SITE-SPECIFIC CRITICAL ACID LOAD ESTIMATES FOR FOREST SOILS IN THE OSBORNE CREEK WATERSHED, MICHIGAN

Anthropogenic acid deposition has the potential to accelerate leaching of soil cations, and in turn, deplete essential nutrients for forest vegetation. The critical load concept is often used to model this process. A critical load is the level of input below which no harmful ecological effects occur to the ecosystem compartment of concern. Many watersheds across the Huron-Manistee National Forests have been determined to be in exceedance of critical acid loads (CAL) by existing national scale estimates that use the simple mass balance (SMB) model. Currently, national scale estimates represent the only available data on this topic for forest managers in Michigan. Because national scale CAL estimates rely on broad assumptions and aggregation of coarse-resolution datasets, the impetus for this research was to test their relevancy at the watershed scale by substituting site-specific field data into the SMB model as much as possible. The Osborne Creek Watershed, in Oceana County, MI, was used as a case study. Field data were collected to estimate soil mineral weathering rates, nutrient uptake rates, and forest growth characteristics at five sites containing sandy, nutrient-poor soils. Using Ti-depletion pedological mass balance modeling, whole profile mineral weathering rates of major base cations (Ca, K, Mg, and Na) ranged from -33 to -98 eq/ha/yr. Three sites showed net positive fluxes of major base cations (negative weathering rates), suggesting a combination of parent material mineralogical heterogeneity, very low weathering rates, and/or significant external base cation sources. Tree nutrient uptake rates ranged widely across the five sites. Deeply-rooted trees penetrating zones with thick lamellae or shallow lithologic discontinuities complicate interpretation of resulting site-specific CAL estimates. Results highlight the variability of soils and tree nutrient demands at the watershed scale and further demonstrate the need for local case studies that critically examine and validate existing national scale CAL estimates before using them to make forest management decisions.