Submitted to: Plant and Soil
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/10/2006
Publication Date: 11/7/2006
Citation: Jenkins, M., Franzluebbers, A.J., Humayoun, S.B. 2006. Assessing short-term responses of prokaryotic communities in bulk and rhizosphere soils to tall fescue endophyte infection. Plant and Soil. 289:309-320. Interpretive Summary: With the concern over the increased emissions of the greenhouse gas carbon dioxide and its connection with global warming, the United States and other nations have been interested in identifying means to enhance the removal of carbon dioxide from the atmosphere. Within the last decade ARS scientists at J. Phil Campbell, Sr., Natural Resource Conservation Center in Watkinsville, GA have identified pastures of tall fescue containing within its blades of grass a particular fungus that appeared to enhance soil’s accumulation of carbon compared to pastures of tall fescue that do not contain this fungus. Previous research by these ARS scientists indicated that the difference between the two types of tall fescue to accrue soil carbon may be related to toxic compounds produced by the fungus of the infected pasture grass that affect the naturally occurring soil bacteria involved in recycling plant material. To determine if the tall fescue infected with the fungus affect the normal bacteria residing in soil a pot experiment was initiated. Pots were filled with two types of soil, a sandy loam, and a clay loam; half of the pots containing the same soil type were planted to fungus infected tall fescue, and the other half to fungus free tall fescue. After certain time intervals of weeks, soil was removed from the pots (bulk soil) and from the roots of the plants (root soil). The two types of bulk and root soils were made into a slurry and prepared for counting the total number of bacteria, and for identifying particular groups of soil bacteria with group specific gene probes that would make the particular group of bacteria visible with a fluorescence microscope and their numbers per unit weight of soil determined. Results of this study indicated that the fungus infected tall fescue compared to the uninfected tall fescue decreased the number of cells corresponding to four bacterial groups that may account for decreased degradation of plant material. Results of this study also appeared to reveal the complicated interaction between plants and soil bacteria by indicating distinct differences in bacterial communities between the two soil types and between bulk and root soils. This study has identified important groups of bacteria that were affected by the fungus infected tall fescue and has helped to better understand the mechanisms of enhanced soil carbon sequestration. This research will be of keen interest to scientists and government agencies dealing with global warming issues, greenhouse gases, and management of agricultural activities.
Technical Abstract: In contrast to endophyte-free (E-) tall fescue, endophyte-infected (E+) tall fescue pastures appear to enhance soil carbon sequestration. A hypothetical mechanism that may account for the enhanced carbon sequestration is that the E+ tall fescue affects the soil microbial community or components of it that are involved in organic carbon turnover. A 60-week mesocosm study with a factorial arrangement of soil type (sandy loam [SL] and clay loam [CL]) and E+ and E- tall fescue was conducted to determine if the soil microbial communities were affected by the presence of the endophyte. Bulk and rhizosphere soil samples were fixed in paraformaldehyde, and homogenized in 0.1% pyrophosphate by mild ultrasonic treatment before being stained with 4’,6-amidino-2-phenylindole (DAPI) for total direct microbial counts, and with a combination of one of a domain or subdivision fluorescent oligonucleotide probe and DAPI for enumerating eubacteria, bacterial subdivisions, and archaea. Results of this study showed that the presence of the endophyte affected the archaeal, and high G+C gram-positive bacterial communities of the bulk CL, and delta-proteobacterial, and planctomycetes communities of the rhizospere SL. In the long-term, decreases in these microbial communities could account for a decrease in soil organic carbon mineralization. Our results indicated that an E+ tall fescue-soil microbial community interaction may indeed be associated with enhanced soil carbon sequestration.