Biological mechanism of effects of root exudates on rhizosphere priming effect：A review

The dynamic balance of soil organic carbon is crucial for soil carbon cycling efficiency and global carbon cycle management. Root exudates，as root-derived external material，interact with soil microorganisms，which would exert crucial impact on the rhizosphere priming effect. By analyzing relevant literature on the rhizosphere priming effect， this article comprehensively reviewed the current research hotspots，future research directions and deficiencies regarding the rhizosphere priming effect. According to the direction and intensity of the rhizosphere priming effect，it summarized the positive priming effect mainly based on the microbial activation theory，the microbial co-metabolism hypothesis，and the nitrogen mining hypothesis；the negative priming effect mainly based on the substrate preference theory，the nutrient competition hypothesis，and the microbial residue recycling theory；and the biological mechanisms of positive/negative priming effect mainly based on the stoichiometry theory. Through analyzing the biological mechanisms of different rhizosphere priming effect，it summed up the influencing factors of the rhizosphere priming effect，described the internal and external connections among different biological mechanisms，and emphasized the impacts of microorganisms，extracellular enzymes，rhizosphere nutrient conditions，ratio of carbon to nitrogen，root structure，and types of root exudates on the priming effect mediated by root exudates. Root exudates and rhizosphere nutrient conditions would interact with the species，activity and quantity of microorganisms，forming a complex network relationship，which ultimately jointly determined the direction and intensity of the priming effect. It is concluded that most of the current researches on the rhizosphere priming effect focused on using cultivation control methods and adding simple low-molecular-weight substrates， such as sugars，organic acids and phenolic acids，to study their impacts and mechanisms on the priming effect. In the future，attention should be paid to the exploration of research methods for the soil organic carbon priming effect under real environmental conditions and strengthening specific research on the biological and non-biological regulation mechanisms of the priming effect. This article provided theoretical reference basis for in-depth research on the mechanism of the priming effect induced by root exudates，impro-ving soil carbon cycling efficiency，and further realizing the national“dual carbon”goals and promoting high-quality agricultural development.