Distribution and turnover of recently fixed photosynthate in ryegrass rhizospheres

The cycling of root-deposited photosynthate (rhizodeposition) 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 C-13 pulse-chase labeling procedure to examine the flow of photosynthetically fixed C-13 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 (Labeling Period 1), or nine days later during the rapid shoot growth stage (Labeling Period 2). Although the distribution of C-13 in the plant/soil system was similar between the two labeling periods, microbial cycling of rhizodeposition differed between labeling periods. Within 24 h of labeling, more than 10% of the C-13 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 C-13 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 C-13 through the microbial biomass was faster in rhizosphere soil than in bulk soil, and faster in Labeling Period 1 than Labeling Period 2. Our results demonstrate the effectiveness of using C-13 labeling to examine microbial dynamics and fate of C associated with cycling of rhizodeposition from plants at different phenological stages of growth. (C) 2003 Elsevier Ltd. All rights reserved.