Title: Soil Carbon Storage and Its Relation to Abiotic and Biotic Factors in the Boise Foothills

Program: Biology MS

Committee Chair: Marie-Anne de Graaff

Committee Co-Chair: Kelly Hopping

Committee: Marie-Anne de Graaff, Kelly Hopping, Derek Pierson, Trevor Caughlin

Abstract: Terrestrial soil sequesters a significant amount of carbon (C), and changes to this pool can impact atmospheric CO2 concentrations and climate change. Thus, it is crucial to understand the drivers of the size of this pool. Changes in plant community composition in semi-arid ecosystems can impact soil C storage. However, little is known about the impact of plant functional type on the quantity and distribution of soil C on a landscape scale relevant to ecosystem management. I conducted a study in the City of Boise’s Open Space Reserves located north of Boise to evaluate how plant functional type affects soil C across soil types, aspects, and elevations. I collected soil samples (0-10 cm, 10-20 cm, and 20-30 cm depth) from four of the most common vegetation types (annual short grassland, annual tall grassland, perennial grassland, and upland shrub) in this landscape. To account for spatial abiotic heterogeneity, I stratified by three soil types (Haploxerolls, Argixerolls, and a combination of Haploxerolls and Argixerolls), two aspects (north-northeast, south-southwest), and two elevation categories (three high elevation reserves and four low elevation reserves). At each sample location, a 10x10m plot was created from which three soil cores were collected, which were subsequently composited, although the different depth increments were kept separate. Soils were dried, sieved (2mm), and roots (>2mm in length) and rocks were removed. I quantified bulk density, analyzed samples for organic soil C concentration (mg C/g soil), and calculated soil C stocks (t/ha). Generalized linear mixed models were used to evaluate the impact of vegetation, aspect, and reserve on soil bulk density on response variables. Soil type did not impact soil C. Averaged across all reserves, upland shrub vegetation coincided with significantly higher soil C concentration and greater C stocks than the other vegetation types. At low elevation reserves, upland shrub-dominated soil had a significantly higher concentration of soil C at all three soil depths, but at high elevation, the concentration of soil C was higher for upland shrubs in the 0-10 soil depth increment only. At low elevation reserves, C stocks were higher for upland shrub-dominated soil only in the top 10 cm of soil.. There were no differences in soil C stocks among plant types at high elevation. Hawkins Range Reserve stored more soil C than any other reserve across all soil depths, potentially due to less anthropogenic disturbance. The trends identified in the distribution of soil C by vegetation type and elevation will inform the City of Boise’s critical land management decisions. Investing resources into shrub restoration and conservation will have a multitude of benefits for soil function, which benefits not only us but the soil as well.