THE SOIL CHEMICAL ENVIRONMENT ALONG A FOREST PRIMARY SUCCESSIONAL SEQUENCE ON THE TANANA RIVER FLOODPLAIN, INTERIOR ALASKA

Alkaline soils exist under forest vegetation on the Tanana River flood plain of interior Alaska. The objectives of this study were to describ e the soil chemical properties and to examine controls oil these chemi cal properties along a forest primary-successional sequence. Soil satu ration pastes were prepared from duplicate sites along the successiona l sequence representing bare alluvium (stage I), open willow (stage II I), poplar alder (stage V), and white spruce (stage VIII). Calcium, Mg , SO4, and ions responsible for alkalinity were the dominant solutes i n the saturation extracts. Soil horizons were generally calcareous (Ca CO3) and therefore alkaline (pH > 7.0) across the successional sequenc e. The CaCO3-containing soil horizons were saturated to supersaturated with respect to calcite. Many soil horizons in the plots of stages II I and V from one site were saturated with respect to gypsum (CaSO4.2H2 O), whereas other stages and sites were generally undersaturated. Phos phate availability in CaCO3-containing soil horizons was apparently co nstrained by CaCO3 solubility and the solubility products of beta-tric alcium phosphate and hydroxyapatite. There was a strong positive relat ionship between soluble salt content and silt concentration in the ear ly stages, suggesting that initial salt content is controlled by the t exture of the alluvial material: the variation in initial texture larg ely accounts for the site to site variation in salts. Evaporation appa rently plays a role in concentrating soluble salts at the surface duri ng the early stages (III and V), but by the late stages (VIII), the bi otic and topographic factors apparently reduce surficial evaporation. This reduction in evaporation, coupled with the production of organic acids, leads to a 42% loss of soluble salts from the soils. This study supports our original hypotheses that physical factors (texture, evap oration) are most important early in the successional sequence and tha t biotic factors (transpiration, acid leaching) are most important lat e in the successional sequence in controlling salt distribution in soi ls on the Tanana River