Verifying Three Types of Methane Fluxes from Soils by Testing the

Sep 7, 2006 - The TDL-PA analyzer detects methane at 1650.957 nm [R (5) line of the ... measurement is not available as a portable system. Additionall...
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Environ. Sci. Technol. 2006, 40, 6425-6431

Verifying Three Types of Methane Fluxes from Soils by Testing the Performance of a Novel Mobile Photoacoustic Method versus a Well-Established Gas Chromatographic One H E R M A N N F . J U N G K U N S T , * ,† RAIMUND SAUTER,‡ ANDREAS LINK,‡ SABINE FIEDLER,§ KARL STAHR,§ AND ULRICH HAAS‡ Plant Ecology, Albrecht-von-Haller-Institute for Plant Sciences, University of Go¨ttingen, Untere Karspu ¨ le 2, D-37073 Go¨ttingen, Germany, Institute of Physics and Meteorology, University of Hohenheim, Garbenstrasse 30, D-70599 Stuttgart, Germany, and Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Strasse 27, D-70599 Stuttgart, Germany

The performance of a novel portable, tunable diode laser, resonant photoacoustic (TDL-PA) analyzer developed for field measurements of CH4 was compared to a commonly applied offline gas chromatographic (GC) method. This comparative study was realized under normal field conditions parallel to long-term weekly GC monitoring of four different soil types with very different methane budgets. The method used for gas-exchange measurements was the well-known closed-chamber technique. The TDL-PA analyzer detects methane at 1650.957 nm [R (5) line of the 2v3 band], guaranteeing high precision without the need for correction procedures. The two techniques correlated well (R2 ) 0.988) over the entire concentration range (0.1533 ppmv CH4) tested at highly varying flux rates between -30 and -12 ppbv CH4 min-1 for uptakes and between 2.5 and 362 ppbv CH4 min-1 for emissions. The two analyzers proved to be interchangeable, leaving the online advantages to the TDL-PA. A suitable CH4 online GC solution for chamber measurement is not available as a portable system. Additionally, the data sampling rate of 2 Hz enables a direct coupling to other infrared gas analyzers with the high time resolution commonly required to determine plant CO2 assimilation rates or soil respiration rates.

1. Introduction Methane (CH4) is the second most important greenhouse gas following carbon dioxide (CO2). As a result of human activities, its average abundance in the atmosphere has doubled since the beginning of the 20th century and is still increasing (1). The global warming potential of one molecule * Corresponding author phone: +49 551 3912178; fax: +49 551 395701; e-mail: [email protected]. † University of Go ¨ ttingen. ‡ Institute of Physics and Meteorology, University of Hohenheim. § Institute of Soil Science and Land Evaluation, University of Hohenheim. 10.1021/es060843b CCC: $33.50 Published on Web 09/07/2006

 2006 American Chemical Society

of methane is about 23 times that of a CO2 molecule (1). Consequently, with respect to the United Nations Framework Convention on Climate Change (2), nations must reduce their net methane emissions to meet the commitments made by signing the Kyoto Protocol. To reduce emissions, the CH4 cycle must be determined in detail. Soils may be major sources as well as sinks for atmospheric CH4, and individual soil types may change CH4 flux directions (3). In order to understand the complexity of CH4 dynamics in ecosystems, measurements providing high-precision data in the field are needed. High sampling rates are required to correctly account for the high temporal variability of CH4 fluxes from soils. At the same time, highly mobile devices are required to meet the needs resulting from the high spatial variability of CH4 fluxes from soils. Furthermore, the equipment must be robust enough to allow use in remote CH4 hotspot areas. In particular, our fragmentary knowledge of tropical wetlands biogeochemistry, recently identified as a major source of uncertainty in understanding the global CH4 cycle, has to be rounded out (5). Open and closed chambers (6), diffusion gradient methods (7), and micrometeorological techniques (8) are utilized for measurements of CH4 fluxes. For long-term stationary monitoring online chamber measurements are reported mainly applying GC, whereas TDL-PA could be used for automatic chamber measurements as well. For micrometeorological techniques highly complex TDL devices are already used in most cases. These stationary methodologies detect the high temporal variability of methane fluxes from ecosystems very well. However, they are not very useful for studies on spatial variability due to restrictions imposed by their size, weight, and power supply. Hence, up-to-date studies on the spatial variability of CH4 fluxes from ecosystems are more or less restricted to offline GC chamber measurements. The principle of this technique is rather simple and therefore successful: a closed volume is placed above the soil-plant continuum, and flux rates can be easily calculated as a function of concentration changes. For single flux calculations, headspace samples are taken at predefined time intervals. However, the main restriction of this method is that the assumed flux linearity between predefined time intervals does not necessarily exist. From highly dynamic fluxes, that is, net ecosystem exchange rates of CO2, it is known that saturation effects in closed chambers may occur within a few minutes, flux directions (uptake or emission) may change even more rapidly, and plants may react immediately to the changed environmental condition that are inherent to closed chambers. The adaptation of net ecosystem exchange rates of CH4 to these chamber effects is highly uncertain. Just recently, living plants were proposed as a large direct CH4 source (9). A mobile methane detector measuring continuously at high precision over a large range (e.g., from -30 to more than 300 ppbv of CH4 min-1) is therefore required in environmental science to test for flux linearity. Further restrictions on offline use are as follows: (i) the risk of leakages or other problems during the transportation from the field to the lab, and (ii) inevitable temperature and pressure correction procedures due to differences between field and laboratory conditions. Additionally, with a continuously measuring mobile online device it is possible to react instantly to special occurrences in the field. The most promising way to develop a portable device for high-precision applications appears to be infrared (IR) absorption spectroscopy, because highly precise gas chromatography (GC) is restricted to a specific volume due to VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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column separation and the need for a gas supply (H2 and O2). Examples of available optical spectrometric systems are nondispersive infrared (NDIR) or photoacoustic (PA) analyzers using broadband emitting light sources in the middle IR (MIR) spectrum, and systems with laser sources such as gas lasers (10, 11), optical parametric oscillators (OPO) (12), or diode lasers (13-15). Even an isotopic ratio measurement of methane in ambient air has been published, demonstrating the high precision possible with IR spectroscopy (16). However, the mobile field applicability of these methods is restricted, either due to technical constraints, such as large power supplies and dimensions and the need for liquid nitrogen cooling, or due to data correction algorithms resulting from cross-interference with other trace gases (e.g., CO2, N2O, and NH3) and water vapor. Still reports on some compact spectroscopic devices have been published and present very promising results of long-term measurements of laboratory air (17-21) and of a dry synthetic gas mixture of different methane concentrations (22-24). The only fieldtested portable devices that we found in print have detection limits above 1 ppmv (25, 26), which does not satisfy the needs of most studies on environmental methane cycling. A recently published review on analytical detection methodologies for methane concludes that adequate selectivity is one of the more pressing needs since the majority of the tested techniques exhibit a response that is not restricted to methane (27). It is pointed out that GC methods are very reliable, but the need to develop small sensing systems effectively rules out the incorporation of column separation on the grounds of manufacturing practicalities. The advantages of spectroscopy were not elucidated in that review. However, diode lasers emitting in the near-infrared spectral range (NIR), in particular, are suitable for the construction of mobile, inexpensive, and compact TDL-PA analyzers. Additionally, tunable diode lasers have the advantage of selective trace gas excitation, an attribute that has been highly demanded (27). The tested TDL-PA analyzer was designed for real-time detection of CH4 within the concentration range 0.02-100 ppmv (20, 21). The analyzer operates at ambient temperatures in near-infrared without cross-sensitivity to other gases (28). To our knowledge, such a device has not been tested under field conditions before. The objectives of this study were (1) to test the performance of this novel, mobile TDL-PA analyzer under field conditions for four different soil types exhibiting three CH4 flux categories (i.e., emission, dynamic equilibrium, and uptake), to compare the data to that concurrently obtained by offline GC, and (2) to identify nonlinear changes in CH4 concentrations in the headspace of the closed chamber.

2. Experimental Procedures 2.1. Site Description. The site Wildmooswald (47° 57′ N, 8° 07′ E) selected for this study is located in the cool humid central Black Forest (Germany) (MAT 6 °C, MAP 1600 mm) at 1090-1099 m asl (3). The site (north-facing upslope, average slope 5%) is covered by a 100-year-old Norway spruce forest [Picea abies (L.) Karst]. Soil types are highly diverse, including well-aerated soils (Endoskeletic and Chromic Cambisol), hydromorphic mineral soils (Humic Gleysol, Histic Gleysol), and organic soils (Fibric Histosol, Sapric Histosol) (29). 2.2. Soils and Associated Flux Types. The different soils of the study site Wildmooswald revealed different CH4 flux types under identical environmental conditions (i.e., the same climatic conditions, vegetation, parent material, and land use history). Methane emissions were monitored from 2000 to 2003 by offline gas chromatographic methodology (3). The methane flux types were classified according to their multiannual flux behavior: (i) continuous net methane 6426

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source, (ii) continuous net methane sink, and (iii) frequently altering methane flux direction. For each type of CH4 flux, one soil was selected. High and low flux rates have been determined for reasons of comparison. The following sites were selected: 2.2.1. Soils as CH4 Sources. Fibric Histosol continuously and persistently emits CH4 (0.1-31.2 mg of C m-2‚h-1). Emission rates were very high compared to the uptake rates. Therefore, nonlinearly increasing concentrations were expected in the chamber headspace due to rapidly declining concentration gradients between soil and the atmosphere in the chamber. The performance of GC and TDL-PA spectroscopy at high CH4 concentrations, significantly above ambient concentrations, could be tested and compared. 2.2.2. Soils as CH4 Sinks. Endoskeletic Cambisol, the most dominant soil type at the study site, was characterized by low CH4 uptake rates (-10 to -64 µg of C m-2‚h-1), representative of temperate European forests (30). In contrast, Chromic Cambisol was characterized by high uptake rates (-11 to -112 µg of C m-2‚h-1). Both soils were selected to evaluate the PA system at low ( 0.95 was set as the limit criterion for accepting the assumption of a linear flux. VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 2. Schematic diagram of PA equipment. (a) End face of the PA cylinder. (b) PA cylinder. The rectangular areas symbolize the nodal planes of pressure of the cell’s second azimuth resonance. (c) Schematic drawing of the complete measuring setup in the field.

TABLE 1. Calibration Values for CH4 Concentrations with Standard Deviation CH4 concn (ppmv) std dev (ppmv)

0.28 0.008

0.51 0.02

1.04 0.03

2.26 0.07

4.68 0.14

10.0 0.3

22.9 0.7

47.8 1.4

102 3.1

TABLE 2. Methane Flux Gradients from Temperate Forest Soilsa soil type Chromic Cambisol Fibric Histosol Endoskeletic Cambisol Histic Gleysol a

calcd intervals (min)

gradient GC (ppbv min-1)

gradient TDL-PA (ppbv min-1)

R 2 of linear regression TDL-PA

30 60 90 30 60 90 30 60 40 60

-22.11 -18.25 -15.63 341.17 345.48 336.20 -23.72 -7.29 8.00 -3.93

-29.67 -23.25 -18.29 361.65 338.44 311.44 -12.21 -12.11 2.50 2.66

0.980 89 0.979 18 0.954 18 0.997 47 0.997 17 0.9954 0.870 76 0.954 03 0.2965 0.535 99

Derived from simultaneous results of the photoacoustic method and gas chromatrography at different time intervals.

3. Results 3.1. General Performance of TDL-PA versus GC System. Fifty GC samples collected from the chamber headspaces above different soil types at varying times were studied, and the results were compared to data obtained by simultaneous real-time TDL-PA determination of CH4 concentration. Individual results correlated well (R2 ) 0.988; y ) 1.0121x 0.0046) for the entire concentration range (0.15-33 ppmv of CH4). The individual results for both systems were in good agreement for high CH4 emission and uptake rates (Figure 3). For high fluxes, both systems exhibited almost identical results, that is, high uptake rates for Chromic Cambisol (Figure 3a) and high emission rates for Fibric Histosol (Figure 3b). At low flux rates, the GC system reached its limits, and 6428

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the advantages of the very precise and continuous TDL-PA system became apparent. The third GC measurement after 30 min had a lower value than the last measurement after 60 min during the low uptake rates for Endoskeletic Cambisol (Figure 3c). Thus, determination of a flux rate with offline GC is uncertain under these conditions. The uncertainty is even higher at the very low emission rates that occur for Histic Gleysol (Figure 3d). The GC measurements for Histic Gleysol (Figure 3d) indicated alternating flux directions, from emission to uptake, whereas the measurements by the TDL-PA system indicated a steady low emission rate. In general, the gradients of CH4 uptake or release calculated from the linear regressions differ between the two

FIGURE 3. Typical methane exchanges between pedosphere and atmosphere from temperate forest soils exhibiting various magnitudes and flux directions. Simultaneous measurements were performed by the laser photoacoustic method (s) and gas chromatography (4). (a) High uptake at Chromic Cambisol (Sep. 17, 2003); (b) high emission at Fibric Histosol (May 27, 2003); (c) low uptake at Endoskeletic Cambisol (July 29, 2004); and (d) dynamic equilibrium at Histic Gleysol (July 29, 2004). systems and time intervals (30, 60, and 90 min). Differences between different time intervals exceeded those observed for different systems (Table 2). A closing interval of 90 min is acceptable for both high CH4 flux types (R2 > 0.95; Table 2). Due to low flux rates, no saturation effects were observed for either of the low flux types. However, a certain time span is needed to determine flux magnitude and direction of the flux in an adequate manner (this is obvious when 30- and 60-min values for R2 are compared in Table 2). 3.2. High Flux Rates. Methane concentrations for Chromic Cambisol converged to a stable minimum (Figure 3a). On the basis of the TDL-PA data, a first-order exponential function could be fitted, indicating an asymptotic limit for CH4 concentration (102 ( 20 ppbv). For Fibric Histosol, a deviation from a linear increase was observed after 80 min as compared to TDL-PA data (Figure 3b). However, the measuring time was too short to enable determination during maximum saturating concentrations. The correlation coefficients were compared; the best correlation for this type of chamber-soil type combination was found for measuring intervals of 40-50 min (R2 ) 0.981 for Chromic Cambisol and R2 ) 0.999 for Fibric Histosol). 3.3. Low Flux Rates. In certain cases, offline GC data hamper the calculation of magnitude and direction of fluxes. Such cases occurred frequently in the course of the 2.5-yearlong field study and represent typical difficulties encountered in offline field GC measurements (Figure 3c,d). TDL-PA data showed clear and statistically significant results for the uncertain cases. An explicit uptake rate was calculated for Endoskeletic Cambisol (Figure 3c), whereas linear flux (R2 > 0.95) was only observed for the entire 60 min of the closing interval (Table 2). This criterion was never met for Histic Gleysol, despite the fact that the TDL-PA data indicated a slight and statistically significant increase. In contrast, the

GC values indicated a dynamic equilibrium (Figure 3d). Additional TDL-PA measurements on this soil type showed either a small drop or increases and, therefore, verified the flux type (data not shown). However, the unambiguous alternating flux directions observed in closing intervals during regular field measurements (3) (1-1.5 h) were not detected by the TDL-PA method.

4. Discussion The mobile TDL-PA system was found to be suitable for field applications, and the results obtained are comparable to those of GC measurements (R2 ) 0.988) over the entire range (0.1533 ppmv) typical for CH4 fluxes from ecosystems. The 3% analytical precision achieved is relatively high. Furthermore, the TDL-PA system can be regarded as being more reliable (e.g., at low flux rates; Figure 3c,d) than offline GC. This is mainly attributed to the differences between ambient pressure and temperature as compared to those under the laboratory conditions used for GC measurements. Such problems are not encountered in the TDL-PA system. Furthermore, the on-site measurements are advantageous, because the risk of leaking gas samples being transferred to the GC is avoided. Therefore, it is possible that in the future TDL-PA approaches may be applied more commonly than off-line GC methods on sites, where online GC measurements are not possible. Additionally, the former method has a much higher time resolution than any GC method. The experimentally proven insensitivity of this TDL-PA system to interference by other gases, such as water vapor, CO2, NH3, or N2O, underlines the advantages of the TDL-PA method. No correction algorithms or other methods are needed. The selective excitation of the gas molecules at a trace level by a single-mode diode laser is a unique advantage of the TDLPA system in comparison to the in situ sampling devices VOL. 40, NO. 20, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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using broadband IR-emitting light sources (such as NDIR or optical filter PA systems). These devices show crosssensitivities to water vapor (or to other trace gases) when detecting CH4 and hence do not achieve the required accuracy due to the correction procedures required. All three types of methane fluxes have been verified. The “at-the-edge” (alternating emission and absorption) type was confirmed only for changing flux direction between measurements, whereas the observed nonlinear flux indicate a real dynamic equilibrium state even during the comparable short closing interval of 1 h. However, this status is less dynamic than offline GC results indicated. A discontinuous and off-site method lacks flexibility; in addition, failures or specific flux events cannot be detected during the field investigation. At low flux rates, one outlier (1 out of 4) obtained by GC probing may preclude the calculation of direction and magnitude of the flux. Except for the “at-the-edge” type, the nonlinear accumulation or depletion of methane within the chamber headspace was only detected after the regular closing interval (90 min). Certainly, the optimized closing interval is site-, chamber-, and season-specific, but during this study a comparable optimal closing interval of about an hour was determined for all four soils. This was the case for strongly emitting Fibric Histosol (Figure 3b) as well as for the low uptake rates of Endoskeletic Cambisol (Figure 3c), where shorter closing intervals would make the calculation of uptake rates less certain (Table 2). Using a mobile TDL-PA device similar to the one being tested improves the quality of gas flux measurement at the ecosystem level. Furthermore, special phenomena such as the uptake limit of Chromic Cambisol for atmospheric CH4 (102 ( 29 ppbv) could be detected (Figure 3a). The TDL-PA system offers the opportunity for field studies at minimum concentrations or uptake rates at elevated concentrations. In summary, this novel system is one of the most convenient devices for monitoring CH4 flux rates in the field, especially for the low flux rates characteristic of most terrestrial ecosystems. Transporting the rather rough (i.e., large and heavy) test prototype along a forested hill slope is easily feasible for two people. For future applications, both the size and weight of the device can easily be reduced. At the moment its transportation container is a large box and its accessories are made of rather heavy materials. Moreover, it can be coupled to any other “closed-path” gas analyzer as frequently used for mobile CO2 measurements of plant assimilation rates or soil respiration rates. Therefore, it could be used to monitor any high or low flux rates from ecosystems and highly dynamic fluxes such as the direct release of large quantities of CH4 by living plants recently proposed (9)

Acknowledgments This study was supported by the Deutsche Forschungsgemeinschaft (DFG) as part of the research training group Strategies To Reduce the Emissions of Greenhouse Gases and Environmental Toxic Agents from Agriculture and Land Use (http://uni-hohenheim.de/∼wwwgkoll/english.htm). We offer special thanks to the three anonymous reviewers, who truly helped to improve this work.

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Received for review April 7, 2006. Revised manuscript received July 28, 2006. Accepted July 31, 2006. ES060843B

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