1. Samuel T. Dunn , Joseph von Fischer
A pulse-label experiment to determine the biophysical kinetics of different carbon pools for methane emissions in a Carex aquatilis dominated wetland
B31D-0449
Samuel T. Dunn , Joseph von Fischer
1. Graduate Degree Program in Ecology, Colorado State University, Fort Collins, CO, USA. 2. Dept. of Biology, Colorado State University, Fort Collins, CO, USA
200
100
Introduction:
Methane is a potent greenhouse gas that is produced by microbes using organic carbon under anoxic conditions. Work by King and Reeburgh [2002] using radioisotope tracers has shown that carbon fixed via photosynthesis is emitted as methane within 24 hours. Given the importance of plant productivity for methane emissions, it is imperative that the underlying dynamics of this process be understood in order to improve the reliability of modeled methane emissions.
The objective of my research is to determine the dynamics governing carbon flow in the rhizosphere leading to methane emission a) b)
Diagram. 2 (a) Conceptual layout of experimental plots within study site. The distance between plots was 2m on all sides.
Picture. 3 (b)
Preparing to add the 13C-Acetate label via injection into the study plot. Injections were made at 3 points to improve distribution
Figure.1 Keeling plot intercepts (d13C) for methane emitted from sample plots over ~26 hours. Values are normalized to the accepted value of d13C-CH4=-47‰ for the control plot. r2 values for all keeling plot intercepts were greater than 0.9
Picture 1. Samples were collected from a high alpine wetland within the Glacial Lakes Ecosystem Experimental Site (GLEES) near Centennial WY. This particular wetland site is listed at 99% Carex aquatilis
Picture. 4 Chamber used in field measurements of CH4, CO2, and d13C-CH4. Chamber was cooled and mixed using a heat exchanger and battery powered fan.
Diagram. 1 Conceptual schematic of the experimental design. Plant tissue includes both above and belowground biomass, “OM” is soil organic matter exuded from root tissue and includes acetate, and “CH4“ is the dissolved methane pool in the rhizosphere.
Picture 2. Chamber measurements were made using a field portable greenhouse gas analyzer (FGGA)(pictured) and a field portable methane carbon isotope analyzer (MCIA) from Los Gatos Research at 5hz.
Discussion:
Preliminary data suggests that the movement of carbon through multiple soil pools can successfully be tracked using stable isotope tracers. In this study, the emission of added 13C as methane was only clear in two of three amendments:
Dissolved 13C-CH4 added directly to the rhizosphere was emitted to the atmosphere within one hour of addition (Figure 1). The isotopic signal decayed following an exponential curve over the next 24 hours (Figure 2). This is most likely due to the physical limitations of diffusion within the rhizosphere prior to release via plant aerenchyma.
Dissolved 13C-Acetate added to the rhizosphere was gradually converted to, and emitted as, 13C-CH4. The increase in the isotopic composition of emitted methane from the labeled acetate is linear and did not reach a peak and decline within 24 hours of addition (Figure 1).
13C-CO2 added to the chamber headspace does not appear to have been converted to methane in clearly quantifiable amounts. However, the slight rise in relative isotopic enrichment of emitted methane around four hours after addition may be due to the isotopic label that was added. Follow up work with additional labeled material in a more controlled setting is planned.
Methane emissions are controlled by physical and biological processes. The temporal importance of different carbon pools for methane emission may be greater than we had originally thought, which means that an understanding of methane transport in the rhizosphere is important for modeling purposes.
Methods:
A series of 0.5 x0.5m plots were established in a high alpine wetland dominated by the sedge Carex aquatilis.
Plant carbon uptake and methane emissions were determined using a field-deployable greenhouse gas analyzer (FGGA)
One day’s worth of emitted CH4-C was added to each of the three experimental plots as 13C-CO2, 13C-Acetate, and 13C-CH4 (Diagrams 1 and 2)
The isotopic composition of emitted CH4 was tracked over a 24 hour period using a field-deployable methane carbon isotope analyzer.
Keeling plot analysis was used to determine the isotopic composition of emitted methane.
Figure. 2 The isotopic composition of methane emitted from the plot amended with 13C-CO2 decreased over a 24 hour period following an exponential decay curve after peaking within an hour of the addition. This trend is indicative of physical processes controlling the release of methane from the rhizosphere.
Future Work:
Due to the historic drought in CO and WY this past summer, field work was unable to be completed once the wetland had dried up.
Ongoing work in the greenhouse will confirm the results presented and increase available data on pool sizes.
A comparative field study using this isotope addition method is being planned for next season and will incorporate plant community composition as a factor.
These data will be used to parameterize the model of methane production, consumption, and transport we are currently developing.
Acknowledgments: I would like to thank the von Fischer lab for their help processing samples and with field work. Additionally I would like to thank Elsie Denton, Whitney Mowll, Tony Vorster, Guy Litt, and Mary Ballard for their assistance with field work. Special thanks to the GLEES and USFS scientists who provided logistical and site support.