Dr. Weixin Cheng

1. Rhizosphere Respiration and Root Demography in Forest Ecosystems: The research tasks include: (1) investigating rhizosphere respiration at one forest site using a new 13C natural tracer method; (2) linking forest rhizosphere respiration with root demography; and (3) developing a tree rhizosphere respiration model. This research will, for the first time, try to separate forest rhizosphere respiration from soil microbial respiration, thereby making possible the measurements of all major belowground C fluxes. This will be a significant step forward in our ability to estimate belowground C sequestration, from decadal time resolution to yearly time resolution by comparing the pool measurement approach with this flux-based approach. 2. Rhizosphere Processes and Soil Organic Matter Decomposition: The soil zone strongly influenced by plant roots, the rhizosphere, plays an important role in regulating soil organic matter decomposition and nutrient cycling. The linkage between rhizosphere processes and soil organic matter decomposition is not well understood. The goal of this project is to better explain the potential mechanisms controlling this linkage by addressing the following research questions: (1) Does soil mineral nutrition affect the direction and magnitude of root-soil interactions? (2) Do different plant species affect rhizosphere processes and soil organic matter decomposition differently? (3) Are there shifts in the magnitude of controlling mechanisms in the rhizosphere through time? In answering these questions, two major mechanisms will be examined: (1) mineral nutrient competition between plants and microorganisms living in the soil; (2) stimulation or depression of microbial growth and metabolism caused by root activities. Completion of this study will improve understanding of rhizosphere processes and soil organic matter decomposition thereby providing valuable information for better management and sustainability of terrestrial ecosystems. 3. Effects of elevated CO2 on nutrient cycling in a sweetgum plantation: The long-term effects of increasing atmospheric CO2 concentration on forest growth and carbon sequestration are highly dependent upon the availability and cycling of nutrients. Until the advent of Free-Air CO2 Enrichment (FACE) systems, there have been no ecosystem-level studies on the effects of elevated CO2 on forest nutrient cycling. The long-term effects of elevated CO2 on nutrient cycling and carbon sequestration cannot be understood from isolated laboratory or even open-top chamber studies, no matter how intensive such studies might be. The purpose of this research is to investigate the effects of elevated CO2 on nutrient cycling and selected belowground processes in the sweetgum FACE facility at Oak Ridge, Tennessee. Current as of 10/1/02