From Isotopes and Biomarkers to Metabolic Flux Tracing: New Insights into Soil C and Nutrient Cycles.

To disentangle fluxes and biogeochemical cycling of OC and nutrients (N, P) in soil a detailed knowledge on the biogeochemical pathways and its controlling factors is required. Over the last decades, biomarker approaches and isotope analyses became the most applied and promising tools to study SOM cycling.

We will firstly review current knowledge on combining biomarker with isotope analyses (¹³C, ¹⁵N, ¹⁸O, ³³P) for identifying the mechanisms and rates of OC and nutrient transformation and cycling. As compound-specific isotope studies on uniformly labeled substances do not allow the differentiation of the intact use of the initial substances from its transformation to metabolites, we will introduce position-specific labeling to trace molecule atoms separately - a prerequisite for metabolic flux tracing. This together with the quantification of isotope incorporation in CO₂ and phospholipid fatty acids, enables tracing specific metabolic pathways of individual microbial communities in situ.

We then discuss changes in the transformation pathways caused by sorption. Sorption of position-specifically ¹³C, ¹⁴C and ¹⁵N labeled Alanine delayed the release of labeled CO₂ and reduced initial respiration rate by 80%. The C-2 position was lost faster than the C-3 position regardless of whether the molecule was used ana- or catabolically. Free Alanine was preferential used by highly competitive free living osmotrophs, while sorbed Alanine was more preferred by microbial groups that build biofilms and extracellular structures fixing hyphae.

Finally, we will give an outlook how these results from position-specific labeling studies can be included into metabolic flux models and combined with methods that enable to directly study microscale soil structures responsible for OC stabilization, like NanoSIMS. Such an adaptation of flux models to microbial regulatory mechanisms will deepen our understanding on the microorganims’ reactions to environmental changes and create the prerequisite for a quantitative prediction of biogeochemical fluxes based on microbial processes.