Understanding the prairie-forest transitional zone in northern Minnesota through variations in soil chemistry

Abstract:

Boundaries between soil types are not discrete, but instead there are transitional zones that exhibit characteristics of soil types that they border. The prairie-forest transitional zone is seen throughout North America and Eurasia. Prairie soils (Mollisols) and deciduous forest soils (Alfisols) demonstrate interesting contrasts in morphology. Understanding variations in chemical properties is key to understanding nutrient cycling and retention, ecosystem development, and furthering the field of soil geography. Research sites are located in northern Minnesota’s eastern forest, western prairie, and the transitional forested zone between. Evidence of clay translocation is a key indicator of Alfisol development. The double layer theory suggests chemical factors allowing clays to be dispersed/flocculated are ionic strength of the solution, relative abundance of Na⁺ and di- or trivalent cations, and pH (Sposito, 1984; van Olphen, 1977). In initial stages of soil formation exchangeable bases (Na⁺, K⁺, Ca⁺⁺, and Mg⁺⁺) occupy 100% of clay exchange sites, but as soil develops are these replaced by acidity ions (H⁺ and Al³⁺) and base saturation decreases. The relationship between exchangeable cations and clay dispersion is understood in lower horizons where Ca⁺⁺ and Mg⁺⁺ are abundant, and clay is flocculated, but this is not well understood in upper horizons. However it is suggested that clay dispersion can occur in upper horizons of pH values between 7 and 5 (van Breeman and Buurman, 2002). CEC values are expected to be much higher in soils where clay is flocculated and base ion concentration is high. Preliminary analyses supports that differences in these chemical factors are key indicators of varying rates of soil development, and explain geographic distribution of soils in this region. Through further lab work and data analysis, the relative importance of these chemical properties will come to light and the drivers prairie-forest soil transition will be better understood.