Sugarcane, as a worldwide cultivated commercial crop, has drawn a great attention towards its nitrogen (N) use efficiency as well as potential effects on associated watersheds. Particularly, there is an increasing concern about potential loss of carbon (C) and nitrogen from sugarcane fields and its environmental impacts on the Australian Great Barrier Reef catchments. Sugarcane field management practices play a vital role in refining soil C and N cycling and improving plant N use efficiency. The common management practices in sugarcane fields such as liming, legume intercropping, green harvest and burning have different effects on soil C and nutrients dynamic. To be specific, liming is widely used as an effective amendment to remediate soil acidity. Moreover, green harvest, which is known as trash blanket practice, is promoted in intensive cropping regions as decomposition of sugarcane trash can sequester C and N under the trash blanket layer. Although, previous studies had investigated the effects of liming and trash blanket on cropping fields, but the understandings of how liming and trash blanket affect Australian sugarcane cropping system, is largely unknown. Additionally, some studies have suggested that sugarcane compost may provide significant amounts of bioavailable C and N to sugarcane cropping system. However, different composition of compost have different influence on soil properties such as soil pH, soil C and N contents. The effects of compost on soil properties may also vary with its application methods as well as soil and vegetation type. Unfortunately, the scientific knowledge regarding the effects of composted sugarcane residue on Australia sugarcane fields remain uncertain. Although several studies have reported the relationship between the soil nutrients cycle and field managements, the mechanistic understanding of how sugarcane field managements affect the N dynamics remained unclear. The objectives of this thesis were to determine: 1) the impacts of liming on the N availability and dynamics in tropical acidic sugarcane soils; and 2) the dynamics of soil C and N in sugarcane cropping fields after application of subsoil organic amendments to sugarcane soil. Two field experiments were designed to achieve these major goals, including: a) Impacts of liming on soil N dynamics in sugarcane fields in Bundaberg; b) Impact of subsoil organic amendments to sugarcane cropping system on long term soil physical, chemical and biological properties in Maryborough, Australia. It was hypothesized that: 1) liming practice in acidic soils would alter soil N dynamics as well as the abundance and activity of soil microorganisms; and 2) subsoil amendment of compost would contribute substantially to the labile C and N supply for soil microbial activity as well as plant uptake and consequently increase sugarcane yield. In a field trial at Bundaberg, Australia, the adjacent sugarcane treatments of 26 months after liming (26ML), 14 months after liming (14ML) and no lime amendment (CK) were selected to investigate the effect of liming practice on soil N bioavailability and microbial activity in a long term subtropical sugarcane cropping system. Liming in both 14ML and 26ML treatments significantly increased soil pH (by 1.2-1.4 units) and exchangeable Ca2+ (> 2 folds) content compared with the CK treatment. The lower concentrations of hot water extractable organic C, total N and NH4+-N in the 14ML treatment compared to the CK and 26ML treatments, was attributed to the absence of trash blanket placement in the former and enhanced N immobilization, by soil microbial community, due to improved soil pH. Liming practice (14ML and 26ML) increased soil microbial biomass C and N contents, particularly in the presence of the trash blanket (26 ML treatment), while decreased soil respiration and qCO2, indicating that acidic stress conditions were relieved in the liming treatments. Soil pH value was shown to be the main factor governing soil enzyme activities, with an overall decrease in all enzyme activities in response to liming practice. In a field trial at Yerra, Maryborough, Australia, Four subsoil amendment treatments, including no amendment as control (CK), gypsum subsoil application as calcium enriched treatment (GP), compost subsoil application as organic ameliorant treatment (CP) and mineral fertilizer subsoil application as fertilizer treatment (FE), were designed to investigate the effect of subsoil amendments on sugarcane soil health. The ameliorants were applied to the subsoil two weeks before sugarcane planting. At the end of the third ratoon harvest, the compost treatment showed a slightly higher soil pH compared to other three treatments. The concentration of NH4+-N in the CP treatment, at the 0-10 cm soil depth, was significantly higher than gypsum (GP) and mineral fertilizer (FE) amended treatments. This may be due to the higher N uptake at the sugarcane root zone (10-25 cm depth) in this treatment and consequently transport of higher amount of N to the topsoil through trash blanket decomposition after sugarcane harvest. The highest concentrations of hot water extractable organic C and N (HWEOC; HWETN) in the CP treatment (10-25 cm depth), can be attributed to the organic matter input and its decomposition within the compost application layer. Soil MBC and MBN contents (0-10 cm depth) generally tend to be higher in the CP treatment in comparison with the other treatments. This may be due to leaching of soluble organic C and N in this treatment, which provide an available energy source for utilization of soil microbial community. The CO2 respiration in the CP and FE treatments, were significantly higher than CK and GP treatments, while GP treatment showed the lowest CO2 respiration in the 10-25 cm depth. At soil depths below 25 cm, soil labile C becomes a limiting factor for soil CO2 respiration, which is supported by significant positive correlation found between HWEOC and soil CO2 respiration. Significant increase in sugarcane yield, over 4 years, has been observed in the CP compared to other treatments, which may be due to the increased labile C and N pools provided by the slow decomposition of subsoil-applied compost.