Carbon Fluxes in the Rhizosphere

Publisher Summary This chapter discusses carbon fluxes in the rhizosphere. Terrestrial ecosystems are intimately connected to atmospheric CO2 levels through photosynthetic fixation of CO2, sequestration of C in biomass and soils, and the subsequent release of CO2 through respiration and decomposition of organic matter. Rhizodeposition is defined as all material lost from plant roots, including water-soluble exudates, secretions of insoluble materials, lysates, dead fine roots, and gases, such as CO2 and ethylene. Plants grown under elevated CO2 conditions often exhibit increased growth and a disproportional increase in C allocation to roots, total rhizosphere respiration, and rhizodeposition. Large quantities of rhizodeposits apparently represent a significant portion of the plant carbon balance and an important source of substrates for soil organisms. Microbial carbon assimilation efficiency is commonly defined as microbial biomass produced as a proportion of total carbon utilized. The timing of root exudation determines how closely rhizosphere processes are linked with plant photosynthesis and aboveground physiology. Many researchers may agree that method development has been, and remains, a key prerequisite for advancing rhizosphere science. In the past few decades, research on carbon fluxes in the rhizosphere have been mostly restricted to cereal crops. If higher plants invest a significant amount of fixed carbon into the rhizosphere to support a portion of the soil biota, then the coevolution between plants and rhizosphere biota must shape the quantity and the quality of rhizosphere C fluxes through selection and adaptation.