Wednesday, August 5, 2009 - 3:20 PM

COS 79-6: Ecological implications of rhizosphere priming effects

Weixin Cheng, Biao Zhu, and Daniel Keck. University of California


Understanding mechanisms embedded in root-soil interactions is a crucial topic in terrestrial ecosystem ecology.  Rhizosphere priming effect, defined as an accelerated rate of soil organic matter (SOM) decomposition by the presence of live roots in comparison with soil incubation without roots, is one of the key mechanisms modulating plant-soil interactions.  Results from numerous experiments have shown that rhizosphere effects may accelerate SOM decomposition by as much as 380% or inhibit decomposition by 50%, depending on the specific plant-soil conditions.  Plant species, types of mycorrhizae, soil types, atmospheric CO2 concentrations, plant growth stages, and photosynthetic rates have been identified as important variables in controlling the magnitude of rhizosphere priming effects.  We have also linked rhizosphere priming effects with two possible mechanisms: (1) accelerated soil microbial turnover rates; and (2) transpiration-induced drying-rewetting cycles.  Overall, we have made some progress towards identifying and understanding rhizosphere priming effects. However, potential ecological implications of rhizosphere priming effects have not been adequately examined.  


One apparent ecological implication of the rhizosphere priming effect is a need to change our existing paradigm of SOM decomposition, so that the rhizosphere’s control on SOM decomposition can be integrated into our general models in a similar way as we do with temperature and moisture.  Because of the common occurrence of significant rhizosphere priming effects, SOM decomposition rates from root-free soils should be considered highly unrealistic in the context of field ecosystem processes.  Furthermore, we may infer that the level of rhizosphere priming should be higher in ecosystems with higher primary production because rhizosphere priming is positively controlled by photosynthetic capability.  Regarding to resource allocation and functional balance, the highest potential rhizosphere priming should occur in ecosystems with an intermediate level of nutrient availability, because nutrient-rich conditions tend to reduce belowground allocation and nutrient-poor conditions tend to reduce the overall primary production, therefore, the driving power for rhizosphere priming is low under both scenarios.