Carbon isotopic contributions

Concerns over atmospheric methane as a greenhouse gas and the large contribution of biomethanogenesis as a source of this gas make it important to determine the relative significance of various components of this activity. A recent paper (8) summarized estimates (28-30) of source fluxes of atmospheric methane based on several carbon isotopic studies and presented new data on natural sources and biomass burning. These data (Table III) show that of a total flux of 594 million tons (Tg) per year, 83% is produced via biomethanogenesis from a combination of natural (42%) and anthropogenic (41%) sources. 

Recurrent is the lack of adequate techniques to assess carbon flows through the plants and microbes into soil organic matter (151). Most important is the development of techniques and protocols to separate rhizosphere from nonrhizosphere soil as well as possibly to facilitate analyses of soil carbon dynamics. The use of carbon isotopes, and, where possible, application of double labeling with C and C, seems inevitable in order to separate the contribution of different substrates to the formation of the soil organic matter pool and to get to an understanding of the ecological advantage of exudates and rhizodeposits. 

The problem is more serious in the case of silicon the "Si isotope contributes only 3.4 % to the Xh-2 signal, whereas Si gives 5.1 % to Xh-1. Neglecting Si would cause an overestimation of the carbon number by 5 per Si present, which is not acceptable. 

The isotopic contribution of various atoms is additive. For low molecular weight compounds, the isotopic contribution originates mainly from the carbon atom as long as no other element with a second isotope of significant abundance is present. For a molecule of Mr 192 the intensity of the m/z 194 ion represents 12% of the [M+H]+ peak (m/z 193 Fig. 1.3A). Chlorine (Cl) has two intense isotopes Cl and Cl (76% and 24% abundance, respectively). Replacing one H by a Cl atom results in a change of the isotopic distribution of the molecule... 

Recently, it was shown that ss-NMR spectroscopy can be used to determine the conformation of EpoB in the solid state [116]. The method relies on the measurement of intramolecular short H-H distances (in the range 1.8-3.0 A) from 2D CHHC correlations under MAS [117]. Regarding the sample preparation, a small amount of 13C labeled compound was diluted with EpoB with natural abundance of carbon isotopes. This reduces the signal to noise but, on the other hand, it excludes contributions from intermolecular H-H polarization transfer. Under these conditions, all CHHC cross-peaks result from intramolecular polarization transfer and reflect intramolecular interproton distances. 

Boschker, H.T.S., de Brower, I.F.C., and Cappenberg, T.E. (1999) The contribution of macrophyte-derived organic matter to microbial biomass in salt-marsh sediments Stable carbon isotope analysis of microbial biomarkers. Limnol. Oceanogr. 44, 309-319. 

The natural abundance of 13C has been exploited in an investigation of carbon isotope effects on loss of methane from metastable butane ions [64]. An inverse isotope effect /12cH //j3ch of about 0.9 was reported. The significance of this result is unclear as there appear to be several reaction pathways contributing to the loss of methane [928]. 

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