Nitrogen (N) is an essential nutrient for ecosystem function and plant growth. The N deposition and biological N fixation (BNF) are two important ways of N inputs into terrestrial ecosystems, particularly native forest ecosystems. The BNF, as a naturally N input process, is an environmentally benign process which typically occurs in symbiotic relationships between plants and rhizobia. However, increasing industrial activities have led to a rapid increase in the amount and rate of N emissions and subsequent increase of N depositions in suburban native forest. Human activities cause N emissions to enter the atmosphere in the form of gases, and are deposited locally and regionally. High levels of N deposition in the soil suppress BNF, reducing reliance of plants on BNF, thereby altering rhizobia populations and activity. Global warming also exacerbates extreme weather conditions causing further soil degradation. Frequent flood events cause N losses from soil through leaching, denitrification, and other pathways. Reasonable land management strategies need to be urgently developed to increase N utilization and cycling in an environmental manner. As a potential soil amendment, biochar addition can control N losses and promote nutrient cycling. As a porous carbon-rich material, biochar can supplement soil carbon (C) pool, promote soil organic matter (SOM) accumulation, and provide a high-quality habitat for plant rhizosphere microorganisms, facilitating microbially mediated soil N transformations. In addition, its adsorption of mineral N is beneficial for long-term utilization by plants and microorganisms and reduced N losses. However, biochar application rate needs to be considered, based on its effectiveness. Currently, biochar as a soil amendment in Australian subtropical forestlands has been poorly studied. It is unclear how biochar application rates directly affect soil N availability, and indirectly affect N transformations in soil-plant-microbe ecosystems. In addition, responses of legume species to external influences can lead to differences in BNF. It is necessary to investigate the responses of different legume species in BNF and microbially mediated soil N transformations to varying biochar application rates under frequent flood conditions. This study was conducted to achieve the following objectives: (1) to quantify the impact of biochar addition on leguminous species growth and BNF, as well as the responses of soil chemical and biological processes to both biochar application and leguminous species; (2) to quantify the effects of biochar application rate and leguminous species on labile C and N pools in the top 20 cm soil profile; and (3) to investigate the short-term transformations of mineral N in soil under different levels of biochar application rates and leguminous species, as well as different soil moisture contents. [...]