A simple method for quantifying dissolved nitrous oxide in tile

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

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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: 5964. 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: 5964. 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 entraıˆ ner 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 production, overlooking emissions associated with water leaching 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 agricultural activities, an effort has to be made to measure losses of dissolved N2O in water draining from 3

Corresponding author (e-mail: [email protected]).

Can. J. Soil Sci. (2013) 93: 5964 doi:10.4141/CJSS2012-021

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 contribute to N2O emissions to the atmosphere (Reay et al. 2003) and may exceed surface emissions (Minamikawa 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 59

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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 subhumid 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 Intergovernmental 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 reduction 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 water 0.0425 mols N2O L 1 water) (Dowdell et al. 1979; Davidson and Swank 1990; Heincke 2001), with its solubility increasing as temperature decreases (Weiss and Price 1980; Heincke and Kaupenjohann 1999). During the winter period in northern latitudes, there is the potential for the formation 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 understand 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 nongrowing 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 autosampler. 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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ROPER ET AL. * NITROUS OXIDE IN TILE DRAINAGE WATER

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 degassing. 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 evacuation, 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 equilibrated at room temperature (228C) before being analyzed 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 Autosampler (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 min1, 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) precolumn 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 L1.

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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 number 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) contained in the original water sample. The calculations were modified from Weiss and Price (1980).

Step 1  Calculate the N2O (mol) in the Headspace The amount of N2O in the pressurized headspace (HS) was calculated according to the equation: N2 OHS 

Psample  x0N2 O  VHS

(1)

RT

where N2OHS is the amount of N2O in exetainer headspace (mol), Psample is the pressure of the headspace (atm), x0 N2O is the molar volume of N2O in the headspace (L N2O-N L1 air), VHS is the volume of headspace (L), R is the ideal gas constant (0.08214 L atm mol1 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  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 coefficients (B, d) that account for the imperfect conditions at gas-water interface (Hayden and O’Connell 1975). N2 ODIS  x0 F VH2 O

(2) 

F K0 (PpH2 O)exp P

  B  2d RT

(3)

B 905:954:1685T-0:0052734T2

(4)

d 65:00:1338T

(5)

where N2ODIS is the amount of N2O dissolved (mol N2O), F is function F (mol air L1), VH2O is the volume of water in exetainer (L), x0 is the molar volume of N2O in dry air (mol N2O mol1 air), K0 is the equilibrium

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constant (mol L1 atm1), 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 mol1), 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 (0:0000299T 2 )(0:00214T)0:0591   100 4:8489 In pH2 O 24:454367:4509 T   T ln 100 (B  2d) 9:4563  0:047596:427105 T RT T

(6)

(7) (8)

Step 3  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 L1) by dividing the moles of N2O by the volume of the sample. N2 OTOT 

N2 ODIS  N2 OHS VH2 O

(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 introducing 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 L1 to determine whether microbial consumption 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 (t0) 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 (P 5 0.001). The rate of decrease was significantly (P 50.01) 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% d1 (k0.02 d1) as compared with 7% d 1 (k0.07 d1) 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 L1 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 L1 (16% loss), while the concentration in the bulk water had dropped to an average of 0.06390.004 mmol N2O L1 (45% loss). There was significantly (P 0.001) 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 minimizing 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 consumption was not part of the decrease. Further examination 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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ROPER ET AL. * NITROUS OXIDE IN TILE DRAINAGE WATER

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Fig. 2. Change in dissolved N2O concentration (mmol N2O L1 water) over an 8-d time period at 228C as influenced by storage location (bulk water in bottle, contained in pipette) and addition of a biosatic agent (HgCl2) to the bulk water. Note that the water in equilibrium with the atmosphere (0.315 mL N2O L 1) at 228C contains 0.047 mmol N2O L 1 water (Weiss and Price 1980). Equations present the exponential fit to the data for each treatment. Error bars are 91 standard deviation.

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 documented loss of N2O from agricultural ecosystems. Bowden, W. B. and Bormann, F. H. 1986. Transport and loss of nitrous oxide in soil-water after forest clear-cutting. Science 233: 867869. Burton, D. L. and Beauchamp, E. G. 1994. Profile nitrous oxide and carbon dioxide concentrations in a soil subject to freezing. Soil Biol. Biochem. 58: 115122.

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