Impact of rhizosphere priming on soil organic carbon dynamics: Insights from the perspective of carbon fractions

In the rhizosphere zone, soil microbes can be stimulated to increase the mineralization of soil organic matter (SOM), which is known as the rhizosphere priming effect (RPE). Soil organic carbon (SOC) can be separated into the coarse fraction (CF-SOC; fast turnover) and the fine fraction (FF-SOC; slow decomposition); however, the turnover of CF-SOC or FF-SOC during the rhizosphere processes remains unclear. Here, C4-plants (maize and sorghum) grew into C3-soil (paddy soil and lou soil), and the dynamic of the carbon fractions (CF-SOC and FF-SOC) and RPEs were monitored using the 13C natural abundance method during three plant growth stages. Positive RPEs were observed in most situations. There was a net SOC loss in both soils at the end of cultivation. The “new” plant-derived C can partially offset the SOC mineralization caused by rhizosphere priming. The formation of “new” FF-SOC was 1.51–60.20 times higher than that of “new” CF-SOC. Growing plants enhanced the sequestration of the “old” CF-SOC in comparison with the control. Moreover, the RPE strengthened the loss of “old” FF-SOC in the lou soil. The shift in microbial variables was found to strongly regulate SOC sequestration, which accounted for 38 % and 70 % of the variations in SOC in paddy and lou soil, respectively. The activated Gram-positive or Gram-negative bacteria were positively associated with soil CF-SOC and negatively correlated with soil “old” FF-SOC, indicating that stimulated rhizosphere bacteria enhanced the formation of “new” or “old” CF-SOC and the loss of “old” FF-SOC by co-metabolism and fast turnover during the priming process. Overall, our study provides insights into C fractions and microbial control of SOC turnover during the rhizosphere processes, which may have significant implications for better understanding the soil C cycle and increasing SOC sequestration.