CONTROL OF SURFACE SOIL CHEMISTRY IN EARLY-SUCCESSIONAL FLOODPLAIN SOILS ALONG THE TANANA RIVER, INTERIOR ALASKA

Surface soils on recently deposited alluvium along the Tanana River, Alaska, have an elevated pH and are high in salts such as calcium carbonate and calcium sulfate. With advancing plant succession surface soil chemistry changes, and when the alder - balsam poplar stage is reached these salt deposits are absent. To test the hypothesis that soil surface salt (salt crust) accumulates as a result of the combined action of capillary rise of groundwater and surface evaporation, a treated plot experiment was undertaken in the early open willow stage on the floodplain where the soil surface was devoid of litter. Treatments, applied in early summer, were salt crust removed, leaf litter added on the surface (evaporation barrier), and polyethylene sheeting installed at a depth of 30 cm (capillary rise barrier). Surface 5-cm soil samples, collected in September, showed significant reductions from salt crust removal and surface litter application in electrical conductance and amounts of bicarbonate, sulfate, calcium, magnesium, potassium, and sodium. In clearings within dense alder - balsam poplar stands on the first terrace, forest floor removal resulted in significantly increased amounts of surface soil sodium, sulfate, calcium, and magnesium. However, this treatment did not result in re-establishment of the salt crust, probably because of a higher ground surface elevation at this terrace and consequent deeper water table. On lower, early-successional terraces salt crusts may form in a period of only several week's if precipitation is low. The advent of a complete litter layer has great significance in the successional sequence on these soils not only because of nutrient cycling as a result of decomposition processes but also because of the probable physical effect of severely decreasing rates of evaporation from the mineral soil surface. In the latter case, although we did not directly measure mineral soil surface evaporation rates with and without litter layers, movement of salt solution injected at 25 cm in the soil profile to the surface was restricted by application of a surface litter layer.