CARBON FLOW FROM ROOTS TO MICROBES TO SOIL HUMIC SUBSTANCES

Principal Investigator:
Mary K. Firestone
Professor
Division of Ecosystem Science
University of California
Berkeley, CA 94720

Duration of Project: 2 years

Project summary

Soil microorganisms mediate decomposition processes and rates of decomposition have been shown to be increased and/or decreased by the presence of living roots. Little is known however, about the mechanisms through which living roots and soil microorganisms impact the conversion of plant material into humic substances. We propose that:

1) Living roots impact the conversion of belowground plant carbon into humic substances.
2) A primary mechanism by which roots affect humification of belowground C is through alteration of microbial community composition and activity.
3) The magnitude and net impact of living roots is partially dependent on N-availability.

A California annual grassland soil will be planted to Avena barbata grown under enriched ¹³CO₂ to uniformly label plant materials. After plants have senesced, surface litter will be removed by clipping and the intact soil-root systems exposed to a 5-month warm dry summer. The intact, labeled belowground system will then be exposed to a four-way treatment matrix: planted x implanted and N-fertilized x unfertilized. ¹³C will be followed through: recognizable root debris; total microbial biomass (PLFA) and microbial community components (PLFA) associated with root debris and bulk soil; microbial biomass in bulk soil by CFI; and SOM fractions (POM, labile humics, labile fulvics, and humin) over a year cycle of plant growth. A complete treatment set will also be carried through the end of the 2-year project for analysis at a later date. We will document microbial community composition over the duration of the experiment in the defined compartments through culture-independent means (T-RFLP-analysis of community DNA). Using ¹³C-analysis, we will follow belowground C as it moves from plant root exudates and debris into the microbial community and its components, and finally into operationally defined soil humic components. We will learn if microbial communities of differing composition and activity (rhizosphere and bulk soil) alter the outcome of humification of below-ground plant debris and how living plants control these effects. Our results will more clearly define the plant and microbial mechanisms involved in humification processes.