Microbial community dynamics associated with rhizosphere carbon flow Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/3x816p691

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  • The cycling of root-deposited photo synthate (rhizodeposition) through the soil microbial biomass is widely recognized as a critical component of ecosystem functioning. Little is currently known about the flow of photosynthate through the microbial biomass however. My research goal was to examine if and how the flow of root-deposited photo synthate through the microbial biomass and individual phospholipid fatty acids (PLFAs) changes during two different developmental growth stages of greenhouse-grown annual ryegrass (Lolium muitWorum Laim). Plants were labeled with ¹³CO₂ during one of two labeling periods: either during a growth stage in transition between active root growth and rapid shoot growth or during a growth stage dominated by shoot growth. In each labeling period, ¹³C was followed throughout the plant/soil (rhizo sphere and bulk soil) system during an 8-d chase period. PLFAs were analyzed on the first and last day of each labeling period. The overall allocation of ¹³C in the plant/soil system was similar in the two labeling periods. There were, however, fundamental differences between the two labeling periods associated with the cycling of rhizodeposition through the microbial biomass and individual PLFAs. In each labeling period, the rhizosphere microbial biomass incorporated a greater percentage of ¹³C-labeled rhizodeposition and turned over luster than the bulk soil microbial biomass. Additionally, in the first labeling period, rhizosphere and bulk soil microbial biomass incorporated more ¹³C and turned over faster than in the second labeling period. The proportion of PLFAs associated with Gram-positive bacteria increased in the rhizo sphere and bulk soils between the first and second labeling period. These Gram-positive organisms were less active in utilizing rhizodeposition in the second labeling period, however. There was a shift in activity within the Gram-negative bacterial community, with organisms containing 16:1ω5 utilizing rhizodeposition more actively in the second labeling period. Fungi utilized a greater proportion of ¹³C-labeled rhizodeposition than bacteria in both labeling periods. These results suggest that some ecological/physiological changes had occurred within the plant/soil system between the first and second labeling periods, such as a change in the quality of rhizodeposition. This is the first study to utilize ¹³C pulse-chase labeling in combination with PLFA analysis to examine the dynamics of soil microbial communities actively involved in rhizo sphere carbon cycling. Application of this approach under a wide range of environmental conditions has the potential to greatly enhance our understanding of rhizosphere carbon cycling.
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