ИСТИНА

Mass spectrometry of stable isotopes and the 13C/12C ratio (d13C value) in particular is widely used in soil zoology and ecology, especially for the study of the flows of energy in food webs. It is usually assumed that d13C values change only little in food chains, i.e. consumers and their food have a similar isotopic composition of carbon. There is however one notable exception. As a rule, saprotrophic soil animals are strongly enriched in 13C relative to plant residues or SOM. This ‘detrital shift’ likely stems from a considerable 13C/12C fractionation by saprotrophic microorganisms. Although the enrichment of saprotrophic fungi in 13C is well documented, the mechanisms behind this phenomenon remain obscure. Both, selective assimilation of 13C-enriched chemical compounds such as cellulose and biochemical metabolic processes can play a role. In the latter case, the increase in the 13C content in fungal tissues should be balanced by the emission of the lighter isotope 12C during respiration. Experiments testing this suggestion gave inconclusive results (Boström et al. 2008). Our study was conducted in the Cat Tien National Park (southern Vietnam). Aboveground nest-building termites were selected as a model object, representing an extremely effective carbon-processing system. Fungi-growing (Termitinae) and wood-feeding termites (Amitermitinae) were chosen as the two contrasting ecological groups. Gas samples were extracted from inside the termite mounds and analyzed for the concentration and d13C values of CO2 and CH4 using a CRDS Picarro G2201-I analyzer. Preliminary results suggest that the CO2 respired by fungal combs of Termitinae is depleted in 13C relative to plant litter and fungal nodules by about 2‰. To our knowledge, this is the first direct evidence of the respiration-driven 13C accumulation in detrital food webs. On the other hand, wood-feeding termites produce large amounts of strongly 13C-depleted CH4.