BOREAS TF-11 CO2 and CH4 Flux data from the SSA-Fen Summary: The BOREAS TF-11 team collected several data sets in their efforts to fully describe the flux and site characteristics at the SSA-Fen site. This data set contains fluxes of methane and carbon dioxide at the SSA fen site measured using static chambers. The measurements were conducted as part of a 2x2 factorial experiment in which we added carbon (300 g m-2 as wheat straw) and nitrogen (6 g m-2 as urea) to four replicate locations in the vicinity of the TF-11 tower. In addition to siting and treatment variables, it reports air temperature and water table height relative to the average peat surface during each measurement. The data set covers the period from the first week of June 1994 through the second week of September, 1994. The data are stored in tabular ASCII files. Table of Contents * 1 Data Set Overview * 2 Investigator(s) * 3 Theory of Measurements * 4 Equipment * 5 Data Acquisition Methods * 6 Observations * 7 Data Description * 8 Data Organization * 9 Data Manipulations * 10 Errors * 11 Notes * 12 Application of the Data Set * 13 Future Modifications and Plans * 14 Software * 15 Data Access * 16 Output Products and Availability * 17 References * 18 Glossary of Terms * 19 List of Acronyms * 20 Document Information 1. Data Set Overview 1.1 Data Set Identification BOREAS TF-11 CO2 and CH4 Flux data from the SSA-Fen 1.2 Data Set Introduction This data set contains fluxes of methane and carbon dioxide at the SSA fen site measured using static chambers. The measurements were conducted as part of a 2x2 factorial experiment in which we added carbon (300 g m-2 as wheat straw) and nitrogen (6 g m-2 as urea) to four replicate locations in the vicinity of the TF-11 tower. In addition to siting and treatment variables, it reports air temperature and water table height relative to the average peat surface during each measurement. The data set covers the period from the first week of June 1994 through the second week of September, 1994. 1.3 Objective/Purpose Much of the area within the boreal forest biome is comprised of wetlands, in which large carbon stores and high water tables drive fundamentally different atmospheric interactions than occur under the other forest types studied by BOREAS. One key difference is in the form carbon is emitted following soil microbial respiration--in wetlands, much of it is emitted as methane. Wetlands are the dominant influence of boreal forests on atmospheric methane. This study was undertaken in order to assess responses of methane emissions in northern wetlands to potential changes in plant productivity, nitrogen availability or both. Whiting and Chanton (1993) recently observed that methane emissions from wetlands across the globe are well related to net primary productivity. This may be for a variety of reasons, including enhanced plant transport, increased methanogenic substrates from root exudates, increased litter input cascading to enhanced substrate availability for methanogenesis, or enhanced C and N mineralization of decomposing residues. Previous work by others and us (Valentine et al. 1994) has shown that substrate availability is a key constraint on methane production in wetlands. The present study was an effort to test whether substrate manipulation results from laboratory studies could be mirrored under field conditions. 1.4 Summary of Parameters The primary focus is on the net fluxes of methane and carbon dioxide measured using 30-minute static chamber enclosures. These were measured at weekly intervals at four replicate platform locations, four treatment levels (control, +C, +N, and +C+N), and two microtopographic positions (hummock & hollow). A total of 32 chamber locations contributed to the set. The data set also includes the height of the water table above the average peat surface, defined as the average height of 16 points on a 5 cm grid within a 30 cm * 30 cm chamber collar. The grid was measured once during the growing season at each location and referenced to a bogwell at each of the four replicate locations; and the water table level in the bogwell was measured at weekly intervals coincident with the flux measurements. 1.5 Discussion These data were collected from a set of small locations within the fen, and therefore no one location represented the entire study site. In fact, the fen in which this work was conducted was characterized by a large-scale gradient of vegetation, microtopography, and hydrology such that the study site itself is only representative of the portion of the fen in which it was located (i.e. the lower 1/3). These data were collected at the same site and over the same time period as Shashi Verma and his team measured methane and carbon dioxide fluxes using eddy correlation. Measurements made using micrometeorological and chamber techniques comprised the two major components of the TF-ll effort. 1.6 Related Data Sets BOREAS TF-11 Biomass Data over the SSA-Fen BOREAS TF-11 CO2 and CH4 Concentration data from the SSA-Fen BOREAS TGB-01/TGB-03 CH4 Chamber flux data over the NSA Fen BOREAS TGB-01/TGB-03 Water Table and Peat Temperature Data over the NSA BOREAS TGB-03 Plant Species Composition Data over the NSA Fen BOREAS TGB-01/TGB-03 NEE Data over the NSA Fen BOREAS TGB-03 CH4 and CO2 Chamber Flux Data over NSA Upland Sites 2. Investigator(s) 2.1 Investigator(s) Name and Title David Valentine Research Associate (through 16 Aug 1996) Assistant Professor (since 01 Sep 1996) Affiliations: Until 16 Aug 1996: Natural Resource Ecology Laboratory Colorado State University Fort Collins, CO Since 01 Sep 1996: Department of Forest Sciences University of Alaska Fairbanks, AK 2.2 Title of Investigation Influence of substrate characteristics and other environmental factors on methane emissions from the BOREAS Southern Study Area fen site. I. Chamber flux data. 2.3 Contact Information Contact 1 --------- David Valentine Department of Forest Sciences University of Alaska Fairbanks, AK (907) 474-7614 (907) 474-6184 (fax) Ffdwv@aurora.alaska.edu Contact 2 --------- Sara Conrad Raytheon STX Corporation NASA/GSFC Greenbelt, MD (301) 286-2624 (301) 286-0239 (fax) Sara.Conrad@gsfc.nasa.gov 3. Theory of Measurements Methane flux was measured using a static chamber technique in which the headspace CH4 concentration is measured at 5 minute intervals over 30 minutes. The slope of concentration change over time is calculated using least-squares regression, and then translated into flux by multiplying by chamber headspace volume and dividing by surface area covered (Klinger et al. 1994). Carbon dioxide flux is measured similarly to methane flux. During the 30 minute enclosure, however, the rate of carbon dioxide uptake decreases. The slope is therefore calculated by fitting the data with a saturating exponential equation of the form Y=a-b(exp(cT)), where Y and T represent CO2 concentration and time, respectively, and a, b, and c are fit parameters. The slope is then calculated as the derivative of the above at time 0, i.e. b*c. As with methane flux, the slope is then translated into flux by multiplying by chamber headspace volume and dividing by surface area covered. The location of the water table relative to the peat surface was measured within a perforated PVC "bog well" using a measuring tape. The height of the well relative to all the other instruments at each platform was measured and checked at the beginning and end of the season. The average peat surface height in each chamber location was measured relative to the bogwell at the end of the growing season. We assumed that the water table was level across all sites, and calculated the water table height at each chamber at each date as (PSo-BWo)- (BWt-BWo), where PS and BW refer to peat surface and water table height relative to the top of the bogwell, respectively, and the subscripts o and t refer to reference and other dates, respectively. 4. Equipment: 4.1 Sensor/Instrument Description 4.1.1 Collection Environment Data were collected near mid-day (+/- 2 h) at weekly intervals for each platform. Flooding at the site at the end of July 1994 prevented data collection for one week. 4.1.2 Source/Platform All chambers sites were fitted with stainless steel chamber collars cut into the surface 10 cm of peat at the beginning of the season. All collars remained in place during the entire summer. 4.1.3 Source/Platform Mission Objectives The purpose of the collars was to provide a consistent seating place to place the chambers. 4.1.4 Key Variables Methane flux (positive = flux to atmosphere) Carbon dioxide flux Water table height above peat surface 4.1.5 Principles of Operation Fluxes of CO2 and CH4 were measured using static chambers. This technique entails measurement of headspace concentration change of CO2 or CH4 over a defined period (30 minutes) using a gas chromatograph. The slope of the change is multiplied by the ratio of chamber volume to covered area to obtain flux data. 4.1.6 Sensor/Instrument Measurement Geometry Not applicable. 4.1.7 Manufacturer of Sensor/Instrument The gas flux chambers were built by investigator, and were made of transparent FEP teflon attached to outside of welded aluminum frame using double-sided tape. Chamber is attached to a permanently installed stainless steel collar at the time of sampling using a series of spring clamps. The gas chromatograph was a Shimadzu GC-8A gas chromatograph equipped with 1 mL sample injection loop, flame ionization detector, and methanizer, manufactured by Shimadzu Scientific Instruments, Inc. 7102 Riverwood Drive Columbia, Maryland 21046 USA Phone: (410) 381-1227 Fax: (410) 381-1222 Toll Free: (800) 477-1227 The Electronic thermometer used for air temperature measurements was an Omega microprocessor thermometer model HH21, manufactured by OMEGA Engineering, Inc. One Omega Drive Stamford, CT 06907-0047 P.O. Box 4047 (800) 826-6342 (203) 359-1660 Fax: (203) 359-7700 4.2 Calibration Flux measurement attempts in which the R^2 for concentration rate change with time dropped below 0.97 were rejected. EXCEPTION: If deletion of a single data point (outlier) raised the R^2 above 0.97, then the measurement based on the six remaining points was kept. The gas chromatograph column oven was operated at 708C, FID temperature was 1508C, and the N2 carrier gas flowed at 35 mL per minute. 4.2.1.1 Tolerance None given. 4.2.2 Frequency of Calibration The gas chromatograph was calibrated at the start of each day using one of two calibration standards, depending on anticipated concentration ranges. Headspace samples from equilibrating porewater gas profile samples had high concentrations of both methane and carbon dioxide, so we used a standard containing 10,000 ppmv (analysisą2%) of both these gases (Scotty IV Can mix 216, obtained from Scott Specialty Gases, Longmont, CO 80501, tel. 303/442-4700). For flux measurements entailing much lower concentrations of methane and carbon dioxide, we diluted the above-referenced Scotty standard 10:1 with ambient air. This was done by loading a stopcock-fitted, 60 mL polypropylene syringe with excess standard, expelling all but 6 mL, then immediately pulling in outside ambient air to make up 60 mL. Calculation of the diluted standard accounted for average concentrations of methane and carbon dioxide in ambient air. Analyses of diluted standards yielded reproducibility across dilutions of better than 3%. All standards were run on the GC until reproducibility was better than 1% over the course of three standard injections from a single syringe. Calibration was rechecked initially every 10 samples, but detector stability was so high that calibration was checked only at the end of the day for most of the season. End of day calibration checks were always within 5% of the starting calibration. 4.2.3 Other Calibration Information None given. 5. Data Acquisition Methods Fluxes of CH4 and CO2 were assessed weekly at each plot using a static chamber fabricated from transparent FEP film supported by an aluminum frame (0.3m*0.3m*0.4m tall). A foam gasket attached to the bottom of each chamber acted to seal the headspace as the chamber was secured onto its collar using four steel spring clamps. Headspace samples (20 mL) were taken with a 35 mL polypropylene syringe fitted with a nylon stopcock at enclosure (time 0) and every five minutes for thirty minutes, yielding a total of seven headspace samples. Headspace temperatures were taken at the beginning and end of each enclosure period using a shaded thermometer. We analyzed the gas samples within six hours on a Shimadzu GC-8 gas chromatograph equipped with a 1 mL sample loop, a methanizer, and a flame ionization detector. We used N2 as the carrier gas at a 35 mL/min flow rate through a Porapak Q column in a 70 8C oven, and a detector temperature of 180 8C. 6. Observations 6.1 Data Notes Chamber flux and other data are sparse or missing for the end of July 1994 because heavy rains raised the water table above the tops of all chamber collars and of most platforms. 6.2 Field Notes None given. 7. Data Description 7.1 Spatial Characteristics 7.1.1 Spatial Coverage All measurements were made along two transects identified by their location relative to the TF-11 micrometeorology tower: a north transect (NA and NB platforms) and a south transect (SA and SB platforms). All measurements were made within 70 m of the TF-ll tower whose North American Datum of 1983 (NAD83) coordinates are 53.80206°N, 104.61798°W. 7.1.2 Spatial Coverage Map Not available. 7.1.3 Spatial Resolution These are point measurements made at the given locations. 7.1.4 Projection Not applicable. 7.1.5 Grid Description Not applicable. 7.2 Temporal Characteristics 7.2.1 Temporal Coverage Data were collected from 08-June-1994 until 15-Sep-1994. 7.2.2 Temporal Coverage Map Not available. 7.2.3 Temporal Resolution Methane and carbon dioxide flux data are optimally collected at sub-daily time intervals. 7.3 Data Characteristics Data characteristics are defined in the companion data definition file (tf11flux.def). 7.4 Sample Data Record Sample data format shown in the companion data definition file (tf11flux.def). 8. Data Organization 8.1 Data Granularity All of the CO2 and CH4 Flux data from the SSA-Fen are contained in one dataset. 8.2 Data Format(s) The data files contain numerical and character fields of varying length separated by commas. The character fields are enclosed with single apostrophe marks. There are no spaces between the fields. Sample data records are shown in the companion data definition file (tf11flux.def). 9. Data Manipulations 9.1 Formulae 9.1.1 Derivation Techniques and Algorithms Flux (nmol m^-2 s^-1) = [(dC/dt * V)/A]*[P/(RT)] * CONVERT, where dC/dt is the slope of gas concentration over time (ppmv/min), V is the chamber volume corrected for water table fluctuations (~45 L), A is the area covered by the chamber (.09 m^2), P is local atmospheric pressure (97.1 kPa), R is the universal gas constant (8.31441 m^3Pa/molK), T is the average chamber headspace temperature ( K), and CONVERT [(1000 nmol umol^-1)/(60 s min^-1)] gives desired flux unit 9.2 Data Processing Sequence 9.2.1 Processing Steps None given 9.2.2 Processing Changes None given. 9.3 Calculations 9.3.1 Special Corrections/Adjustments None given. 9.3.2 Calculated Variables Fluxes of methane and carbon dioxide are calculated using the formula given in section 9.1.1 9.4 Graphs and Plots 10. Errors 10.1 Sources of Error Atmospheric pressure was assumed constant during entire growing season, so flux calculations contain errors equivalent to daily barometric fluctuations. Leaks under and through the chamber, physical disturbance of the peat associated with the measurement, and temperature artifacts associated with heating under the chamber also may have contributed unknown error to the measurements. Finally, all the flux measurements entailed no mechanical automation and involved substantial and intensive operator involvement. As such, each step in the data collection process was subject to human error. All such error identified has been corrected, but not all such error may have been identified. Reliability of the carbon dioxide fluxes is compromised by two factors: First, the time of day (hence irradiance and photosynthesis) of flux measurement varied. Second, the duration of chamber enclosure during flux measurements likely influenced photosynthetic rates because of changes in CO2 mixing ratios, humidity, and temperature. Fluxes obtained when time of day is substantially different should be dropped from any time series analysis. Chamber artifacts likely obviate use of the data as absolute estimates of CO2 exchange, although estimates relative to each other and over time are probably not compromised. 10.2 Quality Assessment 10.2.1 Data Validation by Source The methane flux data were compared to the mid-day eddy correlation data collected by Verma et al. A paired t-test comparing the datasets summarized at a weekly time scale did not find a significant (p<0.1) difference between them. 10.2.2 Confidence Level/Accuracy Judgement The exclusion of the few methane concentration-over-time data yielding R^2 < 0.95 implies that the data presented were not strongly influenced by bubbling events during chamber enclosure periods. While some methane emission may occur through ebullition, the influence of the artificial disturbance associated with the measurement cannot be dismissed or easily quantified. Based on my observations of bubbling at this and other fens, exclusion of fluxes based on enclosure periods with strongly non-linear changes in concentration over time presented the lesser risk of introducing errors. For the reasons presented in section 10.1, the carbon dioxide flux data presented here should be interpretted carefully. While comparisons within the dataset across the study area and through time are probably valid, comparisons with other datasets produced using humidity & temperature controlled cuvettes will likely show discrepancies. 10.2.3 Measurement Error for Parameters All flux enclosures yielding low (<0.95) R^2 for the headspace CH4 concentration against time regression were omitted from the dataset. 10.2.4 Additional Quality Assessments None given. 10.2.5 Data Verification by Data Center Data were examined for general consistency and clarity. 11. Notes 11.1 Limitations of the Data None given. 11.2 Known Problems with the Data See section 10.1 11.3 Usage Guidance See section 10.1 11.4 Other Relevant Information None given. 12. Application of the Data Set Several avenues are being pursued in publications now being produced to answer the following questions: 1. How do CH4 flux measurements compare by technique used in measurement? 2. How and why do CH4 flux measurements vary through time and across the landscape? 3. Does plant productivity limit CH4 emissions? 13. Future Modifications and Plans None given. 14. Software We used only commercially available software, mostly Quattro Pro spreadsheet and the Statistical Analysis System (SAS) 14.2 Software Access Not applicable. 15. Data Access 15.1 Contact Information Ms. Beth Nelson BOREAS Data Manager NASA GSFC Greenbelt, MD (301) 286-4005 (301) 286-0239 (fax) Elizabeth.Nelson@gsfc.nasa.gov 15.2 Data Center Identification See Section 15.1. 15.3 Procedures for Obtaining Data Users may place requests by telephone, electronic mail, or fax. 15.4 Data Center Status/Plans These data are available from the Earth Observing System Data and Information System (EOSDIS) Oak Ridge National Laboratory (ORNL) Distributed Active Archive Center (DAAC). The BOREAS contact at ORNL is: ORNL DAAC User Services Oak Ridge National Laboratory (865) 241-3952 ornldaac@ornl.gov ornl@eos.nasa.gov 16. Output Products and Availability 16.1 Tape Products None. 16.2 Film Products None. 16.3 Other Products Comma delimited ASCII text files. 17. References 17.1 Platform/Sensor/Instrument/Data Processing Documentation None given. 17.2 Journal Articles and Study Reports Klinger, L.F., P.R. Zimmerman, J.P. Greenberg, L.E. Heidt, and A.B. Guenther. 1994. Carbon trace gas fluxes along a successional gradient in the Hudson Bay lowland. J. Geophys. Res. 99:1469-1494. Sellers, P. and F. Hall. 1994. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1994-3.0, NASA BOREAS Report (EXPLAN 94). Sellers, P., F. Hall, H. Margolis, B. Kelly, D. Baldocchi, G. den Hartog, J. Cihlar, M.G. Ryan, B. Goodison, P. Crill, K.J. Ranson, D. Lettenmaier, and D.E. Wickland. 1995. The boreal ecosystem-atmosphere study (BOREAS): an overview and early results from the 1994 field year. Bulletin of the American Meteorological Society. 76(9):1549-1577. Sellers, P., F. Hall, and K.F. Huemmrich. 1996. Boreal Ecosystem-Atmosphere Study: 1994 Operations. NASA BOREAS Report (OPS DOC 94). Sellers, P. and F. Hall. 1996. Boreal Ecosystem-Atmosphere Study: Experiment Plan. Version 1996-2.0, NASA BOREAS Report (EXPLAN 96). Sellers, P., F. Hall, and K.F. Huemmrich. 1997. Boreal Ecosystem-Atmosphere Study: 1996 Operations. NASA BOREAS Report (OPS DOC 96). Sellers, P. J., F. G. Hall, R. D. Kelly, A. Black, D. Baldocchi, J. Berry, M. Ryan, K. J. Ranson, P. M. Crill, D. P. Lettenmaier, H. Margolis, J. Cihlar, J. Newcomer, D. Fitzjarrald, P. G. Jarvis, S. T. Gower, D. Halliwell, D. Williams, B. Goodison, D. E. Wickland, and F. E. Guertin. 1997. BOREAS in 1997: Experiment Overview, Scientific Results and Future Directions. Journal of Geophysical Research 102 (D24): 28,731-28,770. Shurpali NJ, Verma SB, Clement RJ, Billesbach DP. 1993. Seasonal Distribution of Methane Flux in a Minesota Peatland Measured by Eddy Correlation. Journal of Geophysical Research 98(D11):20,649-20,655. Valentine DW, Holland EA, Schimel DS. 1994. Ecosystem and physiological controls over methane production in northern wetlands. Journal of Geophysical Research 99(D1):1563-71. Whiting GJ, Chanton JP. 1993. Primary production control of methane emission from wetlands. Nature 364:794-5. 17.3 Archive/DBMS Usage Documentation None. 18. Glossary of Terms None. 19. List of Acronyms BOREAS - BOReal Ecosystem-Atmosphere Study BORIS - BOREAS Information System DAAC - Distributed Active Archive Center EOS - Earth Observing System EOSDIS - EOS Data and Information System GSFC - Goddard Space Flight Center NASA - National Aeronautics and Space Administration ORNL - Oak Ridge National Laboratory URL - Uniform Resource Locator SSA - BOREAS Southern Study Area 20. Document Information 20.1 Document Revision Date Written: 29-Jan-1997 Last Updated: 06-Oct-1998 20.2 Document Review Date(s) BORIS Review: 06-Oct-1998 Science Review: 20.3 Document ID Valentine, D.W. 1996. Influence of substrate characteristics and other environmental factors on methane emissions from the BOREAS Southern Study Area fen site. I. Static chamber flux data. 20.5 Document Curator 20.6 Document URL Methane flux static chamber wetland TF11_CH4_CO2_Flux.doc 10/09/98