Reforestation and Topography Affect Montane Soil Properties, Nitrogen Pools, and Nitrogen Transformations in Hawaii

Land use changes, such as deforestation and reforestation, modify not only the organisms inhabiting affected areas, but also above‐ and belowground environments. Topography further influences local vegetation and environment. Effects of topography and re‐establishment of N‐fixing koa ( Acacia koa A. Gray) trees in +100‐yr‐old montane grassland on surface soil properties, N pools, and N transformations were assessed using standard and 15 N‐isotope pool dilution methods. Data were collected for soils on slopes and in drainage bottoms located in grassland, under 9‐ to 11‐yr‐old planted koa, and in nearby old‐growth koa‐ohia ( Metrosideros polymorpha Gaud.) forest. Soil under planted koa had significantly lower fine soil bulk density and pH than soil in grassland, and had concentrations of total C, total N, and extractable Mg and C/N ratio intermediate between grassland and forest. Topographic position affected pH and concentrations of total C, total N, and extractable P. Ten years after koa trees were re‐introduced to grassland, the concentration of soil NH 4 + –N had increased above levels found even in intact forest, and the concentration of NO 3 − –N was intermediate between forest and grassland. Ammonium dominated the inorganic N pools in grassland soil and NO 3 − dominated in forest soil. Under planted koa, NH 4 + domination was beginning to give way to NO 3 − domination. Soil microorganisms were potentially strong competitors for inorganic N on grassland slopes where they immobilized nearly all the inorganic N that was produced during short‐ and medium‐term field incubations, thus leaving little for plant growth. Re‐establishment of koa trees increased soil N availability.