Identification of Metabolically Active Rhizosphere Microorganisms by Stable Isotopic Probing of PLFA in Switchgrass

摘要

Root-derived rhizodeposits of recent photosynthetic carbon (C) are the foremost source of energy for microbial growth and development in rhizosphere soil. A substantial amount of photosynthesized C by the plants is translocated to belowground and is released as root exudates that influence the structure and function of soil microbial communities with potential inference in nutrient and C cycling in the ecosystem. We applied the C-13 pulse chase labeling technique to evaluate the incorporation of rhizodeposit-C into the phospholipid fatty acids (PLFAs) in the bulk and rhizosphere soils of switchgrass (Panicum virgatum L.). Soil samples of bulk and rhizosphere were taken at 1, 5, 10 and 20 days after labeling and analyzed for C-13 enrichment in the microbial PLFAs. Temporal differences of C-13 enrichment in PLFAs were more prominent than spatial differences. Among the microbial PLFA biomarkers, fungi and Gram-negative (GM-ve) bacterial PLFAs showed rapid enrichment with C-13 compared to Gram-positive (GM+ve) and actinomycetes in rhizosphere soil. The C-13 enrichment of actinomycetes biomarker PLFA significantly increased along with sampling time in both soils. PLFAs indicative to fungi, GM-ve and GM+ ve showed a significant decrease in C-13 enrichment over sampling time in the rhizosphere, but a decrease was also observed in GM-ve (16: 1.5c) and fungal biomarker PLFAs in the bulk soil. The relative C-13 concentration in fungal PLFA decreased on day 10, whereas those of GM-ve increased on day 5 and GM+ ve remained constant in the rhizosphere soil. However, the relative C-13 concentrations of GM-ve and GM+ve increased on days 5 and 10, respectively, and those of fungal remain constant in the bulk soil. The present study demonstrates the usefulness of C-13 pulse chase labeling together with PLFA analysis to evaluate the active involvement of microbial community groups for utilizing rhizodeposit-C.