[Research advance on molecular ecology of asymbiotic nitrogen fixation microbes.]

Nitrogen is the main limiting factor in the productivity of ecological system, and biological nitrogen fixation is the main nitrogen source in the natural ecosystem. Biological nitrogen fixation includes 3 types: symbiotic, associate and free-living nitrogen fixation. Associate and free-living nitrogen fixations are collectively called asymbiotic nitrogen fixation. Compared with symbiotic system, asymbiotic nitrogen fixation rate is lower, but asymbiotic nitrogen fixation microorganisms can survive and fix nitrogen without forming symbiotic structure with plants, therefore play an important role in nitrogen cycling, especially nitrogen input in ecosystems, due to their wide distribution and high adaptability to different environments. In this review, we mainly introduced the research progress of asymbiotic nitrogen fixation microorganisms in terms of diversity, distribution characteristics, the factors influencing asymbiotic nitrogen fixation efficiencies in soils and phyllosphere, and also highlighted the existing problems and future perspectives in this research field.