Stable isotopic signature of methane from major sources in Germany

Abstract

Measurements of the isotopic composition (δD, δ 13C) of methane originating from major sources in Germany and northern Italy (i.e. dairies and dairy wastes, waste deposits, swamps, natural gas, rice paddies) are presented. All methane samples collected from recent biogenic sources are strongly depleted in δ 13C and δD if compared to the isotopic composition of the substrates, organic material (δ 13C, δD) and ground- and precipitation water (δD). δ 13C of simultaneously produced carbon dioxide, on the other hand, shows a slight isotopic enrichment with respect to the original organic matter. The following systematical effects influencing the isotopic signature of the generated methane have been observed: 1. (1) δ 13C of methane eructated from cows strongly depends on the diet (100% C3 diet: δ 13 C = −65.1±1.7‰ ; 60–80% C4 diet: δ 13 C = −55.6±1.4‰ ). This suggests fractionation factors during methane generation in the rumen being independent from the substrate. 2. (2) The δD values of methane generated in dairy wastes ( δ D = −298±6‰ ) and produced by cows ( δ D = −300±10‰ ) are very similar. This and the absolute δD depletion with respect to the substrates suggests that acetate fermentation is the major methanogenic pathway in all sources from dairy farming - although CH 4 in the rumen of cows is known to be mainly generated via CO 2 reduction. The unexpected δD signature in ruminant methane can perhaps be explained by an additional δD fractionation of the intracellular water of M. ruminantium if compared to the methanobacteria species in marine and freshwater environments. 3. (3) Large variations in the δ 13C of methane generated from different waste deposits were observed. The fractionation factors α CO2 CH4 calculated from parallel δ 13CO 2 measurements are, however, very similar. The large spread of δ 13CH 4 (and δ 13CO 2) is explained by differences in the δ 13C of the organic substrate in the source systems. 4. (4) Methane samples collected from swamps show significant isotopic enrichment relative to the original methane generated if partial oxidation had occurred. Isotope enrichment due to partial oxidation of methane was also observed in a waste deposit in the uppermost aerobic layers. From the concentration profile in the waste deposit investigated we estimated a reduction of the effective methane flux to the atmosphere due to oxidation by more than 60%.