With widespread and frequently disparate effects on ecological systems and environmental quality, we have drastically changed the global nitrogen (N) cycle during the Industrial Revolution and the subsequent 150 years of fast development. Three key processes are responsible for the N cycling in terrestrial ecosystems: atmospheric N deposition, soil N transformations and biological N fixation (BNF). Among other factors, N saturation raises the danger of N leaching, soil acidification and denitrification. It is one of the potential threats that atmospheric N deposition poses to forest health, and concerns are being raised about the significant impacts on plant biodiversity and the state of the worldwide forests. Recently, biochar has been shown to be the most promising soil additive, which can reduce N losses and increase soil fertility. Biochar is a carbon-rich substance made from organic materials via the pyrolysis process. Applying biochar can increase soil water and nutrient availability, especially N, and decrease N losses. The biochar application rate can also impact soil N transformations and how it affects the soil microbial community. There is still a considerable knowledge gap in understanding the specific mechanisms via which biochar influences the soil N cycle and how it affects N availability to microbes and plants. In addition, it is not yet clear how biochar affects soil microbial N transformations, thereby influencing the soil N cycle and organic C pools. The necessary N transformations in the soil are all mediated by soil microorganisms. Although people have been interested in using biochar as a soil amendment to improve soil quality, improve and maintain soil fertility, and increase soil C sequestration, there is still a lack of clear understanding of the characteristics of biochar produced from different raw materials and under different pyrolysis conditions and its interaction with soil. Moreover, biochar can act as a soil amendment to affect nutrient cycling and plant growth by affecting microbial community composition and activity, water holding capacity and pH, as well as improving root growth. Labile C fractions of biochar may also accelerate the decomposition of old soil organic matter through the priming effect. The high-temperature biochar (600°C) used in this study usually has a higher surface area, porosity and alkalinity, thereby enhancing the soil water holding capacity and nutrient use efficiency of the soil and improving soil nutrient uptake by plants. However, the optimal application rate of high-temperature biochar and its relationship with plant-soil interactions have not been studied well. In order to fill these gaps, the following research was conducted to: (1) quantify the impact of biochar addition on soil inorganic N distribution (Chapter 5); (2) investigate how different biochar application rates affect the composition of the soil microbial communities involved in soil N cycling in the field experiment, and unravel potential mechanisms influencing these microbial community changes (Chapter 4 and 6); and (3) evaluate potential mechanisms of soil and plant N dynamics and their influence on plant growth under vary biochar application rates in the soil-plant systems (Chapter 3).