Microb Ecol (1996) 31:57-66
MICROBIAL ECOLOGY © 1996Springer-VerlagNewYorkInc.
Spatial and Temporal Variations of Dissolved Gases (CH4, COs, and 02) in Peat Cores J. Benstead*, D. Lloyd Microbiology Group, School of Pure and Applied Biology, University of Wales College of Cardiff, EO. Box 915, Cardiff CF1 3TL, UK Received: November 22, 1994; Revised: March 28, 1995
Abstract.
Spatial and temporal variations in the concentrations of dissolved gases (CH4, CO2, and 02) in peat cores were studied using membrane inlet mass spectrometry (MIMS). Variations in vertical gas profiles were observed between random peat cores taken from hollows on the same peat bog. Methane concentrations in profiles (0-30 cm) generally increased with depth and reached maximum values in the range of 200-450 IxM CH4 below about 13-cm depth. In some profiles, a peak of dissolved methane was observed at 7-cm depth. Oxygen penetrated to approximately 2-cm depth in the hollows. The sampling probe was used to continuously monitor CH4, CO2, and 02 concentrations at fixed depths in peat cores over periods of several days. The concentration of dissolved CO2 and O2 at 1-cm depth oscillated over a 24-h period with the maximum of CO2 concentration corresponding with the minimum of 02. Diurnal variations in CO2 but not CH4 were measured at 15-cm depth; dissolved CO2 levels decreased during daylight hours to a constant minimum concentration of 4.85 raM. This report also describes the application of MIMS for the measurement of gaseous diffusion rates in peat using an inert gas (argon); the value of D, the diffusion coefficient, was 2.07 × 10-* m2 s -1.
Introduction Water-saturated peatlands are largely anaerobic environments in which gaseous products (CO2, CH4) are released during the degradation of readily available organic matter. The production of C H 4 from these natural wetlands is a major source of atmospheric CI-I4 [3, 10, 12, 18]. Global concentrations of methane have increased at a rate of about l%/year since 1978 [2], although recent studies have indicated an apparent slowing down of this global accumulation of methane [26]. Conse-
*Present address: Darling Marine Center, University of Maine, Walpole, ME 04573, USA Correspondence to: D. Lloyd
58
J. Benstead, D. Lloyd
quently, there is considerable interest in identifying the factors that control CH4 emissions in wetland environments. Estimates of CH 4 emissions from peatlands show large seasonal and spatial variability [4, 7]. Temperature, pH, water table position, and nutrient availability are important factors that influence methane emission [6-8, 19-21, 24, 29]. Generally, completely water-saturated peat ecosystems (hollows) emit more methane than ecosystems in which the water table is several centimeters below the vegetation surface (hummocks), and nutrient-rich fens and marshes emit more methane than nutrient-limited bogs [7]. Another factor affecting CH4 emission from wetlands is methane oxidation. Consumption of CH4 by methanotrophic bacteria significantly limits the flux from methanogenic habitats to the atmosphere [9, 22]. During drought conditions, some terrestrial habitats have been shown to consume atmospheric methane [11]. Studies of CH4 oxidation in sediments have demonstrated that the relative extent of oxidation is variable and dependent in part on benthic photosynthesis [13, 14]. In the latter study, 02 concentrations and penetration depths in peat sediments of the Florida Everglades were shown to be higher under illuminated than under dark conditions; the CH4 concentration profile was shown to change in correspondence with changes in the 02 distribution. Changes in the distribution of these gases have also been demonstrated during movements of the water table [ 1]. This report describes the spatial and temporal variations in dissolved gas concentrations in peat cores using membrane inlet mass spectrometry. This technique allows the continuous and simultaneous measurement of several dissolved gases ( i n c l u d i n g H2, CH4, N2, 02, NOx, CO2, and H2S), directly in a sample using a single probe with minimal disturbance [1, 16, 17]. The present study represents the first report of direct measurements of diurnal changes in the concentration of dissolved gases (CH4, CO2, and 02) at fixed depths in peat cores. Such studies are extremely useful for quantifying the role of the terrestrial wetland environment in regulating changes in atmospheric CO2 and CH4 levels. Membrane inlet mass spectrometry also provided a direct method for the assessment of gas diffusion rates through peat. Materials and Methods Sampling Site and Procedures Peat and surface vegetation samples were collected from the Ellergower Moss site (New Galloway, Scotland), a site dominated by Sphagnumspp. Pore water pH (0-30 cm) averaged 3.7. Cores (30-cm diameter, 30- to 40-cm length) were taken as described by Clymo [5], except that a stainless steel corer (30-cm internal diameter [ID], 60-cm length) was used. The diurnal measurements were made on smaller diameter cores (15-cm) collected as follows: The peat was cut using a stainless steel corer (15-cm ID, 41 cm length), the corer was removed, and a PVC sample tube of the same dimensions was lowered into the cut peat. The sample tube containing the peat was removed from the peat bog, and its base sealed. Cores were kept outdoors and subjected to natural variations of temperature and rainfall until required for measurements. All measurements were determined on peat cores within 6 months of collection. Distilled water was added to peat cores during periods of drought to replace moisture loss that was due to evaporation.
Measurement of Dissolved Gas Concentrations in Peat Cores Partial pressures of CH4, CO> and 02 in peat cores were simultaneously measured using a Hal series quadrupole mass spectrometer (Hiden Analytical, Warrington, UK), fitted with a silicon-covered inlet
Variability of Gases in Peat
59
probe (1.56-mm outside diameter [OD], 0.5-mm ID, 70-cm length) as described by Benstead and Lloyd [1]. Peat cores were brought indoors and allowed to equilibrate at 10°C using a temperaturecontrolled refrigerated water bath. The water table in the peat cores taken from hollows was controlled at the vegetation surface [1]. The mass spectrometer probe was carefully inserted to the required depth in the peat as measured by 0.5 cm intervals marked on the probe and was left to equilibrate for about 30 min. For vertical gas profiles, at least 15 measurements were taken at the steady state. For diurnal studies, the mass spectrometer probe was left at a fixed depth over a period of several days, and measurements were recorded every 5 rain. Liquid phase concentrations were calculated from the partial pressures using mole fractions [28]. Repeatability for CH4 was about + 3%.
Spatial and Temporal Variations The spatial variation in gas concentrations (CH4,CO2,and 02) in an individual peat core was examined by comparing vertical gas profiles. Gas profiles with measurements taken at depths of 1, 5, 10, 15, and 20 cm were determined at five different locations within a peat core taken from a hollow. Variations of vertical gas profiles (0- to 30-cm depth) taken from random hollows on the same peat bog were measured during the period April 1992 to March 1993. Depth profiles of dissolved CH4 were also determined at a hummock. Measurements of gas concentrations are expressed as micromolar values; depth in centimeters refers to the distance below the top of the Sphagnum moss. Two different temporal experiments were performed. In the first experiment, gas profiles on the same peat core taken from a hummock were measured on two occasions 5 months apart. In the second experiment, diurnal variations were examined by continuously monitoring dissolved gas concentrations at a fixed point in a peat core taken from a hollow. Diurnal variations were monitored at both 1- and 15-cm depths over a period of several days.
Diffusion of Argon Diffusion of argon from the surface of a completely water-saturated peat sample (hollow) was monitored using membrane inlet mass spectrometry. The headspace above the core was sealed using a plastic lid, and air was replaced with a mobile gas phase of argon (flow rate, 44 cm 3 min
MICROBIAL ECOLOGY © 1996Springer-VerlagNewYorkInc.
Spatial and Temporal Variations of Dissolved Gases (CH4, COs, and 02) in Peat Cores J. Benstead*, D. Lloyd Microbiology Group, School of Pure and Applied Biology, University of Wales College of Cardiff, EO. Box 915, Cardiff CF1 3TL, UK Received: November 22, 1994; Revised: March 28, 1995
Abstract.
Spatial and temporal variations in the concentrations of dissolved gases (CH4, CO2, and 02) in peat cores were studied using membrane inlet mass spectrometry (MIMS). Variations in vertical gas profiles were observed between random peat cores taken from hollows on the same peat bog. Methane concentrations in profiles (0-30 cm) generally increased with depth and reached maximum values in the range of 200-450 IxM CH4 below about 13-cm depth. In some profiles, a peak of dissolved methane was observed at 7-cm depth. Oxygen penetrated to approximately 2-cm depth in the hollows. The sampling probe was used to continuously monitor CH4, CO2, and 02 concentrations at fixed depths in peat cores over periods of several days. The concentration of dissolved CO2 and O2 at 1-cm depth oscillated over a 24-h period with the maximum of CO2 concentration corresponding with the minimum of 02. Diurnal variations in CO2 but not CH4 were measured at 15-cm depth; dissolved CO2 levels decreased during daylight hours to a constant minimum concentration of 4.85 raM. This report also describes the application of MIMS for the measurement of gaseous diffusion rates in peat using an inert gas (argon); the value of D, the diffusion coefficient, was 2.07 × 10-* m2 s -1.
Introduction Water-saturated peatlands are largely anaerobic environments in which gaseous products (CO2, CH4) are released during the degradation of readily available organic matter. The production of C H 4 from these natural wetlands is a major source of atmospheric CI-I4 [3, 10, 12, 18]. Global concentrations of methane have increased at a rate of about l%/year since 1978 [2], although recent studies have indicated an apparent slowing down of this global accumulation of methane [26]. Conse-
*Present address: Darling Marine Center, University of Maine, Walpole, ME 04573, USA Correspondence to: D. Lloyd
58
J. Benstead, D. Lloyd
quently, there is considerable interest in identifying the factors that control CH4 emissions in wetland environments. Estimates of CH 4 emissions from peatlands show large seasonal and spatial variability [4, 7]. Temperature, pH, water table position, and nutrient availability are important factors that influence methane emission [6-8, 19-21, 24, 29]. Generally, completely water-saturated peat ecosystems (hollows) emit more methane than ecosystems in which the water table is several centimeters below the vegetation surface (hummocks), and nutrient-rich fens and marshes emit more methane than nutrient-limited bogs [7]. Another factor affecting CH4 emission from wetlands is methane oxidation. Consumption of CH4 by methanotrophic bacteria significantly limits the flux from methanogenic habitats to the atmosphere [9, 22]. During drought conditions, some terrestrial habitats have been shown to consume atmospheric methane [11]. Studies of CH4 oxidation in sediments have demonstrated that the relative extent of oxidation is variable and dependent in part on benthic photosynthesis [13, 14]. In the latter study, 02 concentrations and penetration depths in peat sediments of the Florida Everglades were shown to be higher under illuminated than under dark conditions; the CH4 concentration profile was shown to change in correspondence with changes in the 02 distribution. Changes in the distribution of these gases have also been demonstrated during movements of the water table [ 1]. This report describes the spatial and temporal variations in dissolved gas concentrations in peat cores using membrane inlet mass spectrometry. This technique allows the continuous and simultaneous measurement of several dissolved gases ( i n c l u d i n g H2, CH4, N2, 02, NOx, CO2, and H2S), directly in a sample using a single probe with minimal disturbance [1, 16, 17]. The present study represents the first report of direct measurements of diurnal changes in the concentration of dissolved gases (CH4, CO2, and 02) at fixed depths in peat cores. Such studies are extremely useful for quantifying the role of the terrestrial wetland environment in regulating changes in atmospheric CO2 and CH4 levels. Membrane inlet mass spectrometry also provided a direct method for the assessment of gas diffusion rates through peat. Materials and Methods Sampling Site and Procedures Peat and surface vegetation samples were collected from the Ellergower Moss site (New Galloway, Scotland), a site dominated by Sphagnumspp. Pore water pH (0-30 cm) averaged 3.7. Cores (30-cm diameter, 30- to 40-cm length) were taken as described by Clymo [5], except that a stainless steel corer (30-cm internal diameter [ID], 60-cm length) was used. The diurnal measurements were made on smaller diameter cores (15-cm) collected as follows: The peat was cut using a stainless steel corer (15-cm ID, 41 cm length), the corer was removed, and a PVC sample tube of the same dimensions was lowered into the cut peat. The sample tube containing the peat was removed from the peat bog, and its base sealed. Cores were kept outdoors and subjected to natural variations of temperature and rainfall until required for measurements. All measurements were determined on peat cores within 6 months of collection. Distilled water was added to peat cores during periods of drought to replace moisture loss that was due to evaporation.
Measurement of Dissolved Gas Concentrations in Peat Cores Partial pressures of CH4, CO> and 02 in peat cores were simultaneously measured using a Hal series quadrupole mass spectrometer (Hiden Analytical, Warrington, UK), fitted with a silicon-covered inlet
Variability of Gases in Peat
59
probe (1.56-mm outside diameter [OD], 0.5-mm ID, 70-cm length) as described by Benstead and Lloyd [1]. Peat cores were brought indoors and allowed to equilibrate at 10°C using a temperaturecontrolled refrigerated water bath. The water table in the peat cores taken from hollows was controlled at the vegetation surface [1]. The mass spectrometer probe was carefully inserted to the required depth in the peat as measured by 0.5 cm intervals marked on the probe and was left to equilibrate for about 30 min. For vertical gas profiles, at least 15 measurements were taken at the steady state. For diurnal studies, the mass spectrometer probe was left at a fixed depth over a period of several days, and measurements were recorded every 5 rain. Liquid phase concentrations were calculated from the partial pressures using mole fractions [28]. Repeatability for CH4 was about + 3%.
Spatial and Temporal Variations The spatial variation in gas concentrations (CH4,CO2,and 02) in an individual peat core was examined by comparing vertical gas profiles. Gas profiles with measurements taken at depths of 1, 5, 10, 15, and 20 cm were determined at five different locations within a peat core taken from a hollow. Variations of vertical gas profiles (0- to 30-cm depth) taken from random hollows on the same peat bog were measured during the period April 1992 to March 1993. Depth profiles of dissolved CH4 were also determined at a hummock. Measurements of gas concentrations are expressed as micromolar values; depth in centimeters refers to the distance below the top of the Sphagnum moss. Two different temporal experiments were performed. In the first experiment, gas profiles on the same peat core taken from a hummock were measured on two occasions 5 months apart. In the second experiment, diurnal variations were examined by continuously monitoring dissolved gas concentrations at a fixed point in a peat core taken from a hollow. Diurnal variations were monitored at both 1- and 15-cm depths over a period of several days.
Diffusion of Argon Diffusion of argon from the surface of a completely water-saturated peat sample (hollow) was monitored using membrane inlet mass spectrometry. The headspace above the core was sealed using a plastic lid, and air was replaced with a mobile gas phase of argon (flow rate, 44 cm 3 min
