PS 48-42
Beyond the transient: A simple model demonstrates that the rhizosphere priming effect can cause sustained increases in the nitrogen mineralization rate
The rhizosphere priming effect (RPE), by which plants produce root exudates that stimulate microbial decomposition of soil organic matter, is thought to benefit plants by increasing the availability of mineral nitrogen. Whether the RPE is a transient or sustained effect is unclear and has implications for our understanding of adaptive plant strategies and terrestrial carbon storage under global change factors, including elevated atmospheric CO2.
We present a model of coupled carbon and nitrogen cycles that contains the minimal number of pools and fluxes necessary to induce the RPE, including nitrogen-limited carbon fixation, decomposition, and carbon exudates that subsidize microbial growth. For clarity of mechanism, the model deliberately simplifies this extremely complicated process. Total system nitrogen is held constant.
Results/Conclusions
Although the transient response to increased exudates is nitrogen immobilization as the microbial population grows, the sustained response is an increase in the decomposition and net nitrogen mineralization rates once the microbial population stabilizes. This is possible because the fraction of system nitrogen shifts from soil to plants with greater rates of exudation.
Despite the spatial and temporal mechanisms inherent to the RPE, our model demonstrates that priming can elevate the availability of nitrogen indefinitely, even without increasing the total soil nitrogen pool, by increasing the rate of nitrogen cycling. Depending on parameter values, especially the microbial turnover rate and the maximum rate of decomposition, the RPE may lead to increased or decreased total system carbon.