Rhizodeposition of organic C by plants: mechanisms and controls

During their life, plant roots release organic compounds into their surrounding environment. This process, named rhizodeposition, is of ecological importance because 1/ it is a loss of reduced C for the plant, 2/ it is an input flux for the organic C pool of the soil, and 3/ it fuels the soil microflora, which is involved in the great majority of the biological activity of soils, such as the nutrient and pollutant cycling or the dynamics of soil-borne pathogens, for example. The present review first examines the mechanisms by which major rhizodeposits are released into the soil: the production of root cap cells, the secretion of mucilage, and the passive and controlled diffusion of root exudates. In a second part, results from tracer studies ( 43 articles) are analyzed and values of C flux from the plant root into the soil are summarized. On average, 17% of the net C fixed by photosynthesis is lost by roots and recovered as rhizosphere respiration (12%) and soil residues (5%), which corresponds to 50% of the C exported by shoots to belowground. Finally, the paper reviews major factors that modify the partitioning of photoassimilates to the soil: microorganisms, nitrogen, soil texture and atmospheric CO2 concentration.