United States Department of Agriculture Site-specific Critical Acid Load Estimates for Forest Soils in the Osborn Creek Watershed, Michigan Trevor Hobbs, Jason Lynch, and Randy Kolka Forest Service Northern Research Station General Technical Report NRS-171 July 2017 Abstract Anthropogenic acid deposition has the potential to accelerate leaching of soil cations, and in turn, deplete nutrients essential to forest vegetation. The critical load concept, employing a simple mass balance (SMB) approach, is often used to model this process. In an evaluation under the U.S. Forest Service Watershed Condition Framework program, soils in all 6th level watersheds on the Huron-Manistee National Forests (HMNF) in Michigan were assigned the lowest score of “3—Impaired Function” due to exceedance of the critical load of acidity as determined by national-scale estimates. The impetus for this research was to test the relevance of national-scale critical acid load estimates at the 6th level watershed scale by using site-specific field data in the SMB model where possible. The Osborn Creek watershed on the HMNF served 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. Critical acid loads and exceedances were developed under “best” and “worst” case scenarios given the uncertainty in the SMB model. Despite the high likelihood of actual exceedance and some evidence for soil acidification across the watershed, base saturation remains excessively high (>100 percent) at most sites. Other field data suggest that these soils receive significant external inputs of base cations that may outweigh what is produced through weathering onsite within the rooting zone. Trees show no visible signs of decline. Overall, the SMB approach may not adequately capture the complexity of nutrient cycling at all of the sample sites. The variability of soils, weathering estimates, and nutrient uptake rates between and within sites makes extrapolation of these results to other HMNF watersheds difficult to justify. Management programs aimed at improving our understanding of base cycling in complex glacial terrain, as well as mitigating the risks associated with nutrient depletion from frequent timber harvests and fuels reduction practices, are suggested. Cover Art A stylized fisheye photo looking up at the canopy of a sugar maple stand in the Osborn Creek watershed. Photo by Trevor Hobbs, Huron-Manistee National Forests. Quality Assurance This publication conforms to the Northern Research Station’s Quality Assurance Implementation Plan which requires technical and policy review for all scientific publications produced or funded by the Station. The process included a blind technical review by at least two reviewers, who were selected by the Assistant Director for Research and unknown to the author. This review policy promotes the Forest Service guiding principles of using the best scientific knowledge, striving for quality and excellence, maintaining high ethical and professional standards, and being responsible and accountable for what we do. The use of trade or firm names in this publication is for reader information and does not imply endorsement by the U.S. Department of Agriculture of any product or service. Manuscript received for publication 28 November 2016 Published by For additional copies: U.S. FOREST SERVICE U.S. Forest Service 11 CAMPUS BLVD SUITE 200 Publications Distribution NEWTOWN SQUARE PA 19073 359 Main Road July 2017 Delaware, OH 43015-8640 Fax: (740)368-0152 Email: [email protected] Site-specific Critical Acid Load Estimates for Forest Soils in the Osborn Creek Watershed, Michigan The Authors TREVOR HOBBS is a soil scientist with the Huron-Manistee National Forests in Cadillac, MI. JASON LYNCH is an ecologist with the U.S. Environmental Protection Agency, Office of Air and Radiation, Office of Atmospheric Programs in Washington, DC. RANDY KOLKA is a research soil scientist with the U.S. Forest Service, Northern Research Station in Grand Rapids, MN. General Technical Report NRS-171 CONTENTS INTRODUCTION ...............................................................................................................................................................1 Watershed Condition Framework ...............................................................................................................................1 Research Questions ....................................................................................................................................................2 Objectives...................................................................................................................................................................2 BACKGROUND AND MODELING APPROACH ......................................................................................................................2 Modeling Critical Acid Loads for Forest Soils ...............................................................................................................2 Simple Mass Balance Method .....................................................................................................................................3 Input Data for the Simple Mass Balance Equation ..................................................................................................... 12 STUDY AREA ................................................................................................................................................................. 13 Geography ................................................................................................................................................................ 13 Geology .................................................................................................................................................................... 13 Soils and Vegetation ................................................................................................................................................. 15 Surface Water and Groundwater .............................................................................................................................. 15 Climate ..................................................................................................................................................................... 16 METHODS ..................................................................................................................................................................... 16 Field Methods........................................................................................................................................................... 16 Laboratory Analysis .................................................................................................................................................. 18 Data Calculation Methods ......................................................................................................................................... 19 RESULTS AND DISCUSSION ............................................................................................................................................ 21 Site-specific Tree Uptake .......................................................................................................................................... 21 Soil Horizon Characteristics ...................................................................................................................................... 27 Weathering Results ..................................................................................................................................................28 Critical Leaching of Acid Neutralizing Capacity ......................................................................................................... 32 Critical Acid Loads and Exceedances .........................................................................................................................34 MANAGEMENT RECOMMENDATIONS .............................................................................................................................36 Applicability of Results .............................................................................................................................................36 Watershed Condition Framework Rating ..................................................................................................................36 Monitoring Soil Nutrients ......................................................................................................................................... 37 CONCLUSIONS ............................................................................................................................................................... 39 ACKNOWLEDGMENTS ....................................................................................................................................................40 English Conversion Factors Applicable to Text, Tables, and Figures ...........................................................................40 LITERATURE CITED ........................................................................................................................................................ 41 INTRODUCTION Soils play a crucial role in maintaining healthy forest watersheds by providing substrate for forest