CONTROL OF SOIL DEVELOPMENT ON THE TANANA RIVER FLOODPLAIN, INTERIOR ALASKA

Alluvial soils on the Tanana River floodplain near Fairbanks, Alaska, were examined for development of physical and chemical properties in r elation to soil depth and across a 200-year vegetation development seq uence. Development was mediated by ecosystem controls including succes sional time, vegetation, terrace height, soil physical and chemical pr operties, and microclimate. These controls interact and are conditione d by the state factors time, flora, topography, parent material, and c limate, respectively. On early-successional (<5 years) lower alluvial surfaces, terrace height above groundwater, soil particle size, and mi croclimate (through soil surface evaporation) interacted through capil lary rise to produce salt-affected surface soil. Calcium salts of carb onate and sulfate were the principal chemicals encountered in these so ils. Establishment of a vegetation cover between 5 and 10 years introd uced evapotranspiration as a new mechanism, along with capillarity, to control moisture suction gradients. In addition, newly formed surface litter layers further helped eliminate evaporation and formation of h igh salt content surface soil. Continued sedimentation raised terrace elevation, so on older terraces only infrequent flood events influence d soil development. Moreover, in these successional stages, only the h ighest river stages raised groundwater levels, so transpiration and ca pillarity influenced water movement to tree root systems. During the f irst 25-30 years of succession, plant deposition of organic matter and nitrogen, associated with the growth of alder, markedly changed soil properties. Nearly 60% (or 240 g . m-2) of the 400 g. m-2 nitrogen enc ountered at 100 years was accumulated during this early period. After 100 years of vegetation development, soil carbonate content dropped to about half the peak values of about 1600 g. m-2 encountered between 4 and 25 years. By the time white spruce was the dominant forest type a t 180 years, carbonate carbon declined to about 500 g . m-2, one-third that of the 1600 g . m-2 high. By this time surface soil pH declined from high values of 7.5 to between 5.5 and 6.0. Organic carbon continu ed to accumulate to about 6300 g . m-2 in the white spruce stage. twic e that encountered in the alder poplar stage at 25 years. Indices of m oisture retention were most strongly related to either soil particle s ize (low moisture tension and available moisture range) or vegetation- mediated soil organic matter content (high moisture tension). Cation e xchange capacity was most strongly related to a vegetation-mediated in dex of organic matter (OM) content (%N, %C, or %OM).