PARENT SESSION
Symposium #2: Linking Aboveground and Belowground Interactions: Emerging Perspectives on the Feedbacks Between Plant and Soil Communities.
Organized by: SD Frey, J Lussenhop, and D Porazinska
Monday, August 5. 8:00 AM to 11:30 AM. Turquoise Ballroom, TCC.
Effects of soil microbial community composition on nutrient cycling dynamics in rhizosphere soil.
FIRESTONE, MARY*,1, HERMAN, DONALD1, DEANGELIS, KRISTEN1, 1 University of California-Berkeley, Berkeley, CA
ABSTRACT- The effects of plant roots on microbial N-immobilization and mineralization in rhizosphere soil are of dominant importance in plant N-availability, yet are poorly understood. Using a micro-15N pool dilution technique in microcosms planted with Avena barbata and designed for direct access to roots and rhizosphere soil, rates of gross N-mineralization in rhizosphere soil were 10 times than in nearby bulk soil. Total bacterial numbers in soil adjacent to root were significantly higher than those in bulk soil but protozoal biomass was not. We used direct counts of total bacteria to estimate the standing stock of N present in the bacterial biomass in rhizosphere soil. Calculating the flux of N through the microbial biomass as the standing stock of biomass N divided by the gross rate of N-mineralization results in N-flux values that are significantly correlated with protozoal biomass. While protozoal grazing can release N tied up in bacterial biomass, it does not explain the transfer of organic-N to rhizosphere bacteria in the first place. Enhanced rates of gross N-mineralization in soil receiving acyl-homoserine lactone autoinducers indicate that gram-negative bacterial signaling compounds are involved in stimulation of organic N-mineralization activity. We do not yet know how roots stimulate soil heterotrophs to access soil organic N; mechanisms could include: stimulation of specific bacterial populations through substrate supply or root elaboration of bacterial signaling compounds; high C/ low N localized environments; or density-dependent response of bacterial populations. Our work indicates that plant roots actively manipulate nitrogen availability in rhizosphere soil through interaction with soil bacteria.
KEY WORDS: rhizosphere, N-mineralization, bacteria, signaling