1 1 Fig.7 - The cavity ring-down. I.Vigano 1, R.Holzinger 1, A.van Dijk 2 & T. Röckmann 1 1 Institute for Marine and Atmospheric Research, Utrecht University, The Netherlands; 2 Department of Earth Sciences, Utrecht University, The Netherlands. Why isotopic signatures? References: Vigano I., Holzinger R., van Weelden H., Keppler F. & R ö ckmann, 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 R ö ckmann, T.: Methane emissions from terrestrial plants under aerobic conditions, Nature, 439, , doi: /nature04420, 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, ,2004. Results 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  13 C-CH 4 (carbon) and  D-CH 4 (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 CH 4 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 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  13 C bulk content in order to find a relation with the methane emitted. 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  13 C-CH 4 and for  D-CH 4 which can be used to unravel present and past atmospheric methane budgets. 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  13 C-CH 4 and  D-CH 4. Bulk  13 C 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  13 C-CH 4 and 2 ‰ for  D-CH 4. Fig.2- GC-IRMS system (M.Brass & T.Röckmann)  unique Fig.3- The intercept will give us the  isotope of the sources Fig.1- The setup with the UV-lamp irradiating the sample Fig.5-  13 C-CH 4 vs  13 C-bulk. The two groups of plants come out well separated and fall close to a line with slope 1. Fig.4-  13 C 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  13 C 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. Table.1- A list of the dry compounds and dry leaves tested up to now. The average isotopic composition of tropospheric methane is  13 C ATM ~-47.5 ‰ and  D ATM ~-91.5‰. If it is indeed a large source, CH 4 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.  13 C-bulk analysis was also performed to investigate the relation with the  13 C 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 (  13 C 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. photo©MSwanson