Tracing carbon metabolism with stable isotope metabolomics reveals the legacy of diverse carbon sources in soil

Abstract

Tracking the metabolic activity of whole soil communities can improve our understanding of the transformation and fate of carbon in soils. We used stable isotope metabolomics to trace 13C from nine labeled carbon sources into the water-soluble metabolite pool of an agricultural soil over time. Soil was amended with a mixture of all nine sources, with one source isotopically labeled in each treatment. We compared changes in the 13C-enrichment of metabolites with respects to carbon source and time over a 48-day incubation and contrasted differences between soluble versus insoluble sources. Whole soil metabolite profiles varied singularly by time, while the composition of 13C-labeled metabolites differed primarily by carbon source (R2 = 0.68) rather than time (R2 = 0.07) with source-specific differences persisting throughout incubations. The 13C-labeling of metabolites from insoluble carbon sources occurred at a slower rate than soluble sources but yielded a higher average atom % 13C in metabolite markers of biomass (amino acids and nucleic acids). The 13C-enrichment of metabolite markers of biomass stabilized at between 5 - 15 atom % 13C by the end of incubations. Temporal patterns in the 13C-enrichment of TCA cycle intermediates, nucleobases (uracil and thymine), and by-products of DNA salvage (allantoin) closely tracked microbial activity. Our results demonstrate that metabolite production in soils is driven by the carbon source supplied to the community, and that the fate of carbon in metabolite profiles do not tend to converge over time as a result of ongoing microbial processing and recycling. ImportanceCarbon metabolism in soil remains poorly described due to the inherent difficulty of obtaining information on the microbial metabolites produced by complex soil communities. Our study demonstrates the use of stable isotope probing (SIP) to study carbon metabolism in soil by tracking 13C from supplied carbon sources into metabolite pools and biomass. We show that differences in the metabolism of sources influences the fate of carbon in soils. Heterogeneity in 13C-metabolite profiles corresponded with compositional differences in the metabolically active populations, providing a basis for how microbial community composition is correlated with the quality of soil carbon. Our study demonstrates the application of SIP-metabolomics in studying soils and identifies several metabolite markers of growth, activity, and other aspects of microbial function.