. Methods of analysis

Nitrous oxide concentrations were measured using a DIMENSION I (Tremetrics, Austin, TX) gas chro-matograph equipped with a 63Ni electron capture detector. The GC was ®tted with a precolumn (100 cm by 0.2 cm i.d.) and an analytical column (300 cm by 0.2 cm i.d.), both packed with 80±100 mesh Prorapak Q (Alltech, Deer®eld, IL). A mixture of argon (90%)

and methane (10%) was used as carrier gas with a ¯ow rate of 30 cm3 minÿ1. Operating temperatures were 708C (columns), 1008C (valves) and 3508C (detec-tor). Standard N2O samples, used for instrument cali-bration, were prepared from N2O (98%) purchased from Alltech and diluted into N2 gas. Gas samples from the WTM3 columns were ®rst analyzed for their N2O content. Then a gas sample aliquot was shipped to the University of California at Berkeley for 15N2O analysis.

Acetylene was analyzed using a Varian (model 3700) GC (Varian, Walnut Creek, CA) equipped with a FID and a 200 cm glass column packed with Prorapak N 80/100 (Supelco, Bellefonte, PA). Gas ¯ow rates were 30 cm3 He minÿ1, 25 cm3 H minÿ1 and 300 cm3 air minÿ1. Oven temperature was 758C, and detector and injector temperature was 2208C.

2.4. Data analysis

Repeated measure analysis of variance (Littell, 1989) was used to determine the eects of soil series, water

Fig. 1. Nitrous oxide (N2O) concentrations at various depths within

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table management (WTM) on N2O concentration within the soil column and N2O ¯ux from the surface of soil columns. Statistical analysis was conducted for each depth. The data were analyzed with soil series as a block, water table management conditions (WTM) as the treatment factor and sampling date as the repeated measure factor. Nitrous oxide concentration at depth, and N2O eux were the response variables while soil series, WTM, and sampling date were used as class variables in the analysis. Statistical analyses were performed using SAS (SAS Institute, 1988).

3. Results

3.1. Nitrous oxide concentration pro®les

Nitrous oxide concentrations within the soil columns responded to the water table position. For the WTM1 treated columns, the water table was maintained at 50 cm below the surface from d 1 to 92. During that

period, N2O concentrations remained relatively con-stant, being in many instances greater below the water table than in the unsaturated portion of the soil umns (Figs. 1±3). In contrast, the WTM2 treated col-umns had N2O concentrations that varied greatly. The highest concentrations of N2O were recorded between d 21 and d 35 after the water table level had been raised to 10 cm below the soil surface in these col-umns. During that period, N2O concentrations of 6750, 2790 and 10967 mL N2O±N Lÿ1 were recorded in the 20 cm depth of the Blount, Clermont and Hun-tington WTM2 columns, respectively. Between d 51 and 92, the water table was maintained at 50 cm in the

WTM2 columns and N2O concentrations declined

rapidly to concentrations typically found prior to rais-ing the water table. On d 92, the water table level was again raised to 10 cm in all columns (WTM1 included), and maintained in that position until the columns were drained on d 110. Nitrous oxide concen-trations reached a second maxima in the 5±20 cm depth region at around d 105. In general, N2O concen-trations during the second saturation period (d 92 to

the Huntington soil columns with a static (WTM1,w) or a dynamic (WTM2,Q) water table. Thesymbol indicates a signi®cant dier-ence (P< 0.05) between water table management practices at a given sampling date.

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110) were lower than those recorded after the ®rst time of saturation (d 21 to 35).

There was a delay between soil saturation and the maximum N2O production in the upper soil layers. For example, the water table was held at 10 cm between d 9 and 14 but N2O concentrations in the 5± 20 cm depth samplers reached maximum amounts between d 21 and 35 (Figs. 1±3). Similar observations were made when the water table was raised to 10 cm a second time (d 92 to 110).

The 15N enrichment (Fig. 4) of the N2O evolved from the WTM3 columns (5±40 cm depth), ranged from 0.9 to 4.5 at% when the water table was raised the ®rst time, and between 0.4 to 2.4 at% the second time the water table was raised to the 10 cm depth in the soil columns. At the 60 and 80 cm depths, 15N enrichment of the N2O produced was smaller.

In the WTM3 treatment, 15N-labeled KNO3 (13.17 at%) was applied as a narrow band at 10 cm depth below the soil surface, but the bulk of this applied NO3±N stayed in the top 20 cm of the soil columns. Soil solution analysis has shown that at depth <20 cm, the 15N content of NO3 ranged between 8 to 9 at%, whereas in the deeper soil layers, the15N content never exceeded 1.2 at% (P.A. Jacinthe, unpubl. Ph.D., Ohio State University, 1995; Jacinthe et al., 1999).

Upon soil column saturation, N2O was produced at dierent depths and originated from substrate of vary-ing 15N contents. Nitrous oxide sampled in the upper soil layers is a mixture of N2O produced at that par-ticular depth and of N2O evolved from 15N-depleted substrate in the deeper soil horizons. The observed di-lution (relative to isotopic composition of substrate) of 15

N2O in the upper soil layers implies that there was migration of 15N-depleted-N2O from the deeper soil horizons to the soil surface and that reduction of N2O to N2in the lower soil horizons was negligible.

3.2. Nitrous oxide emission from the surface of soil columns

The ¯ux of N2O from the soil columns was not sig-ni®cantly aected by soil series. As with concentration within the soil columns, N2O emission rates responded to the water table ¯uctuations, being greater when the

Fig. 5. Nitrous oxide (N2O) emission rates from the surface of the

Blount (A), Clermont (B) and Huntington (C) columns with a static (WTM1,w) or a dynamic (WTM2,Q) water table. Rates (*) and

15_{N enrichment (}

w) of the N2O emitted from the surface of the

WTM3 columns are given in the insert of graph C. The scale for the

15_{N enrichment is given on the right side of the graph.}

Fig. 4. Nitrous oxide concentrations (*) and 15_{N enrichment (in}

at%) of the N2O pool (r) at various depths within the soil columns

treated with K15_NO

3(WTM3). The scale for the 15N enrichment is

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water table was closer to the soil surface and vice versa (Fig. 5). When a 10 cm below the surface water table was imposed, N2O emissions (between d 21 and 28) from the surface of the Blount, Clermont and Huntington columns reached rates of 125, 201 and 303 mg N2O±N mÿ2 dÿ1, respectively. When the water table was in a lower position, N2O emission decreased and ranged from 2.4 to 7.8 mg N2O±N m2 dÿ1in the Blount, 5.1 to 23.1 mg N2O±N mÿ2 dÿ1 in the Cler-mont, and 5.0 to 44.1 mg N2O±N mÿ2dÿ1in the Hun-tington columns, respectively.

When the water level was raised to 10 cm in all col-umns at d 92, a second spike of N2O emission from the soil surface was observed. However, N2O emissions from the WTM2 columns did not reach the amounts attained between d 21 and 28. Total N2O emitted from the WTM2 after the water table was raised a second time to the 10 cm depth ranged from 13±24% of the total N2O emitted during the whole experiment as opposed to 50±53% after the ®rst raising of the water table level. In comparison to WTM2, between 40 and 47% of the N2O was emitted from the WTM1 col-umns between d 92 and 110.

Emission of N2O from the surface of the15N-treated Huntington columns (WTM3) was similar to that of the WTM2 columns of this soil (Fig. 5c, insert). Cumulative amounts of N2O emitted from the WTM2 and WTM3 Huntington columns were comparable, averaging 565 and 528 mg N2O±N, respectively. Emis-sion rates as well as the15N content of the gas emitted responded to changes in water table position. The 15N content of the N2O released ranged from 0.5±4.5 at% and was related R2₀_:_60±0_:₆₅ _{to the isotopic} com-position of N2O sampled at 5 and 20 cm depths.

3.3. The N2O mole fractions

To assess the eectiveness of the technique used to supply C2H2to the soil columns, the rate of C2H2 mi-gration into selected soil columns was monitored over an 8 h period. The data reported in Table 2 show that after 2 h, the C2H2 concentration in the top 20 cm of the soil columns was in the range 0.56±3.56% (v/v). Ryden et al. (1979) reported quantitative inhibition of N2O reduction with C2H2 concentrations between 0.1 and 10% (v/v). Thus, C2H2concentrations in the 0±20 cm depth were sucient to inhibit the reduction of N2O. In the deeper soil layers (>40 cm), however, C2H2 concentrations remained below inhibitory amounts indicating that, with the technique we used to supply C2H2to the soil columns, inhibition of N2O re-duction by C2H2 was achieved only in the upper soil layers. Should N2O reduction to N2 be an important process in the lower portions of the soil columns, a failure to inhibit the activity of the N2O-reductase at depth could lead to in¯ated mole fractions of N2O and an underestimation of the N2O+N2 ¯ux. Evidence presented later in this paper discounts this possibility, however, and indeed suggests an inherently low amount of N2O-reductase activity at depth.

The mole fractions of N2O, or [N2O]/[N2+N2O] ratios (Table 3) were computed at d 92, 96 and 105.

Table 2

Acetylene (C2H2) concentrations within soil columns exposed to 17.1 21.8% C2H2(v/v) in a closed chamber located at the soil column

surfacea

Depth (cm) C2H2concentration in soil (%, v/v) after

2 h 5 h 8 h

5 3.56 (0.42)b _{5.14 (0.46)} _{4.55 (0.37)}

20 0.56 (0.27) 1.67 (0.55) 1.80 (0.75) 40 0.01 (0.0) 0.07 (0.06) 0.09 (0.01) 60 < 0.01 0.03 (0.01) 0.05 (0.01) 80 < 0.01 < 0.01 < 0.01

a_{Average air-®lled porosity (}_e

a) in the top 0±5 cm soil layer was

0.16 cm3_{air cm}ÿ3_soil.

b_{Values in parentheses are standard deviations.}

Table 3

Mole fractions of N2O in the denitri®cation gases emitted at the surface of the soil columns

Soil [N2O]/[N2+N2O] ratios

d 92 d 96 d 105

WTM1 WTM2 WTM1 WTM2 WTM1 WTM2

Blount 0.72 (0.00)a 0.61(0.38) 0.94 (0.03) 0.96 (0.00) 0.27 (0.17) 0.38 (0.01) Clermont 0.49 (0.24) 0.71 (0.31) 0.97 (0.00) 0.96 (0.00) 0.56 (0.07) 0.23 (0.11) Huntington 0.71 (0.35) 0.44 (0.28) 0.89 (0.10) 0.96 (0.02) 0.66 (0.66) 0.69 (0.41)

Mean (date)b 0.68b 0.95a 0.35c

a

Values in parentheses are standard deviations.

b

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Analysis of variance revealed a signi®cant (PR0.05) eect of sampling date on the mole fractions of N2O, but the eects of soil and water table conditions were not signi®cant. At d 92, the water table had been maintained at 50 cm below soil surface for 92 and 42 d in the WTM1 and WTM2 columns, respectively. On average, 68% of the nitrogenous gas emitted on this day was in the form of N2O. The water table level was then raised to 10 cm in all columns and the mole frac-tions of N2O obtained 3 and 13 d later showed an interesting contrast. On d 96, N2O was the dominant gas emitted, averaging 95% across soils and treat-ments. Whereas on d 105, the mole fraction of N2O decreased signi®cantly to 35% in average, indicating a shift toward a dominance of N2 in the denitri®cation products.

3.4. Soil gas diusivity

The eective diusion coecient (Ds) of N2O in soils was computed using the procedures described in Rolston et al. (1976) as:

Ds ÿkVL=A, 1

where Ds is the soil eective diusion coecient (cm 2

sÿ1), V is the volume of the chamber (cm3), A is the area circumscribed by the chamber (cm2), L is the thickness of the soil layer considered (5 cm) and k (sÿ1) is the slope of the plot of

ln_CsÿC_a=Cs ÿkt 2

in which Csis the concentration of N2O (mL Lÿ1) at 5 cm below soil surface, and Ca is the concentration of N2O (mL Lÿ1) in the chamber at time t (s).

Out of 240 ¯ux measurements made during this study, this method could not be applied in 27 cases. Most of these were in situations where N2O emissions were greater than 200 mg N2O±N mÿ2 dÿ1. In those cases, at some point during the measurement period, Caexceeds Cs and a value for Ds could not be deter-mined from Eq. (2).

Analysis of variance showed a signi®cant eect (P< 0.05) of soil series on soil gas diusivity expressed as Ds. Eective diusion coecients of N2O in the Hun-tington soil were greater mean₃_:₈₁₁₀ÿ3cm2 sÿ1_; range: 0.03 10ÿ3±10.4 10ÿ3 cm2 sÿ1) than in the Blount mean ₁_:₄₉₁₀ÿ3 cm2sÿ1_{; range: 0.01} 10ÿ3±3.310ÿ3cm2sÿ1) and the Clermont soilsmean 1:2510ÿ3 cm2sÿ1_{; range: 0.02} ₁₀ÿ3

±3.710ÿ3 cm2sÿ1). Reported in situ diusion coecients of N2O in a loam soil (0.1410ÿ3±0.2510ÿ3cm2sÿ1) and a silt loam soil (0.59 10ÿ3 to 2.23 10ÿ3 cm2 sÿ1) (Rolston et al., 1976; Grundmann and Chalamet, 1987) are within the range obtained in our study.

Relationships between Ds and air-®lled porosity (ea,

cm3 air cmÿ3 soil) were derived for sampling dates at which both factors were measured. Data for periods of water table ¯uctuations were not included since mass ¯ow of gas due to redistribution of soil water was likely (Rolston, 1986), and consequently gas movement may have been controlled by process other than diu-sion. For the Blount and Huntington soils, relation-ships between the two parameters were: Ds22:1ea4:2 and Ds1:4ea4:5, respectively, suggesting a reason-able conformity to the model Dskma proposed by

Curie (1960). For the Clermont soil, however, no clear relationship emerged. Sallam et al. (1984) noted that there is generally good agreement with models at e_a_>

0.3 but, below this value, ®tting of experimental data to gas diusion models is usually not successful. The

e_a _{values recorded for the Clermont soil were, in}

gen-eral, less than 0.1 (Fig. 6). Therefore, an explanation for the lack of a relationship between Ds andeain the Clermont soil may be that the available pore space was mainly isolated air pockets which did not contrib-ute eciently to gas exchange in this poorly structured soil. The presence of such blocked pores has been

indi-Fig. 6. Air-®lled porosity (e_a) of the surface soil layer (0±5 cm) in the Blount, Clermont and Huntington columns under WTM1 (w) and WTM2 (Q) during the course of the experiment. Air-®lled porosity was computed as:ea=(1ÿPb/Pa)ÿy, wherePbis soil bulk density,Pa

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cated as the major cause for the scatter of experimen-tally-determined diusion coecients (Pennman, 1940), and possibly nonconformity to diusion models.

4. Discussion

The highest concentrations of N2O in the soil col-umns measured in this study were much greater than the maximum concentration (197 mL N2O Lÿ1) measured in surface-frozen soil pro®les in Ontario (Burton and Beauchamp, 1994), and the maximum concentration (550 mL N2O Lÿ1) measured in ethanol-treated soil columns (Weier et al. (1994). However, Hansen et al. (1993) observed N2O concentrations in the order of 1900 mL N2O Lÿ1 in fertilized soil after heavy rainfall and Rolston et al. (1976) reported N2O concentrations as high as 28,000 mL N2O Lÿ1 in a study of NO3±N movement and transformation in soil columns.

When the water table was positioned at 10 cm below surface, the zone of maximum N2O production was much closer to 20 cm than to 5 cm in the Blount and Huntington columns (Figs. 1 and 3). In contrast, N2O concentrations at 5 and 20 cm depths in the Cler-mont columns were generally similar during those periods (Fig. 2). These dierences can be ascribed to soil texture, structure and drainage characteristics as they aect moisture distribution above the water table, N2O production, emission at the soil column surface and reduction of N2O to N2. The Blount and Hunting-ton soils have better internal drainage, and conse-quently soil pore space in the upper layers would become more readily available for gas transport than in the Clermont columns. Supporting this contention are the air-®lled porosity plots (Fig. 6) which showed that, when the water table was at 10 cm, the surface of the Clermont columns was near water saturation (e_a

range: 0.04 to 0.08), contrasting with the surface con-ditions of the Huntington (e_a _{range: 0.15 to 0.20) soil}

columns. All these factors would contribute to the greater in situ N2O concentrations in the 5 cm region of the Clermont columns compared to the Blount and Huntington pro®les.

As observed during our experiment, Gilliam et al. (1978) also reported accumulation of N2O in the bottom of soil columns for several weeks. The persist-ence of N2O in the lower soil horizons is a good indi-cator of low N2O-reductase activity at depth. Activity of this enzyme is controlled by O2 (Tiedje, 1988;

KoÈr-ner and Zumft, 1989) and NO3±N concentration

(Letey et al., 1980) as well as soil pH (Terry and Tate, 1980). It appears, however, that the inhibitory eect of low pH on N2O-reductase is lessened by prolonged anaerobiosis (Terry and Tate, 1980) and high concen-trations of NO3±N. In denitri®cation studies, N2

O-re-ductase activity is purposely inhibited with C2H2, resulting in N2O as sole end product of denitri®cation (Yoshinari et al., 1977).

However, in the context of our study, problems could arise with the C2H2-inhibition technique with respect to its interference with nitri®cation (Mosier, 1980) and a loss of inhibition due to degradation of C2H2 by soil microbes (Yeomans and Beauchamp, 1978; Terry and Leavitt, 1992). In our experiment, C2H2 was used only at d 92, 96 and 105. During that period, a high water table was imposed, thereby creat-ing conditions not optimal for nitri®cation. Our results should not, therefore, be aected by a possible inter-ference of C2H2 with nitri®cation. Terry and Leavitt (1992) reported enhanced degradation of C2H2in soils with history of continuous (1 to 6 weeks) exposure to this gas. They noted that in soils with prior exposure to C2H2, degradation of C2H2 occurs in a 1 week period, whereas in soil samples not previously exposed to C2H2, 3 to 6 weeks of incubation were needed before degradation could be initiated indicating a low indigenous community of C2H2 degraders. In a ®eld evaluation of the C2H2-inhibition technique, Ryden and Dawson (1982) observed eective inhibition of N2O reduction by C2H2 and no signs of C2H2 degra-dation in soils with up to 20 prior intermittent ex-posures to C2H2. These results indicate that the C2H2 -inhibition technique is problematic in soils continu-ously exposed to the gas but, evolution to C2H2 degra-ders and its subsequent degradation are not likely to be a result of our short-term use of C2H2in this exper-iment.

It is clear from the data presented in Table 2, that C2H2 concentrations in the soil columns at depths >40 cm were below inhibitory amounts. If reduction of N2O to N2 was actively occurring at these soil depths, a failure to inhibit this process could lead to underestimation of denitri®cation N loss. However, using the acetylene-inhibition technique we describe in this paper, we found that 24 to 43% of the NO3±N in-itially present in the columns was removed. These values compared favorably with the 40% NO3removal obtained by mass balance in the 15N-treated columns (Jacinthe, 1995; Jacinthe et al., 1999). Moreover, con-centrations of N2O remained high in the lower portion (depth >40 cm) of the soil columns, exceeding in many instances concentrations in the upper soil layers. Persistence of elevated N2O concentrations at depth has been observed by Gilliam et al. (1978) and indi-cates that the activity of N2O-reductase was inherently low and that N2O was not being reduced at a signi®-cant rate.

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decrease in the mole fraction of N2O from 84% at 12 h to 38% at 48 h of incubation. A similar sequence in denitri®cation products evolution was observed in soils (Cady and Bartholemew, 1960; Firestone and Tiedje, 1979; Letey et al., 1980) and in pure cultures (Matsu-bara and Mori, 1968). Data from Rolston et al. (1976) showed that N2 ¯uxes lagged by several days behind N2O emission from soils. Our data indicate that, in the days immediately following the rise of the water table near the soil surface, denitri®cation activity was enhanced and so was N2O emission. But, it took more than 1 week for the saturated soil to become su-ciently anoxic and N2 production to substantially increase. This agrees with Letey et al. (1980) who suggested that, unlike the NO3-reductase, the N2 O-re-ductase develops after a longer period of anaerobiosis. Also, because of the great sensitivity of the N2 O-re-ductase to O2(Tiedje, 1988; KoÈrner and Zumft, 1989), it is conceivable that the presence of residual O2could inhibit its activity resulting in the early dominance of N2O in the denitri®cation products. As the upper soil horizons remained saturated for longer, O2 becomes depleted because its rate of transfer from the overlying soil layer could not keep up with its rate of consump-tion in the saturated region. As the system becomes more anaerobic, denitri®cation shifted to N2O re-duction resulting in lower mole fractions of N2O in the gas euent on d 105.

In the Introduction to our paper, we suggested that the mole fraction of N2O would be less if the water table is maintained deeper in the soil pro®le than near the soil surface. This assumption was not consistently supported. It is true that with the water table near the soil surface, the mole fraction of N2O increased in the short term. But, after several days of anaerobiosis, the mole fraction dropped to values signi®cantly lower (35%) than when a deep water table was maintained (68%). This suggests that the composition of the deni-tri®cation gas emitted at the soil surface may not depend primarily on the ¯ow path length and residence time of N2O within the soil column, but on the

aera-tion status of the most biologically-active surface soil layers.

5. Conclusions

To provide a basis for assessing the potential impact of WTM techniques on ®eld N dynamics and air qual-ity, N2O emission data from natural and managed eco-systems were compiled (Table 4). Comparison of the N2O emission rates in cultivated ®elds with those obtained during this study shows that when the water table was located at or below 50 cm depth, the rates of N2O emission from the soil column surface (1.8 to 44 mg N mÿ2 dÿ1) were in the same range as those recorded in agricultural ®elds and other managed eco-systems. However, when a high water table was imposed, N2O emission rates from the soil column sur-face were 4 to 430 times higher than those obtained under ®eld condition. Although excess amount of NO3±N can be removed with a near-surface water table, the practice could potentially increase atmos-pheric N2O loading. However, the highest rates of N2O emission we measured are of similar magnitude as those observed at spring thaw under natural settings (Christensen and Tiedje, 1990). Also the N2O emission peaks we observed are probably short-lived and their cumulative eect may be limited because the N2O pro-duce cannot exceed available NO3±N.

Randall and Iragavarapu (1995) found that 20% of N fertilizer added to crops is typically lost in tile drai-nage. This represents the potential amount of N that is leached out of the root zone into groundwater. If 40% of this N can be removed using WTM techniques (Jacinthe et al., 1999) and 50% of the denitri®cation products is N2O, then the total amount of N2O evolved would account for 4% of the annual N fertili-zer application. Since between 0.1 and 3.0% of the N fertilizer added to cropland is typically lost as N2O (Eichner, 1990; Jacinthe and Dick, 1997), the WTM technique could, potentially, increase the proportion of

Table 4

Comparison of nitrous oxide emission rates (g N2O±N haÿ 1

dÿ1

) from the surface of soil columns with emission rates from agricultural soils

Range System Reference

0.2±10.3 grass, meadow, mixed forests Seiler and Conrad, 1981

7±49 corn, alfalfa Cates and Keeney, 1987

10.5±123 review of ®eld studies, 1979±1987 Eichner, 1990

3.6±480 0 to 8 d after application of 120 kg N haÿ1and irrigation Ryden et al., 1978

0±5300 Spring thaw emission Christensen and Tiedje, 1990

Up to 25000 soil columns treated with NO3±N and ethanol Weier et al., 1994

6.9±17.6 mean daily ®eld emissions under continuous corn and other crop rotations Jacinthe and Dick, 1997 74.7±326 maximum daily ®eld emissions under continuous corn and other crop rotations Jacinthe and Dick, 1997

18±440 columns with low water table this study

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fertilizer N returned to the atmosphere as N2O. To what extent this practice could have an eect on global N2O budget will depend on the total surface area where WTM practices may be applicable, i.e., where vulnerable shallow groundwater systems are threatened by NO3±N leaching.

As a practical recommendation it appears preferable, when possible, to prolong a high water table in the upper soil layers. This would best be done in the fall after harvest has been completed. As was seen in our study and in those of Firestone and Tiedje (1979) and Letey et al. (1980), prolonged anoxic conditions decreases the proportion of N2O in the N gases emitted. Thus, there is an improvement of the euent gas quality, from a N2O perspective, and the potential negative eects of WTM techniques on air quality would be minimized.

Acknowledgements

The authors gratefully thank Mr. T. Reily for help-ing with collection of soil cores, Dr. J. Streeter for donating the15N used in this study, M. B. Bishop for helping with data analysis, Mr. F. Knox for perform-ing gas analysis and Ms. J. Durkalski for her capable laboratory management. Salaries and research support provided by state and federal funds appropriated to the Ohio Agricultural Research and Development Cen-ter and by USDA-CSREES Grant No. 91-34214-6062.

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