Abstract: Priming effect (PE) is defined as the changes of soil organic carbon mineralization after the inputs of external labile organic matter in short term. The PE is a natural phenomenon that occurs in various terrestrial ecosystems and plays an important role in soil carbon cycling. The responses of soil microbes to the inputs of external labile organic matter are the internal drivers for PE. The microbial mechanisms of PE included: (1) cometabolism of a group of microorganisms and extracellular enzymes in the presence of labile organic matter; (2) labile organic matter provides energy for the special microorganisms to degrade recalcitrant organic matter under nutrient constraint; (3) microorganisms preferentially utilize the readily degradable substrates which match their stoichiometric demands. There is no consensus on those mechanisms and on the dominant soil microbes in the PE. Besides, the decomposition of soil organic matter and external organic matter is affected by the quantity and quality of external organic matter, all of which may influence PE. Soil temperature and moisture also affect the PE, but their importance may be less than external organic matter. In view of the existing controversy and deficiency about the PE studies, we propose that future research should adopt new methods to further clarify the contributions of various microbes to the PE, and should focus on the imbalance between microbial demand and external organic matter in order to promote the integration and development of the PE studies and the theory of ecological stoichiometry.

Key words: urban vegetation, formation mechanism, adaptive strategy, vegetation classification.