Publication Date

12-2016

Date of Final Oral Examination (Defense)

4-13-2016

Type of Culminating Activity

Thesis

Degree Title

Master of Science in Hydrologic Sciences

Department

Geosciences

Supervisory Committee Chair

Jennifer Pierce, Ph.D.

Supervisory Committee Member

Shawn Benner, Ph.D.

Supervisory Committee Member

Nancy Glenn, Ph.D.

Supervisory Committee Member

Mark Seyfried, Ph.D.

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 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 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.

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