Linking microbial community structure and allocation of plant-derived carbon in an organic agricultural soil using 13CO2 pulse-chase labelling combined with 13C-PLFA profiling

We conducted a 13CO2 pulse-chase labelling experiment in a drained boreal organic (peat) soil cultivated with perennial crop, reed canary grass (RCG; Phalaris arundinacea) to study the flow of carbon from plants to soil microbes. Both limed and unlimed soils were studied, since liming is a common agricultural practice for acidic organic soils. Soil samples taken within three months after the labelling and three times in the following year were used for the δ13C analysis of microbial phospholipid fatty acids (PLFAs), root sugars and root lipids. We estimated the contribution of carbon from root exudates to microbial PLFA synthesis. The flow of carbon from plants to microbes was fast as the label allocation in PLFAs had a peak 1–3 days after labelling. The results showed that fungi were important in the incorporation of fresh, plant-derived carbon, including root sugars. None of the main microbial PLFA biomarker groups (fungi, Gram-positive bacteria, Gram-negative bacteria, arbuscular mycorrhizal fungi) was completely lacking label over the measurement period. One year after the labelling, when the labelled carbon was widely distributed into plant biomass and soil, bacterial biomarkers increased their share of the label allocation. Liming had a minor effect on the label allocation rate into PLFAs. The mixing model approach used to calculate the root exudate contribution to microbial biomass resulted in a highly conservative estimate of utilization of this important C-source (0–6.5%, with highest incorporation into fungi). In summary, the results of this study provide new information about the role of various microbial groups in the turnover of plant-derived, fresh carbon in boreal organic soil. ► We applied 13CO2 pulse-chase labelling in combination with δ13C analyses of PLFAs in an organic soil. ► Photosynthetically fixed, fresh carbon was quickly taken up the microbial community. ► Fresh carbon was found in all of the microbial groups. ► Fungi were the most effective in processing the fresh, plant-derived carbon, including root sugars. ► Liming had a minor effect on the allocation of fresh carbon into the microbial PLFAs.