Rhizosphere priming effect: Its functional relationships with microbial turnover, evapotranspiration, and C–N budgets

Understanding soil organic matter (SOM) decomposition and its interaction with rhizosphere processes is a crucial topic in soil biology and ecology. Using a natural 13C tracer method to separately measure SOM-derived CO2 from root-derived CO2, this study aims to connect the level of rhizosphere-dependent SOM decomposition with the C and N balance of the whole plant–soil system, and to mechanistically link the rhizosphere priming effect to soil microbial turnover and evapotranspiration. Results indicated that the magnitude of the rhizosphere priming effect on SOM decomposition varied widely, from zero to more than 380% of the unplanted control, and was largely influenced by plant species and phenology. Balancing the extra soil C loss from the strong rhizosphere priming effect in the planted treatments with C inputs from rhizodeposits and root biomass, the whole plant–soil system remained with a net carbon gain at the end of the experiment. The increased soil microbial biomass turnover rate and the enhanced evapotranspiration rate in the planted treatments had clear positive relationships with the level of the rhizosphere priming effect. The rhizosphere enhancement of soil carbon mineralization in the planted treatments did not result in a proportional increase in net N mineralization, suggesting a possible de-coupling of C cycling with N cycling in the rhizosphere.