The Importance of the Snowmelt Pulse to Podzolization.

Perhaps the best way to glean insight into pedogenesis is to examine sites where the morphologic products of that process are best expressed. To implement this approach, we instrumented a Typic Durorthod in the eastern Upper Peninsula of Michigan, where Spodosols are extremely well developed. For two years, zero-tension lysimeters installed here captured water leaving the O, E and B horizons, and buried sensors determined volumetric water contents and soil temperatures. During this time we also measured snowpack depths and determined snow water equivalents. These field data were used to calibrate the  hydrologic model BROOK90. We applied 1961-2013 meteorological data from four NWS stations along a N-S transect that spans the range of podzolization strength in Michigan as inputs to the model.  Across the transect, both field and modeled data show that the soils remain dry throughout the summer and slowly wet up in fall. The more strongly developed soils in the north are slightly wetter at this time, facilitating breakdown of fresh litter and enhancing production of soluble organic materials in the O horizon. These materials are translocated into the mineral soil by a reliable and persistent “pulse” of cold snowmelt water in spring. This pulse comprises over half of the annual flux of water at 100 cm depth, even though it occurs in ca. 15% of the time. Snowmelt fluxes are larger and are more temporally compressed in the north, where podzolization is strongest. By storing precipitation in the snowpack, especially in the north, inputs of water to the soil are temporally compressed. This flux (pulse) of meltwater is utilized mainly when soluble organic materials are most readily available for translocation; this is the essence of podzolization in this region. Thus the most strongly developed Spodosols occur where sandy soils are found in association with thick, persistent snowpacks. Both the modeled and field data provide excellent insight into what drives podzolization in this region, and support earlier work that emphasized the importance of snowmelt flux/percolation in podzolization.