Plant, soil, and environmental factors controlling rhizosphere effects on soil organic matter decomposition

Plant roots and their associated rhizosphere processes are major gateways for nutrients and water.  At the global scale, rhizosphere processes utilize approximately 50% of the energy fixed by photosynthesis in terrestrial ecosystems, contribute roughly 50% of the total CO₂ emitted from terrestrial ecosystems, and mediate virtually all aspects of nutrient cycling.  This presentation summarizes our current understanding of the rhizosphere effect on soil organic matter (SOM) decomposition.  The rhizosphere effect on SOM decomposition has been investigated through comparisons of SOM-derived CO₂ efflux rates in planted treatments with efflux rates from unplanted treatments under similar temperature and moisture conditions.  Carbon isotope methods are essential in these investigations.  Results from several experiments indicate that the rhizosphere effect may accelerate SOM decomposition by as much as 380% or inhibit decomposition by 50%, depending on the kinds of plant-soil couplings and experimental conditions.  Plant species, soil type, atmospheric CO₂ concentrations, plant growth stages, and photosynthetic rates, separately or in concert, modulate the magnitude of the rhizosphere effect.  The level of the rhizosphere effect is positively correlated with either leaf biomass or the amount of nitrogen in live roots or gross nitrogen mineralization rates.  Soil moisture significantly controls the degree of the rhizosphere effect.  The rhizosphere effect is long-lasting and obscures the response of soil organic matter decomposition to warming.  Overall, these data strongly support a rhizo-centric view about the functioning of terrestrial ecosystems.