Position-specific labeling: A novel biogeochemical tool to trace C transformations in soils
O 3.2 in Research Yields: Ideas Pursued to the End
10.10.2013, 15:45-16:00, H6, GEO
Understanding soil organic matter (SOM) dynamics is one of the most important issues in the scope of C sequestration, climate change and maintenance of soil fertility and ecosystem sustainability. Isotope biogeochemistry developed as one of the most promising fields during the last 70 years to trace C in soils. Besides investigating the natural abundance of 13C and 14C, the application of isotopically enriched substances allowed a huge progress in understanding the sources, transformation, translocation, sequestration and losses of C in soil. Labeled plant residues as well as individual substances like sugars, amino acids or carboxylic acids and their oligo- and polymers were applied in SOM studies. The investigation of turnover and transformations of low molecular weight organic substances (LMWOS) became a key topic in soil biogeochemistry as all high molecular substances pass this stage during their decomposition.
Nearly all previous studies used uniformly labeled organic substances i.e. all C atoms in the molecules were labeled with 13C or 14C. However, this classical approach did not allow to distinguish whether the initial substance was involved in a certain processes, or whether the substance was first transformed and its metabolites entered this process.
Here we introduce and overview a novel tool of isotope applications – the position-specific labeling – to trace the biogeochemical fate of individual C atoms in the molecules. We show the advantages of position-specific 13C and 14C labeling to investigate sorption, microbial uptake and decomposition as well as the formation of new microbial compounds. We present results from a representative spectra of LMWOS of sugars, amino acids and carboxylic acids. Position-specific labeling enabled to distinguish the fate of initial substance and its metabolites. Such metabolite tracing allowed to evaluate contribution of individual functional groups of one substance to various processes in soil. Furthermore, we coupled position-specific 13C labeling with compound-specific 13C analysis of various microbial metabolites to trace the utilization of individual C atoms by the microbial community in soils. Formation of microbial cell membranes was investigated by GC-C-IRMS measurement of phospholipids fatty acids (PLFA) as well as cell wall formation was observed by LC-O-IRMS measurement of the microbial amino sugars. We highlight the perspective of this coupling to reconstruct the microbial metabolization pathways in soils and sediments. In combination with the investigation of biomarkers like PLFA, specific transformation of ecophysiological groups within the microbial community can be traced. We found that LMWOS are mainly transformed according to major cellular C metabolization pathways but could also identify individual pathways driven under special conditions or by special microbial groups. More complex substances like fatty acids are rarely incorporated into basic C metabolism but preferentially used as entire building block by microbes. In summary, specifics in the transformation of individual molecule positions can be identified and C transformation pathways of various substances reconstructed.
To reflect the differences between the fate of individual C atoms independent of the concentration and pools of the substances in soil we introduced the divergence index. The divergence index reveals the convergence or divergence of C from individual molecule positions during microbial utilization and stabilization. It enables identification of specific molecule positions which reflect characteristic changes in the C transformation pathways in soils. Thus, observing single C atoms and their transformation pathways is a unique tool to gain new information about mechanisms and kinetics of biogeochemical processes in soils and sediments and therefore will strongly improve our understanding and generalization of carbon fluxes.
Export as iCal:
