Abstract: Recent researches have noted that biochar significantly increases soil carbon storage, improves soil fertility and maintains balanced soil ecosystem. Biochar also acts as soil fertilizer or amendment to enhance plant growth and increase crop yield by supplying and retaining soil nutrients. Biochar is also widely used in agricultural soils as soil conditioner. Furthermore, there is experimental evidence that soil microbial communities/activities which sustain soil health and functions are directly affected by biochar addition to soils. The full range of the processes and consequences of the effects of biochar remains poorly documented. In a pot maize cultivation experiment, different quantities of charcoals from burnt maize and rice straws were applied and the maize seedling height, biomass and rhizosphere soil nitrogen transformation, and the related microorganisms investigated. The research results showed that 60 g·kg^-1 maize biochar and 40 60 g·kg^-1 rice biochar significantly decreased maize seedling height (P < 0.05). Rice biochar presented more apparent effects than maize biochar. Aboveground biomass of maize seedling was also significantly reduced under 60 g·kg^-1 maize biochar or 20 60 g·kg^-1 rice biochar application. Under 60 g·kg^-1 maize biochar application, rhizosphere soil water content and microbial biomass N obviously improved. As the application level of the two types of biochar increased, the content of total nitrogen/nitrate and nitrogen fixation rate also appreciably increased, reaching peak levels at 60 g·kg^-1 maize or rice biochar application. Ammonia nitrogen content significantly improved under 40 g·kg^-1 maize biochar treatment. Moreover, each application level of the two types of biochar limited total bacterial count. It also promoted at different degrees of growth of nitrogen-fixing bacteria and cellulose degrading bacteria in rhizosphere soils, with 60 g·kg^-1 maize biochar as the most effective treatment. Overall, proper biochar application enhanced nitrogen cycle and transformation by influencing community structure of related microorganisms in rhizosphere soils. The effect of maize biochar was more obvious than that of rice biochar. The effects of biochar on nitrogen efficiency in soils were investigated from three aspects-crop growth, rhizosphere soil nitrogen transformation and the related microorganisms. The results detected the mechanisms of the effects of biochar application to soils on nitrogen availability. This provided the basis for the mechanisms of increasing soil carbon sequestration and decreasing the amount of nitrogen fertilizer input under biochar application to mollisols. This was profoundly significant in terms of protecting the fertility of black soils.