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

Soil Biology & Biochemistry 32 (2000) 371±382
www.elsevier.com/locate/soilbio

Bioremediation of nitrate-contaminated shallow soils and waters
via water table management techniques: evolution and release of
nitrous oxide
Pierre-Andre 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
Department of Food, Agricultural and Biological Engineering, The Ohio State University, Columbus, OH 43210, USA

b

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 a€ected 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 di€erent WTM schemes were
studied: static (WTM1) and dynamic (WTM2). We repeated WTM2 using 15N 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 mÿ2 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 mÿ2 dÿ1). 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 (P R 0.05) 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
in the most biologically-active portion of the soil pro®le. # 2000 Elsevier Science Ltd. All rights reserved.
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

* Corresponding author. Tel.: +1-330-263-3877; fax: +1-330-2633658.
E-mail address: dick.5@osu.edu (W.A. Dick).

by raising the water table. As a result, oxygen is
rapidly depleted in the soil pores, thus creating conditions 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 dinitrogen (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. Therefore, by creating an anoxic environment in these
layers, denitri®cation activity would be stimulated and

0038-0717/00/$ - see front matter # 2000 Elsevier Science Ltd. All rights reserved.
PII: S 0 0 3 8 - 0 7 1 7 ( 9 9 ) 0 0 1 6 3 - 7

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P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

move through soil horizons of di€erent gas di€usivity,
porosity, redox potential, soil water content, organic
matter content and N2O-reductase activity. Several of
these factors a€ect the rate of gas migration and residence time in the soil pro®le. The relative proportion
of N2O and N2 escaping from the soil surface would,
consequently, be a€ected.
Smith (1980) indicated that the ratio of N2O-to-N2
emitted during denitri®cation depends on the balance
between the rate of N2O di€usion from sites where it
is produced and the rate of N2O reduction to N2. A
low rate of N2O di€usion rate favors N2O reduction.
Letey et al. (1980) noted that when N2O di€usion 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 N2 before it escapes to the
atmosphere.

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 e€ect 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 portion 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 warming. 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 techniques would represent environmentally sound ways of
removing NO3±N from soil.
Application of WTM techniques creates a watersaturated subsoil overlain by unsaturated soil of varying biochemical and physical properties. Denitri®cation

gases, produced in the saturated soil, di€use upward
to the surface due to concentration gradients. In their
migration toward the soil surface, these gases must

Table 1
Physical and chemical properties of soils
Parameter

Location
Taxonomy
Drainage class
Bulk densitya (g cmÿ3)

Soil depth (cm)

0±10

Soil series
Blount

Clermont

Huntington

Union County, OH
Aeric Ochraqualf
somewhat poorly drained
1.53

Jennings County, IN
Typic Glossaqualf
poorly drained
1.56

Pike County, OH
Fluventic Hapludoll
well drained
1.30

pH

0±5
5±10
10±15
15±20b

5.5
6.1
4.9
5.4

5.1
5.2
4.9
5.2

7.2
7.3
7.5
7.5

Organic C (g kgÿ1 soil)

0±5
5±10
10±15
15±20b

27.9
22.3
11.5
11.8

23.3
6.0
6.4
8.7

31.0
24.4

20.1
21.8

Nitri®cation potentialc (mg NO3±N kgÿ1 weekÿ1)

0±5
5±10
10±15
15±20b

31.0
12.3
14.3
16.7

Denitri®cation potentiald (mg N2O±N kgÿ1 dÿ1)

0±5
5±10
10±15

15±20b

a

0.93
1.44
0.53
0.89

0.15
0.10
0.10
0.08
0.88
0.05
0.02
0.01

89.7
85.3

60.8
53.3
5.78
2.14
1.25
0.88

Bulk density was determined for the 0±10 cm soil depth.
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
Measured after 48 h of incubation of NO3±N amended soil samples under N2 atmosphere and without addition of external C.
b

P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

We hypothesized that the overall e€ects of applying
WTM strategies to stimulate NO3±N removal and to
a€ect atmospheric N2O loading will depend on individual 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 agricultural 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) cylinders were used in our study. Procedures for soil column 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 di€erent 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 management 2 (WTM2) treatment 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

373

d 110 when the columns were allowed to drain freely
from the bottom. The dynamic water table management treatment (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 columns 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 constructed 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 material, 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 silicone 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 outside 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 monitored using a closed chamber consisting of two parts
(Jacinthe and Dick, 1997). The chamber's lower section was made of a 30 cm long by 15 cm dia PVC
pipe. One end of this PVC pipe was beveled to facilitate 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 column. The top portion of the chamber consisted of a
PVC endcap ®tted with a gas sampling port. The
chambers remained in place throughout the experiment.
When collecting a gas sample for N2O emission, the
chamber was capped and air samples were taken at 0,
45 and 90 min. Sampling times were 0, 30 and 60 min

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P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

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 additional 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 N2 and 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 prolonged period of a static water table level. The second
day (d 96) was selected to represent results immediately 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 techniques 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 C2H2 addition. 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 minÿ1 for 6±8 min.
After such time, the C2H2 partial pressure inside the
chamber headspace was 17.1 2 1.8% (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 C2H2 to the rate with C2H2.
2.3. Methods of analysis
Nitrous oxide concentrations were measured using a
DIMENSION I (Tremetrics, Austin, TX) gas chromatograph 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%)

Fig. 1. Nitrous oxide (N2O) concentrations at various depths within
the Blount soil columns with a static (WTM1, w) or a dynamic
(WTM2, Q) water table. The  symbol indicates a signi®cant di€erence (P < 0.05) between water table management practices at a given
sampling date.

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 (detector). Standard N2O samples, used for instrument calibration, 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 e€ects of soil series, water

P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

Fig. 2. Nitrous oxide (N2O) concentrations at various depths within
the Clermont soil columns with a static (WTM1, w) or a dynamic
(WTM2, Q) water table. The  symbol indicates a signi®cant di€erence (P < 0.05) between water table management practices at a given
sampling date.

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 e‚ux 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

375

Fig. 3. Nitrous oxide (N2O) concentrations at various depths within
the Huntington soil columns with a static (WTM1, w) or a dynamic
(WTM2, Q) water table. The  symbol indicates a signi®cant di€erence (P < 0.05) between water table management practices at a given
sampling date.

period, N2O concentrations remained relatively constant, being in many instances greater below the water
table than in the unsaturated portion of the soil columns (Figs. 1±3). In contrast, the WTM2 treated columns 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 columns. 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 Huntington 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 raising 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 concentrations reached a second maxima in the 5±20 cm
depth region at around d 105. In general, N2O concentrations during the second saturation period (d 92 to

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P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

Fig. 4. Nitrous oxide concentrations (*) and 15N enrichment (in
at%) of the N2O pool (r) at various depths within the soil columns
treated with K15NO3 (WTM3). The scale for the 15N enrichment is
given on the right side of the graphs.

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 40 cm), however,
C2H2 concentrations remained below inhibitory
amounts indicating that, with the technique we used to
supply C2H2 to the soil columns, inhibition of N2O reduction 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.

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 dÿ1 in the
Blount, 5.1 to 23.1 mg N2O±N mÿ2 dÿ1 in the Clermont, and 5.0 to 44.1 mg N2O±N mÿ2 dÿ1 in the Huntington columns, respectively.
When the water level was raised to 10 cm in all columns 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 columns between d 92 and 110.

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

Blount
Clermont
Huntington
Mean (date)b
a

d 96

d 105

WTM1

WTM2

WTM1

WTM2

WTM1

WTM2

0.72 (0.00)a
0.49 (0.24)
0.71 (0.35)

0.61(0.38)
0.71 (0.31)
0.44 (0.28)

0.94 (0.03)
0.97 (0.00)
0.89 (0.10)

0.96 (0.00)
0.96 (0.00)
0.96 (0.02)

0.27 (0.17)
0.56 (0.07)
0.66 (0.66)

0.38 (0.01)
0.23 (0.11)
0.69 (0.41)

0.68b

0.95a

0.35c

Values in parentheses are standard deviations.
Reported means for each sampling date are computed across soils and treatments. Means within a row followed by the same letter are not
signi®cantly di€erent at P < 0.05.
b

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P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

Analysis of variance revealed a signi®cant (P R 0.05)
e€ect of sampling date on the mole fractions of N2O,
but the e€ects 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 fractions 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 treatments. 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 di€usivity
The e€ective di€usion coecient (Ds) of N2O in
soils was computed using the procedures described in
Rolston et al. (1976) as:
Ds ˆ ÿk…VL=A†,

…1†

where Ds is the soil e€ective di€usion coecient (cm2
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
…2†

Fig. 6. Air-®lled porosity (ea) 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, where Pb is soil bulk density, Pa
soil particle density, and y is the volumetric moisture content.

in which Cs is 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,
Ca exceeds Cs and a value for Ds could not be determined from Eq. (2).
Analysis of variance showed a signi®cant e€ect (P <
0.05) of soil series on soil gas di€usivity expressed as
Ds. E€ective di€usion coecients of N2O in the Huntington soil were greater …mean ˆ 3:81  10ÿ3 cm2 sÿ1 ;
range: 0.03  10ÿ3±10.4  10ÿ3 cm2 sÿ1) than in the
Blount …mean ˆ 1:49  10ÿ3 cm2 sÿ1 ; range: 0.01 
10ÿ3±3.3  10ÿ3 cm2 sÿ1) and the Clermont soils …mean
ˆ 1:25  10ÿ3 cm2 sÿ1 ; range: 0.02  10ÿ3±3.7  10ÿ3
cm2 sÿ1). Reported in situ di€usion coecients of N2O
in a loam soil (0.14  10ÿ3±0.25  10ÿ3 cm2 sÿ1) 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 di€usion. For the Blount and Huntington soils, relationships between the two parameters were: Ds ˆ 22:1ea 4:2
and D s ˆ 1:4ea 4:5 , respectively, suggesting a reasonable conformity to the model … D s ˆ km
a † 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 ea >
0.3 but, below this value, ®tting of experimental data
to gas di€usion models is usually not successful. The
ea values recorded for the Clermont soil were, in general, less than 0.1 (Fig. 6). Therefore, an explanation
for the lack of a relationship between Ds and ea in the
Clermont soil may be that the available pore space
was mainly isolated air pockets which did not contribute eciently to gas exchange in this poorly structured
soil. The presence of such blocked pores has been indi-

ln‰…Cs ÿ Ca †=Cs Š ˆ ÿkt

P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

cated as the major cause for the scatter of experimentally-determined di€usion coecients (Pennman, 1940),
and possibly nonconformity to di€usion models.

4. Discussion
The highest concentrations of N2O in the soil columns 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 ethanoltreated 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 Clermont columns were generally similar during those
periods (Fig. 2). These di€erences can be ascribed to
soil texture, structure and drainage characteristics as
they a€ect moisture distribution above the water table,
N2O production, emission at the soil column surface
and reduction of N2O to N2. The Blount and Huntington soils have better internal drainage, and consequently 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 (ea
range: 0.04 to 0.08), contrasting with the surface conditions of the Huntington (ea 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 persistence of N2O in the lower soil horizons is a good indicator of low N2O-reductase activity at depth. Activity
of this enzyme is controlled by O2 (Tiedje, 1988; KoÈrner 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 e€ect of
low pH on N2O-reductase is lessened by prolonged
anaerobiosis (Terry and Tate, 1980) and high concentrations of NO3±N. In denitri®cation studies, N2O-re-

379

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 creating conditions not optimal for nitri®cation. Our results
should not, therefore, be a€ected by a possible interference of C2H2 with nitri®cation. Terry and Leavitt
(1992) reported enhanced degradation of C2H2 in 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 e€ective inhibition of
N2O reduction by C2H2 and no signs of C2H2 degradation in soils with up to 20 prior intermittent exposures to C2H2. These results indicate that the C2H2inhibition technique is problematic in soils continuously exposed to the gas but, evolution to C2H2 degraders and its subsequent degradation are not likely to
be a result of our short-term use of C2H2 in this experiment.
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 initially present in the columns was removed. These
values compared favorably with the 40% NO3 removal
obtained by mass balance in the 15N-treated columns
(Jacinthe, 1995; Jacinthe et al., 1999). Moreover, concentrations 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 indicates that the activity of N2O-reductase was inherently
low and that N2O was not being reduced at a signi®cant rate.
Variations in the mole fraction of N2O between d 96
and 105 are consistent with current understanding of
the sequence of gas evolution during biological denitri®cation. Blackmer and Bremner (1978) reported a

380

P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

Table 4
Comparison of nitrous oxide emission rates (g N2O±N haÿ1 dÿ1) from the surface of soil columns with emission rates from agricultural soils
Range

System

Reference

0.2±10.3
7±49
10.5±123
3.6±480
0±5300
Up to 25000
6.9±17.6
74.7±326
18±440
150±5000

grass, meadow, mixed forests
corn, alfalfa
review of ®eld studies, 1979±1987
0 to 8 d after application of 120 kg N haÿ1 and irrigation
Spring thaw emission
soil columns treated with NO3±N and ethanol
mean daily ®eld emissions under continuous corn and other crop rotations
maximum daily ®eld emissions under continuous corn and other crop rotations
columns with low water table
columns with high water table

Seiler and Conrad, 1981
Cates and Keeney, 1987
Eichner, 1990
Ryden et al., 1978
Christensen and Tiedje, 1990
Weier et al., 1994
Jacinthe and Dick, 1997
Jacinthe and Dick, 1997
this study
this study

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 (Matsubara 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 N2O-reductase develops after a longer period of anaerobiosis.
Also, because of the great sensitivity of the N2O-reductase to O2 (Tiedje, 1988; KoÈrner and Zumft, 1989),
it is conceivable that the presence of residual O2 could
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 consumption in the saturated region. As the system becomes
more anaerobic, denitri®cation shifted to N2O reduction resulting in lower mole fractions of N2O in the
gas e‚uent 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 denitri®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 quality, N2O emission data from natural and managed ecosystems 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 ecosystems. However, when a high water table was
imposed, N2O emission rates from the soil column surface 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 atmospheric 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 e€ect may be limited because the N2O produce cannot exceed available NO3±N.
Randall and Iragavarapu (1995) found that 20% of
N fertilizer added to crops is typically lost in tile drainage. 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 fertilizer 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

P. Jacinthe et al. / Soil Biology & Biochemistry 32 (2000) 371±382

fertilizer N returned to the atmosphere as N2O. To
what extent this practice could have an e€ect 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 e‚uent
gas quality, from a N2O perspective, and the potential
negative e€ects of WTM techniques on air quality
would be minimized.

Acknowledgements
The authors gratefully thank Mr. T. Reily for helping with collection of soil cores, Dr. J. Streeter for
donating the 15N used in this study, M. B. Bishop for
helping with data analysis, Mr. F. Knox for performing 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 Center and by USDA-CSREES Grant No. 91-34214-6062.

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