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

1993; Parsons et al., 1993; Reiners et al., 1994; Parsons and Keller, 1995). Land use changes, such as deforestation and reforestation, modify Re-establishing trees, especially fast-growing N-fixing not only the organisms inhabiting affected areas, but also abovespecies, in deforested grassland landscapes can reverse and belowground environments. Topography further influences local changes in soil structure, chemical properties, and N vegetation and environment. Effects of topography and re-establishment of N-fixing koa (Acacia koa A. Gray) trees in 100-yr-old transformations induced by land use change. Fisher montane grassland on surface soil properties, N pools, and N transfor(1995) reported that within 4 yr of planting in abanmations were assessed using standard and 15N-isotope pool dilution doned pasture, native Costa Rican trees reduced soil methods. Data were collected for soils on slopes and in drainage bulk density and increased cation concentrations, and bottoms located in grassland, under 9to 11-yr-old planted koa, and some N-fixing species significantly increased total N in nearby old-growth koa-ohia (Metrosideros polymorpha Gaud.) concentration. Montagnini and Sancho (1990) found forest. Soil under planted koa had significantly lower fine soil bulk that within 2.5 yr of planting, soils under N-fixing species density and pH than soil in grassland, and had concentrations of total had greater organic matter content, total N, and exC, total N, and extractable Mg and C/N ratio intermediate between changeable K and Mg than pasture soils. After 4 yr, net grassland and forest. Topographic position affected pH and concentranitrification potentials were greater under plantations tions of total C, total N, and extractable P. Ten years after koa trees were re-introduced to grassland, the concentration of soil NH4 –N had of N-fixing species than under non N-fixing species, and increased above levels found even in intact forest, and the concentrathat net nitrification rates were comparable with those tion of NO3 –N was intermediate between forest and grassland. Amunder older secondary forests (Montagnini and Sancho, monium dominated the inorganic N pools in grassland soil and 1994). Rhoades et al. (1998) found NO3 pools were four NO3 dominated in forest soil. Under planted koa, NH4 domination times greater and nitrification rates five times faster unwas beginning to give way to NO3 domination. Soil microorganisms der N-fixing trees than under pasture grasses. Scowcroft were potentially strong competitors for inorganic N on grassland and Jeffrey (1999) reported significantly increased N slopes where they immobilized nearly all the inorganic N that was availability in surface soil as a result of grassland reforesproduced during shortand medium-term field incubations, thus leavtation with koa, a N-fixing tree endemic to Hawaii. Thus, ing little for plant growth. Re-establishment of koa trees increased re-establishing tree cover in anthropogenic grasslands soil N availability. can affect soils, but it is not the only factor at work. Interactions between recovering forest vegetation and physiography can influence changes in soil properL use changes, such as conversion of forest to ties and processes. Studying high elevation grassland in grazed grassland, can drastically change plant speHawaii, Scowcroft and Jeffrey (1999) showed that soils cies composition, and dominant life forms, which in turn located in drainage bottoms supported greater amounts markedly affect soils and soil processes (e.g., Neill et al., of herbaceous biomass and had greater amounts of total 1999; Chen et al., 2000; McGrath et al., 2001). Reiners et N, extractable NH4 , P, and Ca than soils on slopes. al. (1994) reported decreased acidity, decreased effective Similar microrelief influences on spatial variability of cation exchange capacity (CEC), increased bulk density, vegetation and soil properties have been observed in a decreased soil porosity, greater concentrations of NH4 , number of temperate grassland ecosystems (Barnes and lower concentrations of NO3 , and lower rates of net Harrison, 1982; Schimel et al., 1985; Aguilar and Heil, N-mineralization following conversion of lowland tropi1988; Burke et al., 1989a, 1989b; Frank et al., 1994; cal rainforest to pasture grasses. Piccolo et al. (1994) found Frank and Groffman, 1998; Corre et al., 2002; Verchot that soils under pasture grasses had smaller extractable et al., 2002). Typically these studies found that moisture inorganic N pools than forest soils, NH4 dominated the content, total C, N, and P, gross NH4 mineralization inorganic pools under grasses, and net N-mineralization and immobilization, N availability, and denitrification and net nitrification were slower under grasses. Pasture potential increased systematically along a gradient from grasslands created from lowland tropical forests have hilltops to drainage bottoms (for opposite effects see shown N limitations to plant productivity (Keller et al., Yonker et al., 1988; Turner et al., 1997). Studies in forested landscapes also have shown topoP.G. Scowcroft and J.E. Haraguchi, Inst. of Pacific Islands Forestry, graphic effects on soil chemical properties and N transPacific Southwest Res. Station, USDA Forest Service, 1151 Punchformation, but changes across toposequences are more bowl St., Suite 323, Honolulu, HI 96813; N.V. Hue, Dep. of Tropical variable than for grasslands. Silver et al. (1994) found Plant and Soil Sciences, Univ. of Hawaii at Manoa, 1910 East-West that exchangeable base cation concentrations and pH Road, Honolulu, HI 96822. Received 10 Sept. 2002. *Corresponding author ([email protected]). increased from ridge tops to riparian valleys, while soil Published in Soil Sci. Soc. Am. J. 68:959–968 (2004).  Soil Science Society of America Abbreviations: APE, atom percent enrichment; asl, above sea level; CEC, cation-exchange capacity; SOM, soil organic matter. 677 S. Segoe Rd., Madison, WI 53711 USA