Date of Award

Spring 2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Forestry and Environmental Studies

First Advisor

Burke, Ingrid

Abstract

The global soil carbon (C) pool contains more than twice as much C as the atmosphere, and a large portion of terrestrial C as a whole. Within the global soil C pool, drylands contain 20% of the soil C, in addition to covering 40% of the earth’s surface. Because they are both widespread and a significant reservoir of C, drylands are an important component of the global C cycle. Despite the importance of drylands, ecosystem function can be difficult to predict, with drylands switching from a C source to sink on an interannual basis. This uncertainty in net C flux creates difficulty projecting dryland contributions to atmospheric C and global change, particularly with expected future changes in annual precipitation and seasonality, changes in magnitude of nitrogen (N) deposition, and widespread land use change. The overall goal of my dissertation research is to improve our understanding of ecosystem structure and ecosystem function for semiarid drylands. In particular, I am interested in asking questions about the response of several dryland ecosystem types to changing water and N additions, similar to those predicted to occur under multiple scenarios of global change. I used field and lab-based methods to explore the structure and function of the shortgrass steppe, mixed-grass prairie, and sagebrush steppe – ecosystem types that are among the most common semiarid drylands in the western U.S. I assessed the composition of and the differences among these dryland ecosystem types in their ‘natural’ state, and under treatments of added water and N, altering the availability of these two components in situ.In Chapter 1, I investigated multiple facets of ecosystem structure and function across three dryland ecosystem types in the Great Plains of the western United States. I conducted this study to better understand patterns in structure and function within semiarid drylands and to understand how those patterns relate to the lesser-studied sagebrush steppe. As expected, plant and soil microbial communities varied by site. Surface soils were largely similar in their soil C content, while subsurface soils (> 5 cm depth) varied widely in their C content; the lower depth soils at the mixed-grass prairie had more than twice the soil C of the other two dryland types in all size fractions studied. I found that each of the sites functioned largely similarly with respect to aboveground net primary production, soil respiration, and soil N mineralization. The data suggest that functional attributes may be similar across dryland ecosystem types, at least when constrained to similar climates and soil textures, as in my study. In Chapter 2, I explored the effects of increasing water and N availability on ecosystem structure across the shortgrass steppe, mixed-grass prairie, and sagebrush steppe. I was particularly interested in changes within the plant and soil microbial communities. β-diversity within plant and microbial communities were largely resistant to change. However, there were three notable changes within functional types and indicator species: grasses increased in abundance at the mixed-grass prairie under high N additions with and without water, saprotrophic fungi increased in abundance at most locations under combined water and N (of differing levels), and arbuscular mycorrhizae decreased across all sites with greater decreases generally accompanying higher loadings of N. Indicator species within the cyanobacteria and lichenized fungi experienced increases in abundance at the shortgrass steppe. While these results show no major widespread shifts in β-diversity, changes in those plant, fungal, and bacterial indicator species may portend future changes in biomass production, decomposition, and nutrient cycling. In Chapter 3, I assessed the effects of changing water and N availability on ecosystem function in the three dryland systems. I measured the responses of aboveground net primary productivity (ANPP), soil respiration, and soil C to these treatments. All dryland systems experienced similar responses to N additions and combined high N with water additions, with increased ANPP and soil respiration. Soil C showed no significant differences after treatment. My results demonstrate co-limitation of ecosystem function by water and N over the course of this study in these three semiarid drylands of the western Great Plains.

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