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Controls on the Presence, Concentration, Storage, And CONTROLS ON THE PRESENCE, CONCENTRATION, STORAGE, AND VARIABILITY OF SOIL INORGANIC CARBON IN A SEMI-ARID WATERSHED by Christopher Allen Stanbery A thesis submitted in partial fulfillment of the requirements for the degree of Master of Science in Hydrologic Sciences Boise State University December 2016 © 2016 Christopher Allen Stanbery ALL RIGHTS RESERVED BOISE STATE UNIVERSITY GRADUATE COLLEGE DEFENSE COMMITTEE AND FINAL READING APPROVALS of the thesis submitted by Christopher Allen Stanbery Thesis Title: Controls on the Presence, Concentration, Storage, and Variability of Soil Inorganic Carbon in a Semi-Arid Watershed Date of Final Oral Examination: 13 April 2016 The following individuals read and discussed the thesis submitted by student Christopher Allen Stanbery, and they evaluated his presentation and response to questions during the final oral examination. They found that the student passed the final oral examination. Jennifer Pierce, Ph.D. Chair, Supervisory Committee Shawn Benner, Ph.D. Member, Supervisory Committee Nancy Glenn, Ph.D. Member, Supervisory Committee Mark Seyfried, Ph.D. Member, Supervisory Committee The final reading approval of the thesis was granted by Jennifer Pierce, Ph.D., Chair of the Supervisory Committee. The thesis was approved by the Graduate College. ACKNOWLEDGEMENTS: The National Science Foundation Reynolds Creek Critical Zone Observatory [EAR 1331872] and Boise State University funded this study. We would like to thank the Agricultural Research Service and Reynolds Creek Experimental Watershed staff for scientific and logistical support. Additionally, we are grateful to Jenna Duffin, Marion Lytle, Lucas Spaete, Alison Good, Christopher Thornton, Benjamin Bruck, Jason McKinnon, Peter Youngblood, and Hayden Lewis for their considerable assistance in this project. iv ABSTRACT Soil inorganic carbon (SIC) constitutes approximately 40% of terrestrial soil carbon and is an integral part of the global carbon cycle; however, the controls on the storage and flux of inorganic carbon are poorly understood. Soil forming factors controlling SIC storage and flux include climate, organisms, relief, parent material, and time (Jenny, 1941). Rainfall is a primary factor controlling SIC accumulation in arid and semi-arid regions, but the hierarchy of controls on SIC development is complex. The Reynolds Creek Experimental Watershed in southwestern Idaho is an ideal location to study factors influencing SIC, as the carbon pool transitions from predominately inorganic carbon in the lower elevations, to organic carbon at higher elevations. This study builds upon fundamental studies in soil science that define and describe precipitation controls on the ‘pedocal’ (calcic) to ‘pedalfer’ (non-calcic) soil transition (e.g. Marbut, 1935; Jenny, 1941) by both defining the precipitation boundary in Reynolds Creek, and quantifying the amount of carbon storage within calcic soils. We collected soil samples from soils developed under a wide range of soil- forming regimes: 1) along a precipitation gradient, 2) within different vegetation communities (sagebrush species (Artimesia spp), bitterbrush (Purshia tridentata), greasewood (Sarcobatus vermiculatus), and juniper (Juniperus occidentalis)) 3) from different parent materials (granite, basalt, other volcanics, and alluvium) and 4) from terrace surfaces of different ages. Our results show SIC does not accumulate above a threshold of ~500 mm mean annual precipitation, and variability in SIC below that value v is significant. Soil inorganic carbon content from ~1 m deep soil pits and cores at 71 sites shows that 64 sites contained less than 10 kg/m2 SIC, 5 sites contained between 10-20 kg/m2, and 2 sites had between 24 and 29 kg/m2. Random forest modeling and multiple linear regression of the environmental controls on SIC indicate that precipitation is the primary control on SIC accumulation, where increased precipitation correlates with lower amounts of SIC. Elevation is an effective predictor of SIC, as it is strongly auto- correlated with precipitation and vegetation. Parent material consistently ranks as an important predictor in random forest analysis; however, we were unable to quantify the importance of wind-blown dust in the soil profiles, which we believe plays a vital role in SIC accumulation. Despite a recognition of different stages of carbonate development and accumulation rates between gravelly and non-gravelly soils, studies often ignore carbonate coatings on gravels in measurements of soil inorganic carbon (SIC). By quantifying and differentiating the fine (<2 mm) and coarse (>2 mm) fractions of SIC in the Reynolds Creek Experimental Watershed in southwestern Idaho, we show that gravel coatings contain up to 44% of total SIC at a given site. Among the 26 soil sites examined throughout the watershed, an average of 13% of the total SIC is stored as carbonate coasts within in the gravel fraction. We measured a high level of pedon-scale field variability (up to 220%) among the three faces of 1 m3 soil pits. Analytical error associated with the modified pressure calcimeter (0.001-0.014%), is considerably less than naturally occurring heterogeneities in SIC within the soil profile. This work highlights and quantifies two sources of uncertainty in studies of SIC needed to inform future research. First, in gravelly sites, the >2 mm portion of soils may store a large vi percentage of SIC. Second, SIC varies considerably at the pedon-scale, so studies attempting to quantify carbon storage over landscape scales need to consider this variability. This study creates a framework for understanding SIC in Reynolds Creek that may be applied to future work. vii TABLE OF CONTENTS ACKNOWLEDGEMENTS: .............................................................................................. iv ABSTRACT .........................................................................................................................v TABLE OF CONTENTS ................................................................................................. viii LIST OF TABLES ............................................................................................................. xi LIST OF FIGURES .......................................................................................................... xii CHAPTER ONE: QUANTIFYING STORAGE OF INORGANIC SOIL CARBON ON GRAVELS AND DETERMINING PEDON-SCALE VARIABILITY..............................1 1. Introduction ..........................................................................................................1 1.1. Background: ..........................................................................................3 1.2. Study Area: ...........................................................................................6 2. Methods: ..............................................................................................................7 2.1. Field methods: .......................................................................................7 2.2. Lab methods: .........................................................................................7 2.3. Uncertainty analysis: .............................................................................9 3. Results: .................................................................................................................9 3.1. Soil inorganic carbon storage within the gravel fraction of soils: ........9 3.2. Analysis of variability: ........................................................................11 4. Discussion: .........................................................................................................17 4.1. Gravel inorganic carbon:.....................................................................17 4.2. Pedon-scale variability: .......................................................................18 viii 5. Conclusions: .......................................................................................................19 CHAPTER TWO: CONTROLS ON THE PRESENCE AND CONCENTRATION OF SOIL INORGANIC CARBON IN A SEMI-ARID WATERSHED .................................21 1. Introduction: .......................................................................................................21 1.1. Background: ........................................................................................23 1.2. Study Area: .........................................................................................27 2. Methods: ............................................................................................................29 2.1. Field methods: .....................................................................................29 2.2. Lab methods: .......................................................................................30 2.3. GIS and data analysis: .........................................................................33 2.4. Dust collection and analysis: ..............................................................34 3. Results: ...............................................................................................................35 3.1. Variation in Soil Inorganic Carbon with Precipitation: ......................35 3.2. Using random forest analysis to determine primary controls on SIC .40 3.3. Predicting soil inorganic carbon using statistical analysis of possible environmental factors: ...............................................................................43 3.4. Changes in carbon storage with soil depth .........................................45
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