Comparative analysis of planted and unplanted controls for assessment of rhizosphere priming effect

The rhizosphere priming effect (RPE) is increasingly being considered to be an important regulator of soil organic matter (SOM) decomposition and nutrient turnover, with potential importance for the global CO₂ budget. As a result, studies on the RPE have rapidly increased in number over the last few years. Most of these experiments have been performed using unplanted soil as the control, which could potentially lead to incorrect assessment of the RPE. Therefore, we performed a greenhouse experiment to investigate how the choice of control (i.e., unplanted control and planted control) influenced the quantification of RPE on SOM decomposition and gross nitrogen (N) mineralization, and to link this to differences in microbial and abiotic soil properties between the two controls. In the planted control, planted seedlings were cut at soil surface 5 d before measurement of the RPE. The RPE on SOM decomposition was positive in pine soil and almost 2-fold higher when calculated from the planted control than from the unplanted control. In spruce soil, a negative RPE on SOM decomposition was found when calculated from the planted control, while the RPE was positive when calculated from the unplanted control. No RPE on gross N mineralization was found when calculated from the planted control, while a positive RPE of more than 100% was found when calculated from the unplanted control. The microbial biomass and growth rate were lower, while the inorganic N content was higher in the unplanted control than in the planted control. The microbial community composition and potential enzyme activity in the planted treatment and planted control were similar, but they differed significantly from those in the unplanted control. The results showed that the RPE varied widely depending on the choice of control; thus, we suggest that a planted control, in which the aboveground plant parts are removed only a few days before the measurement of RPE, should be used as the control when elucidating the RPE on belowground C and N cycling responses to environmental change.