Weekly Soil Solution Nutrient Concentrations in Bonanza Creek Experimental Forest for the summers of 1985-1988

The chemical composition of soil solution reflects solubility and ion exchange equilibria between a number of 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 and to assess the effect of vegetation clearing in these sites. Soil solution samples were collected on a weekly basis using porous cup soil solution samplers located at 20, 50, and 150 cm below the soil surface. In addition, ground water and river water samples were collected at several sites which 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 highest concentrations at the ground water level. Aluminum, and Ca showed decreasing concentrations with depth from the surface. Silicon displayed no specific depth trends. Ammonium was the only ion which was more concentrated in river water than in soil solution. Soil solution pH tended to show no specific depth trends. Conductivity of the soil solution was generally lower at deeper depths and was much lower in the river water. Sulfate, potassium, calcium, and manganese decreased in concentration from the early successional stages to the later successional stages, although some year to year variability did occur. All chemical parameters except zinc displayed at least one significant change in concentration due to vegetation clearing. These differences can be summarized broadly as effects on non-biologically cycled nutrients in the open shrub willow stage (III) and changes in the biological cycling of nutrients in the poplar-alder and mature white spruce stages (V and VIII, respectively). The stage V sites displayed the greatest response to treatment. In general the Stage V-A sites displayed significant increases, as a result of clearing, in concentrations of NH4, PO4, SO4, K, Na, Ca, Mg, Mn, Fe, Al, Si, HCO3, and conductivity in 1987 and 1988; with the exception of Fe and Si in 1988. Chloride decreased in the poplar-alder stage V-A sites in 1987 but increased in 1988. The V-B poplar-alder sites showed no effect of treatment in 1988 and only NO3, SO4, Ca, Mg, and conductivity increased in 1987. Chloride, Al and Si decreased as a result of treatment in 1987 on the stage V-B poplar-alder sites. Nitrate, NH4, Cl, and pH increased, while PO4, K, Na, Ca, Mg, Si, HCO3, and conductivity all decreased as a result of clearing in the mature white spruce site VIII-AT. These trends were similar in 1988. At the VIII-BT site 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 did show significant decreases in 1987 in the VIII-BT site which were repeated in 1988. The results of this study combined with the results of other studies detailing the soil environment and control of evaporative movement of water to the soil surface support the hypotheses that; (1) soil solution salt concentrations were sufficiently high to affect seedling germination and growth on the early successional sites, (2) the genesis and maintenance of surface salt crusts are controlled by the soil physical and chemical environments encountered on early successional mineral seed beds, and (3) the disappearance of salt crusts and reduction in mineral soil salt concentrations is controlled by forest succession which mediates the changing soil physical, chemical, and biological environment.