Transcript Document

I.Vigano1, R.Holzinger1, A.van Dijk2 & T. Röckmann1
1Institute for Marine and Atmospheric Research, Utrecht University, The Netherlands; 2Department of Earth Sciences, Utrecht University, The Netherlands.
Why isotopic signatures?
Introduction
Following the discovery that plants emits methane
(Keppler et al., 2006) and the recent findings on the role of
UV radiation (Vigano et al., 2007, submitted manuscript), we
have determined the 13C-CH4 (carbon) and D-CH4
(deuterium) isotopic signatures of methane emitted from
different dry plant matter.
The work of Keppler et al. indicated that plants having the
same photosynthetic pathway (C3 or C4) emit methane with
a carbon isotope signature that is characteristic for those two
major groups.
In this work we report the isotope signatures of CH4 emitted
following UV irradiation. We used strong UV radiation (5
times more than natural) to produce high emission rates in
order to precisely characterize the isotopic composition of
1
the source.
Using a standard method for all samples we got different
isotopic signatures from different plants, showing the high
natural variability. This isotopic variability is not chaotic but
it groups the plant-species depending on their
photosynthetic system (C3, C4, CAM).
It is well known that C3, C4 and CAM plants incorporate
carbon with different isotopic discriminations (Hobbie et al.,
2004), and in our work we have analyzed the 13C bulk
content in order to find a relation with the methane emitted.
 unique
When methane is emitted from different sources, the
isotopic signature is characteristic of these different
sources.
In this study we wanted to determine the isotopic content
of the methane emitted by plants under UV irradiation.
From our results, methane emitted from plant matter has
a species-specific isotopic content for 13C-CH4 and for
D-CH4 which can be used to unravel present and past
atmospheric
methane
budgets.
1
13
 C
13
 C
Methods and experimental set-up
A UV source (OSRAM Vitalux lamp) irradiates a leak tight 100ml Quartz vial containing the plant material for a
certain time (from 15 up to 180 min.). Before every run the sample is pre-flushed with compressed air at ambient
methane level (~1.9 ppm). When the lamp is turned on the amount of methane increases proportionally with the
irradiation time. After irradiation, the gas sample is transferred to a 100ml evacuated glass bottle (Fig.1). These
samples are successively injected into our GC-IRMS system (Fig.2) for high precision analysis of 13C-CH4 and
D-CH4. Bulk 13C analysis were performed by A. van Dijk at the Earth Sciences Dept. of Utrecht University. The
GC-IRMS gives isotope values and concentrations derived from the peak area of the chromatogram. The source
signatures are then determined from the y axis intercept in a Keeling plot ( value versus inverse peak area). The
method allows to determine single signatures within 2h with a precision of 1‰ for 13C-CH4 and 2‰ for D-CH4.
Plant signature
0.000
0.020
1/Peak Area
0.040
0.060
0.080
0.100
0.120
-40


C -C H 4
-45
y = 167.2x - 64.53
-50
-55
-60
-65
-70
Results
Fig.1- The setup with the UV-lamp
irradiating the sample
Fig.2- GC-IRMS system
(M.Brass & T.Röckmann)
Fig.3- The intercept will give us the isotope
of the sources
Plants-Compounds
Lignin
Pectin
Cellulose
Perennial ryegrass (Lolium perenne)
Sweet vernal grass (Anthoxanthum odoratum L.)
Madagascar Dragon Tree (Dracaena Marginata)
Oil tree (Pentaclethra macroloba)
Weeping Fig (Ficus benjamina)
(Crinum lily)
Planes (Platanus orientalis L.)
Maple (Acer pseudoplatanus)
Rosmary (Rosmarinus officinalis)
Ash (Fraxinus xanthoxyloides)
Salvia (Salvia argentea)
Ginger (Costus scaber)
Barbados nut (Jatropha curcas)
Schefflera actinophylla (Octopus tree)
Fig.7 - The cavity ring-down.
Palma Imperial (Ceratozamia mexicana)
Bananas (Musa Acuminata)
Scarlet star (Guzmania lingulata)
Snake plant (Sanseviera trifasciata)
(Tilandsia Xerographica)
Spanish moss (Tilandsia Usneoides)
Maize (zea mais)
Switchgrass (Panicum virgatum)
Sorgum bicolor
Orzaga (Atriplex halimus)
Fig.4- 13C vs D plot. Plants can be very well separated in their two main photosynthetic
groups, showing characteristic isotope fingerprints of the methane emitted upon irradiation with
UV light. Average 13C values of methane are -71.4‰ and -55.1‰ for C3 and C4/CAM plants
respectively. Average D values are -489‰ and -383‰ for C3 and C4/CAM plants respectively.
Fig.5- 13C-CH4 vs 13C-bulk. The two
groups of plants come out well separated
and fall close to a line with slope 1.
Amaranthus cruentus
Sorgum drummondii
Lemon grass (Cymbopogon flexuosus)
Cotton flower (Gossipium hirsutum)
Bamboo (Phyllostachys aurea)
Lignin
Pectin
Cellulose
C3 /1g
C3 /2g
C3 /3t
C3 /4t
C3 /5t
C3 /6t
C3 /7
C3 /8
C3 /9
C3 /22
C3 /10
C3 /11t
C3/27
C3 /12t
C3 /13t
C3 /14t
CAM /15
CAM /16
CAM /17
CAM /18
C4 /20
C4 /19g
C4/23
C4/24
C4/25
C4/26
C4/28
cotton
CX/21
Table.1- A list of the dry compounds
and dry leaves tested up to now.
The average isotopic composition of tropospheric methane is 13CATM~-47.5 ‰ and DATM~-91.5‰. If it is indeed a large source, CH4 emissions from UV interaction with biomass
can leave a strong isotope signal. In particular the emissions from C3 plants are strongly depleted. We have analyzed several dry leaves and basic plant compounds (Table 1). The
signatures are in good agreement with the values reported by Keppler et al. from the living plants experiments. 13C-bulk analysis was also performed to investigate the relation with
the 13C of methane (Fig.5). First data indicate a good correlation of both signatures, defined by the two major photosynthetic groups, but further experiments are required to make this
observation more solid. Overall, plants can be easily grouped and separated in C3 and C4-CAM by the isotopic composition (13C vs D, Fig.4) of the methane emitted under UV
light. This can be used to further investigate the role of this source to the atmosphere.
References:
Vigano I., Holzinger R., van Weelden H., Keppler F. & Röckmann, T.:
Effect of UV radiation and temperature on the emission of methane from plant biomass and structural components, Biogeoscience submitted.
Keppler, F., Hamilton, J. T. G., Brass, M., and Röckmann, T.:
Methane emissions from terrestrial plants under aerobic conditions, Nature, 439, 187-191, doi:110.1038/nature04420, 2006.
photo©MSwanson
Hobbie A., Werner E., Roland A.:
Intramolecular, compound-specific, and bulk carbon isotope patterns in C3 and C4 plants: a review and synthesis, New Phytologist, 161-2, 371-385,2004.