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 floodplain of interior Alaska. The objectives of this study were to describe the soil chemical properties and to examine controls on these chemical properties along a forest primary-successional sequence. Soil saturation pastes were prepared from duplicate sites along the successional sequence representing bare alluvium (stage I), open willow (stage (III), poplar-alder (stage V), and white spruce (stage VIII). Calcium, Mg, SO4, and ions responsible for alkalinity were the dominant solutes in the saturation extracts. Soil horizons were generally calcareous (CaCO3) and therefore alkaline (pH > 7.0) across the successional sequence. The CaCO3-containing soil horizons were saturated to supersaturated with respect to calcite. Many soil horizons in the plots of stages III and V from one site were saturated with respect to gypsum (CaSO4.2H2O), whereas other stages and sites were generally undersaturated. Phosphate availability in CaCO3-containing soil horizons was apparently constrained by CaCO3 solubility and the solubility products of beta-tricalcium phosphate and hydroxyapatite. There was a strong positive relationship between soluble salt content and silt concentration in the early stages, suggesting that initial salt content is controlled by the texture of the alluvial material; the variation in initial texture largely accounts for the site to site variation in salts. Evaporation apparently plays a role in concentrating soluble salts at the surface during the early stages (III and V), but by the late stages (VIII), the biotic 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, evaporation) are most important early in the successional sequence and that biotic factors (transpiration, acid leaching) are most important late in the successional sequence in controlling salt distribution in soils on the Tanana River floodplain.