Submitted to: Soil Biology and Biochemistry
Publication Type: Peer reviewed journal
Publication Acceptance Date: 10/6/2003
Publication Date: 11/18/2003
Citation: Butler, J.L., Bottomley, P.J., Griffith, S.M., Myrold, D.D. 2003. Distribution and turnover of recently fixed photosynthate in ryegrass rhizospheres. Soil Biology and Biochemistry. 36:371-382. Interpretive Summary: The soil has the largest reservoir of organic carbon in the terrestrial biosphere. Soil microorganisms rely on these reserves to live. These microbes also play an important role as recyclers of soil carbon and thus influence plant production by affecting soil nutrient levels. Much of the organic matter found in the soil comes directly from plants, either from plant tissue or carbon substances that pass through roots. The amount and quality of plant carbon reaching the soil can influence soil microbial activity and hence primary plant production. The objective of this study was to follow the flow of carbon from the plant, through the roots and into the rhizosphere. We hypothesized that the quantity and quality of carbon moving into the soil from plant roots would be different at different plant developmental stages. We found that carbon fixed during plant photosynthesis rapidly entered the soil and was consumed by soil microorganisms. This occurred faster during a time when root growth dominated over shoot growth.
Technical Abstract: The cycling of root-deposited photosynthate through the soil microbial biomass can have profound influences on plant nutrient availability. Currently, our understanding of microbial dynamics associated with rhizosphere carbon (C) flow is limited. We used a 13C pulse-chase labeling procedure to examine the flow of photosynthetically fixed 13C into the microbial biomass of the bulk and rhizosphere soils of greenhouse-grown annual ryegrass (Lolium multiflorum Lam.). To assess the temporal dynamics of rhizosphere C flow through the microbial biomass, plants were labeled either during the transition between active root growth and rapid shoot growth (P1), or 10 days later during the rapid shoot growth stage (P2). Although the distribution of 13C in the plant/soil system was similar between P1 and P2, microbial cycling of rhizodeposition differed between P1 and P2. Within 24 h of labeling, more than 10% of the 13C retained in the plant/soil system resided in the soil, most of which had already been incorporated into the microbial biomass. From day 1 to day 8, the proportion of 13C in soil as microbial biomass declined from about 90 to 35% in rhizosphere soil and from about 80 to 30% in bulk soil. Turnover of 13C through the microbial biomass was faster in rhizosphere soil than in bulk soil, and faster in P1 than P2. Our results demonstrate the effectiveness of using 13C labeling to examine microbial dynamics and fate of C associated with cycling of rhizodeposition from plants at different phenological stages of growth.