Measurements of the isotopic composition (deltaD, deltaC-13) 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 strong depleted in deltaC-13 and deltaD if compared to the isotopic composition of the substrates, organic material (deltaC-13, deltaD) and ground- and precipitation water (deltaD). DeltaC-13 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) deltaC-13 of methane eructated from cows strongly depends on the diet (100% C3 diet: deltaC-13 = -65.1 +/- 1.70 parts per thousand; 60-80% C4 diet: deltaC-13 = -55.6 +/- 1.40 parts per thousand). This suggests fractionation factors during methane generation in the rumen being independent from the substrate. (2) The deltaD values of methane generated in dairy wastes (deltaD = -298 +/- 6 parts per thousand) and produced by cows (deltaD = -300 +/- 10 parts per thousand) are very similar. This and the absolute deltaD depletion with respect to the substrates suggests that acetate fermentation is the major methanogenic pathway in all sources from dairy farming - although CH4 in the rumen of cows is known to be mainly generated via CO2 reduction. The unexpected deltaD signature in ruminant methane can perhaps be explained by an additional deltaD fractionation of the intracellular water of M. ruminantium if compared to the methanobacteria species in marine and freshwater environments. (3) Large variations in the deltaC-13 of methane generated from different waste deposits were observed. The fractionation factors alpha(CO2/CH4) calculated from parallel delta(CO2)-C-13 measurements are, however, very similar. The large spread of delta(CH4)-C-13 (and delta(CO2)-C-13 is explained by differences in the deltaC-13 of the organic substrate in the source systems. (4) Methane samples collected from swamps show significant isotopic enrichment relative to the original methane generated if partial oxidation bad 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%.
