Translocation and turnover of rhizodeposit carbon within soil microbial communities of an extensive grassland ecosystem

A substantial amount of photosynthesized plant-C is allocated belowground in grassland ecosystems where it influences the structure and function of the soil microbial community with potential implications for C cycling and storage. We applied stable isotope probing of microbial PLFAs and repeated soil sampling in a grassland over a period of 1 year to assess the role of microbial communities in the cycling of rhizodeposit-C. Pulse-labeling with (CO2)-C-13 was performed in a grassland site near Gent (Belgium). Soil samples were taken 24 h, 1 week, 1 month, 4 months, 9 months and 1 year following labeling and analyzed for C-13 in soil, roots and microbial PLFAs. C enrichment of PLFAs occurred rapidly (within 24 h) but temporally varied across microbial groups. PLFAs indicative for fungi and gram-negative bacteria showed a faster C-13 uptake compared to gram-positive bacteria and actinomycetes. However, the relative C-13 concentrations of the latter communities increased after 1 week, while those of fungi decreased and those of gram-negative bacteria remained constant. PLFA C-13 mean residence times were much shorter for fungi compared to bacteria and actinomycetes. Our results indicate temporally varying rhizodeposit-C uptake by different microbial groups, and faster turnover rates of mycorrhizal versus saprotrophic fungi and fungi versus bacteria. Fungi appeared to play a major role in the initial processing and possible rapid channeling of rhizodeposit-C into the soil microbial community. Actinomycetes and gram-positive bacteria appeared to have a delayed utilization of rhizodeposit-C or to prefer other C sources upon rhizodeposition.