Soil-solution chemistry in relation to forest succession on the Tanana River floodplain, interior Alaska

Abstract

The chemical composition of soil solution reflects the demand of soil biological processes and the solubility and ion-exchange equilibria between physical and biological components of the soil. The objectives of this study were to document soil-solution chemistry for representative phases of the primary successional sequence on the Tanana River floodplain near Fairbanks, Alaska, and to assess the effect of physical versus biological control on solution chemistry in these sites. Soil-solution samples were collected weekly using porous-cup soil-solution samplers located 20, 50, and 150 cm below the soil surface. In addition, groundwater and river water samples were collected at several sites that represented the successional stages typical of the Tanana River floodplain of interior Alaska. Magnesium, HCO3, Cl, Na, K, NO3, and PO4 showed the highest concentrations in the 50-cm layer at each site. Manganese, Fe, and Zn showed the highest concentrations at the groundwater level. Aluminum and Ca showed decreasing concentrations with depth from the surface. Silicon displayed no specific depth trends. Ammonium was the only ion that was more concentrated in river water than in soil solution. Soil-solution pH showed no specific depth trends. Conductivity of the soil solution was generally lower at greater depths and was much lower in the river water. Sulfate, K, Ca, and Mn decreased in concentration from the early successional stages to the later successional stages, although some year to year variability did occur. All measured concentrations except Zn displayed at least one significant change in concentration due to vegetation clearing. These differences can be summarized broadly as effects on nonbiologically cycled nutrients in the open willow stage (III) and changes in the biological cycling of nutrients in the poplar–alder and mature white spruce stages (V and VIII, respectively). Significant increases in concentration of Fe and Mn were found at III-B-T (stage III–site B–treated (i.e., cleared) plot) while Na, Mn, Fe, Ca, Mg, Si, and SO4 increased at III-A-T. At V-A-T significant increases were found in concentrations of NH4, NO3, PO4, SO4, K, Na, Ca, Mg, Mn, Fe, Al, Si, HCO3, and conductivity in 987. These same trends were repeated in 1988 with the exception of NO3, Fe, and Si, which showed no significant differences. As a result of clearing both V-A-T and V-B-T, Fe and Si significantly decreased in concentration at 50 cm, which was the opposite trend found at 150 cm. The Cl concentration at 50 cm decreased at V-A-T in 1987 but increased in 1988 as a result of clearing. V-B-T showed no effect of clearing in 1988 and only NO3, SO4, Ca, Mg, and conductivity increased in 1987. Chloride, Al, and Si decreased in concentration as a result of clearing in 1987 at V-B-T. Nitrate, NH4, Cl, and pH increased, while PO4, K, Na, Ca, Mg, Si, HCO3, and conductivity decreased, as a result of clearing in VIII-A-T. At VIII-B-T, NO3, K, Ca, Mg, Cl, HCO3, and conductivity increased as a result of clearing in 1987, while only NO3 increased in 1988. Bicarbonate, Cl, and pH showed significant decreases in 1987 and 1988 at VIII-B-T. The results of this study, combined with the results of other studies of these salt-affected floodplain soils, detailing the soil environment and control of evaporative movement of water to the soil surface support the hypotheses that: (i) the genesis and maintenance of surface salt crusts are controlled by the soil physical and chemical environments encountered on early-successional mineral soils and (ii) the disappearance of salt crusts and reduction in mineral soil salt concentrations are controlled by forest succession, which mediates the changing soil physical, chemical, and biological environment.