Modification of Snowmelt Chemistry by Forest Floor and Mineral Soil, Northern Michigan

Abstract

AbstractPrecipitation, snowpack, snowmelt, forest floor percolate, soil water, and streamwater chemistry were studied throughout winter for 7 yr in a small (176 ha) Northern Michigan watershed vegetated by northern hardwoods. The objective was to see how precipitation and snowmelt chemistry is modified before it reaches the stream. Intermittent snowmelt occurred throughout winter, and 65 to 78% of total snowpack loss of H+, NH+4, NO−3, and SO2−4 occurred before peak snowpack and snowmelt. From 70 to 95% of snowmelt H+, NH6+4, and NO−3 was retained in the forest floor and shallow mineral soil by exchange, adsorption, or biological uptake. Snowmelt SO2−4 concentration was enriched in forest floor percolate and shallow soil water probably by organic mineralization, low adsorption, and perhaps desorption. The significant relationship between dissolved organic carbon (DOC) and SO2−4 concentration in forest floor percolate and shallow soil water indicates organics may contribute to reduced SO2−4 adsorption. Sulfate and organic anions were major contributors to base cation (CB) flux in the forest floor while SO2−4 was related to CB flux in shallow soils. A decline in mineral soil and streamwater DOC concentration and increase in streamwater HCO−3 relative to concentrations in forest floor percolate suggests the relative importance of organic decomposition decreased and soil mineral weathering increased with soil depth. High streamwater Ca2+, Mg2+, and HCO−3 concentrations indicated that soil mineral weathering was a major process altering snowmelt chemistry at the watershed level.