Artigo científico

1. SHORT COMMUNICATION
A simple method for quantifying dissolved nitrous
oxide in tile drainage water
Jennifer D. Roper1
, David L. Burton1,3
, Ali Madani2
, and Glenn W. Stratton1
1
Department of Environmental Sciences, Dalhousie University, Truro, Nova Scotia, Canada B2N 5E3; and
2
Department of Engineering, Dalhousie University, Truro, Nova Scotia, Canada B2N 5E3.
Received 7 March 2012, accepted 6 November 2012.
Roper, J. D., Burton, D. L., Madani, A. and Stratton, G. W. 2013. A simple method for quantifying dissolved nitrous oxide
in tile drainage water. Can. J. Soil Sci. 93: 59Á64. It is often assumed that the N2O produced from nitrification and
denitrification in soil systems is lost primarily as a gas from the soil surface. However, the dissolution and eventual
degassing of N2O in water leaching through, and draining from, agricultural fields is also a significant loss pathway. The
quantification of this pathway of N2O loss has been limited by available methodologies for measuring dissolved gases in
drainage water. Here a simple method is presented, which allows for the collection of tile drainage water samples using
standard automated water sampling equipment that maintains the dissolved gases. Tile drainage water was collected in 1 L
ISCOTM
water sampling bottles outfitted with modified 10 mL volumetric pipettes. The pipettes provide a means of
reducing the water:atmosphere interface for water held within the pipette thus reducing the N2O exchange with the
atmosphere. The water samples are removed from the pipette using long slender needles attached to a 20-mL syringe,
drawing 5 mL of water from within the bulb of the pipette. The dissolved N2O in the water samples was measured by
headspace analysis using a gas chromatograph. A laboratory trial determined that retaining the water in the pipette bulbs
resulted reduced N2O degassing such that N2O concentration did not decrease significantly in the first 24 h after filling of
the bottle.
Key words: Dissolved gas measurement, nitrous oxide, drainage water, greenhouse gas
Roper, J. D., Burton, D. L., Madani, A. et Stratton, G. W. 2013. Simple me´ thode pour doser l’oxyde nitreux dissous dans
l’eau draine´ e par des tuiles. Can. J. Soil Sci. 93: 59Á64. On pre´ sume souvent que le N2O re´ sultant de la nitrification et de la
de´ nitrification dans le sol se perd essentiellement sous forme de gaz qui s’e´ chappe a` la surface du sol. Cependant, la
dissolution du N2O dans l’eau puis le de´ gazage de cette dernie` re quand elle s’infiltre dans le sol par lixiviation ou drainage
des champs agricoles peut aussi entraıˆner des pertes importantes de ce gaz. Jusqu’a` pre´ sent, les me´ thodes existantes qui
recueillent automatiquement les e´ chantillons d’eau servant a` doser le volume des gaz dissous dans l’eau de drainage
restreignaient la quantification de ces pertes. Les auteurs proposent une me´ thode simple permettant de recueillir l’eau
draine´ e par des tuiles avec du mate´ riel standard d’e´ chantillonnage automatique tout en re´ duisant les pertes de gaz dissous.
L’eau draine´ e est recueillie dans des bouteilles ISCOMC
d’un litre pourvue a` l’exte´ rieur d’une pipette volume´ trique de
10 mL modifie´ e. La pipette diminue l’interface entre l’eau qu’elle contient et l’atmosphe` re, ce qui re´ duit les e´ changes de
N2O. L’eau e´ chantillonne´ e est retire´ e de la pipette avec une longue aiguille fixe´ e a` une seringue de 20 mL. On pre´ le` ve ainsi
5 mL d’eau de l’ampoule de la pipette, puis on mesure la quantite´ de N2O dissous en analysant l’espace libre par
chromatographie gazeuse. Un essai en laboratoire a e´ tabli que garder l’eau dans l’ampoule de la pipette atte´ nue le de´ gazage
du N2O au point que la concentration de ce gaz ne diminue pas de manie` re significative durant les 24 heures qui suivent le
remplissage de la bouteille.
Mots cle´s: Doser le volume de gaz dissous, oxyde nitreux, l’eau draine´ e, gaz a` effet de serre
Studies of the fate of N in soil often assume that surface
flux emissions adequately estimate the soil N2O produc-
tion, overlooking emissions associated with water leach-
ing from the soil profile (van Bochove et al. 2001;
Grandy et al. 2006; Phillips 2007; Reay et al. 2009). To
fully quantify N2O emissions associated with agricul-
tural activities, an effort has to be made to measure
losses of dissolved N2O in water draining from
agricultural landscapes (Dowdell et al. 1979; Minami
and Fukushi 1984; Haag and Kaupenjohann 2001;
Reay et al. 2009). The degassing of N2O dissolved in
agricultural drainage water has been shown to contri-
bute to N2O emissions to the atmosphere (Reay et al.
2003) and may exceed surface emissions (Minamikawa
3
Corresponding author (e-mail: dburton@dal.ca).
Abbreviations: ECD, electron capture detector; HgCl2, mercuric
chloride; HS, headspace; K, solubility; NO3
-N, nitrate nitrogen;
NH4
-N, ammonium nitrogen; N2O, nitrous oxide; TCD, thermal
conductivity detector
Can. J. Soil Sci. (2013) 93: 59Á64 doi:10.4141/CJSS2012-021 59
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2. et al. 2010). The N2O produced in the soil profile
dissolves in the soil solution and is transported to
subsurface and, where drainage systems are present, is
discharged to surface waters, where the N2O rapidly
degasses from solution and is released to the atmosphere
(Bowden and Bormann 1986; van Bochove et al. 2001;
Sawamoto et al. 2005). This indirect source of N2O
is easily overlooked, in that it is often temporally
and spatially displaced from the expected site of
production and its quantification is difficult (Hasegawa
et al. 2000).
Subsurface drainage is a management practice in
agricultural fields used to remove excess water from
the landscape in order to increase trafficability and
enhance crop productivity (Drury et al. 1993). It has
been shown to be a highly beneficial practice in Atlantic
Canada, which receives approximately 1200 mm of
precipitation annually, primarily in the fall and spring
(Madani and Brenton 1995). The addition of fertilizer
(organic and inorganic) to agricultural lands in sub-
humid regions contributes to the production of N2O
(Minami and Fukushi 1984). Ronen et al. (1988) state
that approximately 30% of the N applied to agricultural
soils can be lost through leaching. Similarly, guidelines
for the reporting of dissolved N2O associated with
drainage water are included as EF5g in the Intergovern-
mental Panel on Climate Change (IPCC) guidelines for
indirect N2O emissions assume the fraction of applied N
that leaches (FracLeach) from the root zone in humid
regions is 0.30 (IPCC 2006; Rochette et al. 2008).
Soluble C compounds and NO(
3 -N applied to the
soil surface can leach downward, supplying substrate
(electron donors) and terminal electron acceptors for
denitrification to occur at depth within the soil. The
N2O produced at depth can easily be dissolved in soil
water or groundwater (Sawamoto et al. 2005). Tile
drainage expedites the movement of water from the
field, reducing the opportunity for biochemical reduc-
tion of dissolved N2O in the soil profile (Mehnert et al.
2007).
Nitrous oxide is highly soluble in water (at 58C 1.0
mL N2O-N mL(1
water00.0425 mols N2O L(1
water)
(Dowdell et al. 1979; Davidson and Swank 1990;
Heincke 2001), with its solubility increasing as tempera-
ture decreases (Weiss and Price 1980; Heincke and
Kaupenjohann 1999). During the winter period in
northern latitudes, there is the potential for the forma-
tion of continuous ice layers that can restrict the
diffusion of N2O to the atmosphere, trapping N2O at
a depth where it can dissolve in cold water (Davidson
and Swank 1990; Burton and Beauchamp 1994).
Although it has been recognized as a component of
N2O emissions from agricultural systems, dissolved N2O
in drainage water is still poorly understood and seldom
quantified. In particular, we do not adequately under-
stand the impact of land management decisions, such as
the choice of tillage system, which could affect water
movement, profile N dynamics and the potential for
N2O emissions in tile drainage water during the non-
growing period. One of the major limitations to
quantification of this source is a practical, reliable,
and automated means of collecting water samples in a
manner that preserves dissolved gas concentrations.
This paper describes a simple method that allows for
the collection of drainage water samples using standard
automated water sampling equipment that maintains the
dissolved gases and thereby allows automated collection
of water sampled for determination of dissolved N2O in
agricultural drainage waters.
MATERIALS AND METHODS
Sampling Apparatus
Tile water is commonly collected in 1-L ISCOTM
water
sampling bottles using an automated sampler (ISCO
6700 Portable Sampler, ISCO Inc., Lincoln, NE). The
configuration of this type of sample container allows
exposure of the sample to the atmosphere and degassing
of dissolved gasses. To maintain the integrity of the
water sample for dissolved N2O quantification, the
potential for the sample to degas must be minimized.
To accomplish this in a simple, cost-effective manner, a
10-mL volumetric pipette, cut at the fill line to ensure
that water fills the pipette beyond the bulb, was placed
inverted in each of the sampling bottles in the auto-
sampler. A portion of the glass was cut from the upper
part of the pipette to insure the pipette fit completely
within the bottle and there was no interference with the
workings of the autosampler and to ensure the water
filled to the narrow portion of the pipette (Fig. 1). The
pipette was inverted for three reasons: the pipette can
be modified by simply cutting it; to speed the rate
of filling; and to minimize turbulence during filling.
Fig. 1. Diagram of modified volumetric pipette within an
ISCOTM
water sample container, illustrating the retrieval of
water from within the bulb of the pipette using a syringe. The
internal diameter of the pipette stem at the water line is 0.13
cm2
and the pipette contains approximately 10 mL of water.
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3. The approximately 9 mL of water contained in the
pipette has a significantly reduced the surface area (0.13
cm2
) compared with the larger sample container (Fig. 1).
The water samples were extracted from the pipettes
using a 20-mL syringe fitted with a 20-gauge 30.5-cm
Popper†
deflected noncoring septum penetration needle
(Fisher Scientific), drawing 5 mL of water from within
the bulb of each of the pipettes. Care was taken to
slowly withdraw the water sample to minimize degas-
sing. Four millilitres of the sample was injected into a
12-mL exetainer (Labco International, UK), which had
previously been evacuated and brought to atmospheric
pressure with ultrapure grade helium. Prior to evacua-
tion, 50 mL of 0.02 M mercuric chloride (HgCl2) solution
was added to the exetainers to inactivate microbial
function and thus prevent further gas production or
consumption in the water sample following injection
into the vial (Elkin 1980; Ueda et al. 1993; Reay et al.
2003). The biostatic agent HgCl2 was chosen as it
inhibits microbial activity in water samples (Reay
et al. 2004), does not alter the physical and chemical
characteristics of the solution (Trevors 1996), and does
not contain N.
The dissolved N2O in the tile drainage water sample
contained in the exetainer was measured by a headspace
method. Vials containing water samples were equili-
brated at room temperature (228C) before being ana-
lyzed by gas chromatography, as noted below. The
laboratory temperature was recorded during the analysis
of dissolved N2O concentration. The value for the
N2O solubility (K0) was adjusted for temperature
according to the procedure presented by Weiss and
Price (1980).
Gas Analysis
Gas analysis was performed using a Varian Star 3800
Gas Chromatograph (Varian, Walnut Creek, CA) fitted
with an electron capture detector (ECD), thermal
conductivity detector (TCD) and a Combi-PAL Auto-
sampler (CTC Analytics, Zwingen, Switzerland). The
Combi-PAL injects 2.5 mL into the gas chromatograph
to fill two 0.5-mL sampling loops that load gas onto
ECD and TCD/FID flow streams. The ECD was
operated at 3808C, 90% Ar, 10% CH4 carrier gas at
20 mL min(1
, Haysep N 80/100 pre-column (0.32 cm
diameter)50 cm length) and Haysep D 80/100 mesh
analytical columns (0.32 cm diameter)200 cm length)
in a column oven operated at 708C. The pre-column was
used in combination with a four-port valve to remove
water from samples. The TCD was operated at 1308C,
pre-purified He carrier gas at 30 mL min(1
, Haysep N
80/100 mesh (0.32 cm diameter)50 cm length) pre-
column followed by a Porapak QS 80/100 mesh (0.32 cm
diameter)200 cm length) analytical column maintained
at 708C. Nitrous oxide was quantified based on ECD
response for concentrations up to 50 mL N2O-N L(1
and on TCD response for concentrations greater than 50
mL N2O-N L(1
.
Calculations
The calculation of total dissolved N2O in tile drainage
waters was achieved by first determining the N2O in the
headspace (N2OHS) of the ExetainerTM
in equilibrium
with the water sample (headspace analysis). This num-
ber was used to calculate the concentration of N2O
dissolved (N2ODIS) in the sampled water assuming
equilibrium. The sum of these two amounts (N2OHS'
N2ODIS) represents total dissolved N2O (N2OTOT) con-
tained in the original water sample. The calculations
were modified from Weiss and Price (1980).
Step 1 Á Calculate the N2O (mol) in the Headspace
The amount of N2O in the pressurized headspace (HS)
was calculated according to the equation:
N2OHS 0
Psample ) x0
N2O ) VHS
R ) T
(1)
where N2OHS is the amount of N2O in exetainer
headspace (mol), Psample is the pressure of the head-
space (atm), x0
N2O is the molar volume of N2O in the
headspace (L N2O-N L(1
air), VHS is the volume of
headspace (L), R is the ideal gas constant (0.08214 L
atm mol(1
8K(1
), and T is the laboratory temperature
(8K).
The sample pressure (Psample) was calculated as a ratio
of the volume of the initial headspace at atmospheric
pressure to the volume of the headspace remaining after
the injection of water (0.012/0.008 atm).
Step 2 Á Calculate the Amount of N2O (mol)
Dissolved in Water (N2ODIS)
The method of Weiss and Price (1980) was modified and
used to calculate the number of moles of N2O dissolved
in the aqueous phase (N2ODIS). To predict phase
equilibria between the aquatic and gaseous phase at
pressures above atmospheric, deviations from the ideal
gas law may need to be taken into account. Under
standard conditions 1 mole of N2O occupies 0.7%
less volume than 1 mole of idea gas (Weiss and Price
1980) and therefore, depending on the desired precision,
correction for non-ideal behavior may be required. This
is done using the temperature-dependent virial coeffi-
cients (B, d) that account for the imperfect conditions at
gas-water interface (Hayden and O’Connell 1975).
N2ODIS 0x0
)F )VH2O (2)
F 0K0 )(P(pH2O))exp
P)
B ' 2d
R ) T
(3)
B0(905:95'4:1685)T-0:0052734)T2
(4)
d065:0(0:1338)T (5)
where N2ODIS is the amount of N2O dissolved (mol
N2O), F is function F (mol air L(1
), VH2O is the volume
of water in exetainer (L), x0
is the molar volume of N2O
in dry air (mol N2O mol(1
air), K0 is the equilibrium
ROPER ET AL. * NITROUS OXIDE IN TILE DRAINAGE WATER 61
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4. constant (mol L(1
atm(1
), P is the total barometric
pressure (atm), pH2O is the vapour pressure of water
(atm), B is the second virial coefficient (cm3
mol(1
), d is
the cross virial coefficient (cm3
mol(1
), R is the gas
constant (0.08214 L atm mol(1
8K(1
), and T is the
absolute temperature (8K).
The value for N2O solubility (K0) was adjusted for
temperature according to the procedure presented by
Weiss and Price (1980). Therefore, the adjusted values in
Eq. 3 are calculated using Eqs. 6 through 8.
K0 0(0:0000299)T2
)((0:00214)T)'0:0591 (6)
In pH2O024:4543(67:4509)
100
T
(4:8489
)ln
T
100
(7)
(B ' 2d)
R ) T
0
(9:4563
T
'0:04759(6:427)10(5
T (8)
Step 3 Á Combine Headspace and Dissolved N2O and
Express as a Function of the Volume of the Water
Sample
Total N2O dissolved in the water sample (mol) is the
sum of N2O in the headspace (N2OHS) determined using
Eq. 1 and N2O dissolved in the water at equilibrium
(N2ODIS) calculated from Eqs. 2 through 8. The final
sum was expressed as a concentration (N2OTOT; mol
N2O L(1
) by dividing the moles of N2O by the volume
of the sample.
N2OTOT 0
N2ODIS ' N2OHS
VH2O
(9)
Eight-day Laboratory Study
To determine the relative rate of degassing from the
water contained in the modified pipette relative to bulk
water, an 8-d laboratory study was conducted using
15 L of water collected from tile drains from a nearby
experimental site and placed in a 20-L glass carboy. The
concentration of dissolved N2O was increased by intro-
ducing 5 mL of 100% N2O into the headspace and
allowing to equilibrate at 48C for a 2-d period. The water
was then transferred to bottles with and without 0.25
mmol HgCl2 L(1
to determine whether microbial con-
sumption of N2O was occurring. Six pipettes were placed
in each bottle. At each sampling time a 4-mL aliquot of
water was collected from within the pipette and a second
4-mL aliquot of water sampled was collected directly
from the water in the open bottle. There were five
replicates for each measurement. The pipette that was
sampled was removed from the bottle after sampling.
Samples were collected immediately upon the water’s
addition (t00) to the apparatus and at 0.5, 1, 2, 4 and 8 d
after addition. The 4-mL water samples were injected
in 12-mL exetainers containing HgCl2 as described
above, and headspace analysis was performed by gas
chromatography at the end of experiment after at least
24 h of equilibration for all samples.
Statistical Methods
The experiment was set up as a two-factor completely
randomized design with the factors being sampling
location (bulk water vs. pipet bulb), (9) biostatic agent
in bulk water with repeated measures (storage times).
Exponential curve fits were performed on averages from
each sampling time. Statistical analysis was performed
using JMP 10 (SAS Institute, Inc., Cary, NC).
RESULTS AND DISCUSSION
Over the 8-d storage period, the concentration of N2O
dissolved in the water decreased significantly (P5
0.001). The rate of decrease was significantly (P50.01)
different depending on the sampling location, but was
not significantly influenced by the addition of HgCl2 to
the water (Fig. 2). Water contained in the pipette lost
dissolved N2O at a rate of 2% d(1
(k00.02 d(1
) as
compared with 7% d(1
(k00.07 d(1
) for the bulk
water (Fig. 2). For the first 24 h of storage there was no
significant change in the concentration of dissolved N2O
in samples stored in the pipette. The average N2O
concentration on the first day from the pipettes and
the bulk water contained in the bottles were 0.1089
0.003 and 0.10390.001 mmol N2O L(1
water, relative
to 0.11490.002 at the time of filling. After 8 d of
storage the average N2O concentration was 0.0979
0.004 mmol N2O L(1
(16% loss), while the concentra-
tion in the bulk water had dropped to an average of
0.06390.004 mmol N2O L(1
(45% loss). There was
significantly (P00.001) less loss of N2O from water
contained in the pipette when compared with water
contained in the open bottle.
These findings indicate that the rate of degassing of
dissolved N2O can be significantly reduced by minimiz-
ing the sample:atmosphere interface using a modified
pipette enclosure. Loss still occurs, but at a reduced rate,
providing the opportunity for delaying the collection of
sample for up to 24 h without significant change in
dissolved N2O concentration. The addition of a biostatic
agent to the bulk water had no effect on the rate to
decrease in dissolved N2O suggesting that loss was
primarily the result of desorption and biological con-
sumption was not part of the decrease. Further exam-
ination of the rate of loss as a function of temperature
and dissolved N2O concentration may result in a
predictable rate of loss allowing for the opportunity to
further delay sample collection.
There are several potential sources of error that need to
considered. Leakage of gas from the vial can result not
only in a loss of N2O, but also in an error in the assumed
pressure of the headspace. Leakage from ExetainersTM
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5. has been shown to be small (Glatzel and Well 2008);
however, direct measurement of vial pressure or the use
of an internal standard to correct for the mass of the gas
in the headspace would provide a more direct means of
correcting for this error, but would result in a more
complex method and/or analytical requirement. This
error can be minimized by limited reuse of exetainers
caps (Bfive punctures) and minimizing the time of
storage in the Exetainers. The volume of water injected
into the vial is also of importance and should be done as
accurately as possible. Caution also needs to be exercised
in the handling and disposal of HgCl2; appropriate safety
documentation should be consulted. The use of a less
toxic biostatic agent should also be considered.
SUMMARY
The method presented offers a simple, reproducible, and
reliable method for measuring dissolved N2O that is
compatible with automated water-sampling systems. The
apparatus described is simple to assemble from readily
available materials. The use of this apparatus provides a
practical means of collecting samples using an automated
water-sampling device without the need for immediate
recovery for dissolved gas analysis. The availability
of simple, practical means of collecting representative
samples of tile drainage water should allow researchers to
obtain better data on this important yet poorly docu-
mented loss of N2O from agricultural ecosystems.
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Fig. 2. Change in dissolved N2O concentration (mmol N2O L(1
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Equations present the exponential fit to the data for each treatment. Error bars are 91 standard deviation.
ROPER ET AL. * NITROUS OXIDE IN TILE DRAINAGE WATER 63
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