Carbon Fluxes in the Rhizosphere

Terrestrial ecosystems are intimately connected to atmospheric CO2 levels through photosynthetic fixation of CO2, sequestration of C into biomass and soils, and the subsequent release of CO2 through respiration and decomposition of organic matter. Considering all the pools and fluxes of C within ecosystems, C-cycling belowground is increasingly being recognized as one of the most significant components of the carbon cycle (e.g., Zak and Pregitzer 1998). Globally, the input of C to the soil has been estimated to be as great as 60×1015 g yr−1, approximately one order of magnitude larger than the global annual rate of fossil fuel burning and other anthropogenic emissions, which is at 6×1015 g yr−1currently (Post et al. 1990). Thus, small changes in the equilibrium between sinputs and decomposition could have a significant impact on atmospheric CO2 concentrations, which may either exacerbate or reduce the consequence of burning of fossil fuels (Schimel 1995). Belowground CO2 efflux can be partitioned into two distinct processes: (1) rhizosphere respiration or root-derived CO2, including root respiration and microbial respiration utilizing materials released from live roots and (2) microbial decomposition of soil organic matter (SOM), or soil-derived CO2. While the two processes act separately, they may also be linked through rhizosphere interactions, which may exert a stimulative (priming effect) or a suppressive influence on SOM decomposition (Cheng 1999). As a measure of main energy use for the acquisition of belowground resources (e.g., nutrients and water), rhizosphere respiration may range from 30 to 80 percent of total belowground CO2 efflux (Hanson et al. 2000) in various ecosystems. Root-associated C fluxes represent a major portion of the input to and the output from the belowground C pool (Schimel 1995).