Soil Soluble Organic Nitrogen (SON) is considered a potential source of N for utilization by microorganisms and plants, and plays a key role in N cycling in forest ecosystems. Land-use change can have significant impacts on soil SON and associated microbial processes. In this study, three adjacent natural and plantation forest sites (Toorbul, Toolara, and Blackswam) in southeast Queensland, Australia, were selected to investigate the effects of vegetative conversions on soil SON and microbial properties. Soils studied were all sandy, with total carbon (C) in surface layer (0-10 cm) ranging from 1.4% to 2.1%, total N from 0.045% to 0.088%, and pH value from 4.1 to 5.5. Soils in these three sites belong to Acrisol, Alisol and Podzol (FAO 1974), respectively. Natural forests are dominated by Eucalypt microcorys, E. pilularis, E. racemosa, E. siderophloia and E. intermedia, while adjacent plantation forest species include 20-25 year old Pinus elliottii var elliottii and P. caribaea var hondurensis. In general, soils under natural forests had higher soil total C and N contents with lower C:N ratio compared with those under plantation forests, which was related to lower C:N ratio in leaf litters under natural forests. The NH4+-N was the predominant form of mineral N (67-96%) while concentrations of NO3—N were low in most soils under both natural and plantation forests. Concentrations of NH4+-N in soils under natural forest were greater than under the adjacent plantation forests. Significant amounts (12.9-42.7 mg N kg-1 or 15-47 kg N ha-1 at the 0-10 cm depth) of SON (extracted by 2 M KCl) were present in soils under both natural and plantation forests, comprising 34-53% of total soluble N and 2.9-5.6% of total soil N. Concentrations of SON in soils under natural forests were generally higher than under plantation forests, while the trend in Soluble Organic C (SOC) was the same. The similar trends were also found in concentrations of NH4+-N, SON and SOC in hot water extracts of soils under both natural and plantation forests. Microbial biomass C and N and microbial activity (CO2 respiration) were generally greater in soils under natural forest compared with those under plantation forests. Results from the Biolog GN2 plate and EcoPlate reading also showed the vegetative conversion had altered soil microbial diversity. Pearson correlation analysis has showed strong relationships among soil total C and N, SON extracted by 2 M KCl and hot water, and microbial biomass C and N and respiration. Results from this study have demonstrated that shifts in tree species could change the nature of organic inputs into soil, leading to altered soil microbial community and associated microbial processes and then liability of soil N in forest ecosystems.