The high flux of C through the pool of LMWOS clearly defines them as a crucial C pools in the SOC cycle. Many previous studies have analyzed the rates and turnover of LMWOS in soil, but the underlying mechanisms and pathways of C transformation of LMWOS remains unknown. Therefore, these studies were focused on biogeochemical pathways of three main groups of LMWOS in soil: amino acids, monosaccharides and organic acids. Tracing their transformations was achieved by combining for the first time position-specific 13C-labeling with compound-specific isotope analysis (CSIA).
The application of individual LMWOS revealed that entry steps into basic C me-tabolism account for specifics in the C partitioning between microbial cata- and anabo-lism: substrates entering citric acid cycle were preferentially mineralized (>80% in 10 days) whereas e.g. monosaccharides entering glycolysis were preferentially allocated to anabolic pathways and incorporated into microbial biomass (less than 70% mineralized in 10 days).
Position-specific 13C- and 14C-labeling provided a unique submolecular approach to reconstruct the main pathways of C transformation in soil and their specifics in individual microhabitats (e.g. at mineral surfaces or at the plant-root interface). The divergence in-dex (DI) was developed and proven to be a valuable tool to compare the position-specific fate of individual substances independent of the used isotopic approach or experimental design used or the pool investigated. 13C incorporation in various microbial compound classes was traced by compound-specific isotope analysis: fatty acids by GC-C-IRMS and amino sugars by IC-O-IRMS. Therefore, a new instrument coupling was applied and purification and measurement methods for soil amino sugar δ13C analysis were estab-lished and evaluated.
Basic microbial C metabolism with glycolysis, pyruvate dehydrogenase oxidation and citric acid cycle could be traced in soil under field and laboratory conditions. Oxidi- zing, catabolic pathways are ongoing in soils in parallel to constructing, anabolic path-ways (like gluconeogenesis): for example, up to 55% of the glucose allocated to amino sugar synthesis was not intact glucose but derived from glucose metabolites allocated by gluconeogenesis backflux towards amino sugar formation. Consequently, substrates en-tering glycolysis are intensively recycled within the cellular C pool, which was shown by a continued decrease of their divergence index.
Specific tracers for individual biosynthetic pathways were identified, which allowed transformations to be followed within these side branches of the basic C metabolism: 1) the pentose phosphate pathway was detected by a combination of hexose and pentose
13C labeling; and 2) turnover within the cellular lipid pool was proven by 13C labeling of
short- (acetate) and long-chain (palmitate) precursors of PLFA. Within 10 days, 65% of the incorporated palmitate was transformed (e.g. by desaturation, elongation or branch-ing) to other fatty acids and the fingerprint of the palmitate 13C-derived fatty acids ap-proached the PLFA pattern of the present microbial community. Knowledge of these fast fatty acid transformations is crucial for the application of fatty acid fingerprints and their isotopic values for palaeoenvironmental reconstructions.
An intensive turnover was not only shown for lipids but also for cell wall polymers.
Metabolic recycling activity and turnover was much higher for bacteria than for fungi, which was proven by both biomarker groups – PLFA and amino sugars. Therefore, this thesis experimentally revealed one underlying, mechanistic reason for the previously ob-served specifics in C turnover of the slow (fungi-based) and the fast (bacteria-based) cycling branch of the soil food web. For the first time, position-specific labeling was cou-pled with compound-specific isotope analysis of microbial biomarkers. This combination provides a novel opportunity to trace simultaneous, biosynthetic pathways of individual microbial groups in diverse microbial communities of soils.
Furthermore, variations of environmental factors, like substrate concentration, were identified as the main regulatory factors for C allocation within microbial metabolism:
Concentration gradients characteristic for soils from C-poor bulk soils to hotspots in-volved a shift of C allocation within metabolic pathways from C starvation pathways via maintenance pathways towards pathways that are characteristic for cells under growth conditions. Sorption, as a soil-specific process reducing the bioavailability of a substrate, affected microbial metabolism: the stronger a substrate is sorbed, the more of its C is allocated towards anabolism, e.g. is found in the microbial products. Understanding these shifts in metabolic pathways is crucial for the SOC cycle, as C allocation towards anabo-lism is the prerequisite for the formation and stabilization of microbially-derived SOM.
Three soil-specific processes were traced in parallel with microbial utilization: sorp-tion, exoenzymatic LMWOS utilization and plant uptake. Sorpsorp-tion, as well as desorpsorp-tion, occurred as intact molecules and did not account for LMWOS transformations. Exoen-zymes caused a stepwise oxidation of the LMWOS C backbone. However, their kinetics could not compete with microbial uptake systems. Consequently, extracellular transfor-mations can only be relevant in specific soil microhabitats, which are inaccessible for microorganisms. Intact uptake of amino acids by plants was assessed by dual-isotope position-specific 13C- and 15N-labeling. This new approach revealed the overestimation of intact amino acid uptake due to methodological constraints of previous studies and showed that less than 1.5% of the applied amino acids were taken up intact by plants.
Consequently, none of the investigated abiotic or biotic processes could compete with
microorganisms for LMWOS utilization and the microbial metabolism determines the fate of LMWOS C in soils.
This thesis established position-specific 13C- and 14C-labeling as a unique tool to trace LMWOS transformation processes in soils. Using this novel approach, the base for detailed mechanistic understandings of microbial LMWOS transformations and subse-quent SOM formations were created. Combination of position-specific labeling with dual-isotope labeling and the improvement of CSIA towards the position-specific detection of the isotope label within the newly formed transformation products will be a future task.
These techniques will further deepen the understanding of microbial C transformations and their controlling factors and improve prediction as well as manipulation of C alloca-tion and stabilizaalloca-tion in soils.