Time Dimension of the Rhizosphere Priming Effect.

The rhizosphere priming effect, defined as the stimulation or suppression of soil organic matter decomposition by live roots and associated rhizosphere organisms when compared to the decomposition rate from rootless soils under the same environmental conditions, has emerged as a key mechanism in modulating soil organic matter decomposition. However, the temporal scales associated with the rhizosphere priming effect have not received adequate attention. This presentation attempts to discuss three key questions: (1) How long does the rhizosphere priming effect persist? (2) How quickly does the rhizosphere priming effect respond to a change of a key controlling variable? (3) What are the likely key variables that affect the temporal patterns of the rhizosphere priming effect?  Relating to the first question, the priming effect has commonly been viewed as primarily relating to mineralization of labile soil organic matter, and therefore, it should not last much longer than a few months or years. However, recent results indicate that priming may actually accelerate the decomposition of stabilized soil organic matter and even black carbon. Therefore, the rhizosphere priming effect may persist for decades and even longer time scales if the necessary conditions for priming are maintained.  A preliminary set of empirical evidence tends to support this line of argument about the persistent nature of the rhizosphere priming effect. For the second question, some experimental evidence suggests that the rhizosphere priming effect is intimately coupled to canopy level plant activities (e.g., photosynthesis) with response times ranging from hours to days possibly by regulating the supply of photosynthates to root exudation. For the third question, results from experiments under controlled conditions have indicated that environmental variables (e.g., light, temperature, and soil moisture), plant species, seasonality, and soil fertility can individually and in concert substantially influence the temporal dimensions of the rhizosphere priming effect. Overall, the rhizosphere priming effect has the potential to substantially regulate both the short-term and the long-term ecosystem dynamics.