Position-Specific Labeling and Tracing: New Isotopic Tool to Trace the Fate of C in Soil.

The fate of carbon (C) in soil remains one of the most important questions but most hidden part of C cycle in terrestrial ecosystems. Despite general knowledge about the C pools in soil, the fluxes and recycling as well as transformations of C between and within the pools remains unknown and are one of the most intrigues mysteries for scientific speculations and ideas. So, the static picture about the pools does not provide suitable knowledge about the dynamic fluxes. The existing methods to trace processes: fluxes and transformation in soil are mainly based on the application of ¹³C and ¹⁴C labeled substances. Over the 70 years of successful history of isotopes in soil science, nearly all studies applied uniformly labeled substances. These studies made huge progress to understand C fluxes, but stacked at the level of microbial metabolism in soil. To overcome this problem and make next steps, we started to apply position-specific ¹³C and ¹⁴C labeling and connected the tracing with compound specific ¹³C analysis of the main group of microbial biomarkers: PLFA, amino sugars, neutral sugars, etc. This cutting-edge combination allows 1) tracing specific anabolic pathways in diverse microbial communities in soils and 2) identification of specific pathways of individual functional microbial groups.

Based on the application of the position-specifically labeled key metabolites (glucose, ribose, pyruvate, alanine, palmitate) under real soil conditions, we quantified the catabolic and anabolic pathways: glycolysis, citric acid cycle, gluconeogenesis, and anaplerotic pathway. In contrast to the microbial pure cultures, these pathways are ongoing in soil simultaneously, reflecting huge genetic and metabolic diversity because of i) very broad range of microbial species, ii) strong C limitation, iii) huge differences of microbial habitats in soil, iv) continuously changing environmental conditions. Examples for disentangling metabolic pathways by microbial utilization of sorbed substances and effects of enviroenmental conditions will be presented.

We conclude that coupling position-specific ¹³C and ¹⁴C labeling of key metabolites with compound specific ¹³C tracing in main group of biomarkers provide new tool to investigate microbial processes of C cycle in soil and to disentangle unidirectional fluxes, recycling, transformation and stabilization.