The nitrogen (N) cycle is one of the key biogeochemical processes in terrestrial ecosystem which is interlinked with some important soil N transformations, notably soil N mineralization, nitrification and denitrification through both biotic and abiotic mechanisms. Global climate change and soil management practices have disrupted the soil N cycling processes. Soil types, plant species and vegetation diversity and management practices are also associated with changing N cycling processes. Prescribed burnings are applied periodically for fuel reduction to reduce the risk of wildfires. Nevertheless, high frequency burning may limit N availability, and make soil more N and water limited. Biochar is a carbon-rich product produced from organic materials using pyrolysis process and has the potential to modify soil N transformations and N retentions by improving soil-plant water relationships and N availability. In the soil-plant system, biochar can recover and retain N by controlling efficient inorganic N use as well as by recycling the N in a balanced manner which determines the flow, direction and availability of soil N and directly influences N use efficiency by plants and controls the productivity of terrestrial ecosystem. However, the effects of biochar on soil N processes are determined by biochar characteristics and soil properties. Biochar pyrolysis temperatures directly influence biochar properties, for example, the surface area and porosity of biochar increase with increasing pyrolysis temperature, leading to enhanced soil N retention and improve water holding capacity (WHC). Additionally, biochar application rate is another factor to consider because large amounts of applied biochar can impact the N cycle by altering soil pH, enhancing N immobilization, leading to changes in N availability. However, the quantitative information about the relationships between biochar pyrolysis temperature and the mechanisms regulating soil N cycling processes as well as biochar application rates and the mechanisms determining whether and how biochar application rates would affect soil N cycling processes have not been well studied especially under field conditions. Therefore, my thesis aimed to examine how biochar application would enhance soil N transformations and N retention by improving soil-plant water relationships and N availability in the N cycling processes in the forest soils of southeast Queensland, Australia. Briefly, in Chapter 2, I assessed how pyrolysis temperature would affect biochar properties and subsequently soil N transformations through a short term laboratory incubation study at two moisture levels. In Chapter 3, I examined how pyrolysis temperature dependent biochar would influence soil labile C and N pools and microbial biomass under the same experimental condition of Chapter 2. To evaluate the interactions between soil N transformations and soil- plant-biochar systems under field conditions, in Chapter 4, I investigated the soil-plant-biochar relationships by examining how biochar and understorey Acacia species would affect the biological N fixation (BNF) and water use efficiency (WUE) of understorey Acacia species as well as soil C and N pools 15 months after biochar application post fire (nearly 3 years after controlled prescribe burning) in suburban native forest of subtropical Australia. Finally, in Chapter 5, I set up a laboratory incubation study to assess the effects of biochar application rates and understorey Acacia species (Acacia leiocalyx and Acacia disparrima) on soil N pools and transformations post fire soil following a 5-day laboratory incubation at two moisture levels (i.e. 60% and 90% WHC). In Chapter 2, I set up a laboratory incubation for 5-days with pine wood (Pinus radiata) biochar at a rate of 5% (w/w) which was produced under six pyrolysis temperatures (e.g. 500, 600, 650, 45 700, 750 and 850°C). I used 15N natural abundance (δ15N) of inorganic N (NH4+-N and NO3--N) to assess the potential of biochar materials in facilitating forest soil N transformations at two soil moisture levels of 50% and 65% WHC. This study revealed that pyrolysis temperature had significant effects on biochar total N and δ15N. Cumulative nitrification and N mineralization were significantly lower in the biochar amended soils than those of the control soil, with significantly lower δ15N of NH4+-N and δ15N of NO3--N in the biochar amended soil. In this 51 study, nitrification was the key driver of soil N mineralization. Additionally, cumulative nitrification and N mineralization responded non-linearly to the increase in pyrolysis temperature. In this study, an optimum pyrolysis temperature range of 600-700°C was identified for improving soil nitrification and N mineralization under the laboratory incubation conditions whereas the greater cumulative nitrification and N mineralization were found at the 65% WHC compared with those at 50% WHC. In Chapter 3, I measured water extractable organic C (WEOC) and total N (WETN), hot water extractable organic C (HWEOC) and total N (HWETN), microbial biomass C (MBC) and N (MBN) as well as mineral N (NH4+-N, NO3--N) to understand the key mechanisms and identify the main indicators of soil quality which can determine soil labile C and N pools through a 5- day laboratory study using different pyrolysis temperature dependent biochar (500-850°C) at two moisture levels of 50% and 65% WHC. This study showed that WETN was significantly lower in the biochar amended soils compared with those of the control soil. WETN was the most sensitive indicators for determining the changes in soil labile C and N pools and had a significant positive correlation with soil MBN, suggesting that microbial biomass would be able to use water extractable N as energy sources for metabolism purposes. Biochar application significantly reduced soil NO3--N and increased N retention in Yarraman soil by enhancing N immobilization due to increased C input from biochar. The 65% WHC had generally greater soil labile C and N pools compared with those of the 50% WHC. In Chapter 4, I used the soil and foliar samples from a suburban forest where pine wood biochar (600°C) was applied at the rate of 0, 5 and 10 t ha-1, 20 months after prescribed burning. I collected the samples months after biochar application. I used N and C isotope compositions (δ15N and δ13C) to assess the BNF and WUE of two understorey Acacia species (A. leiocalyx and A. disparrima). I also examined soil C and N pools and their stable isotope compositions (δ15N and δ13C). This study revealed that biochar did not affect the BNF and WUE 15 months after biochar application. However, BNF varied significantly between the Acacia species and were significantly greater for A. leiocalyx compared with those of A. disparrima; suggesting that understorey A. leiocalyx was more effective in improving N recovery after prescribed burning via BNF as reflected in more N availability in the soil compared with that of A. disparrima in the suburban native forest of subtropical Australia. Soil NH4+-N was significantly lower in the biochar amended soils compared with that of the control soil due to the surface adsorption and N immobilization. The significant positive relationship between soil δ15N (10-83 20 cm) and foliar δ15N highlights that the mechanisms influencing soil δ15N also influence plant δ15N through N uptake. In Chapter 5, I collected soil from a depth of 0-5 cm from biochar treated forest soil 15 months after biochar application and conducted a 5-day laboratory incubation study. I used δ15N approaches to assess and measure the soil N pools and N transformations. Biochar application at the rate of 10 t ha-1 had lower soil NH4+-N and lower cumulative ammonification in the soil which would help improve N availability by reducing mineral N losses. Day 5 NH4+-N was significantly greater at 90% WHC compared with that of 60% WHC whereas Day 5 δ15N of NH4+-N showed an opposite result, indicating a negative liner relationship between NH4+-N and δ15N of NH4+-N . These results demonstrated that at the greater soil moisture level of 90% WHC, N availability in the form of NH4+-N was greater due to greater N mineralization of organic N with lower δ15N, leading to lower δ15N of NH4+-N. Hence, δ15N of organic and inorganic N in the forest soil could be a useful tool for distinguishing the contributions of different processes such as BNF from soil N transformations as well as N loss mechanisms in the soil N cycle.