Title

Consequences of Vegetation Change on the Dynamics of Labile Organic Matter and Soil Nitrogen Cycling in a Semiarid Ecosystem

Date of Award:

5-2009

Document Type:

Dissertation

Degree Name:

Doctor of Philosophy (PhD)

Department:

Biology

Advisor/Chair:

John M Stark

Abstract

Sagebrush-dominated ecosystems are being transformed by wildfire, rangeland improvement techniques, and exotic plant invasions. These disturbances have substantial effects on the composition and structure of native vegetation, but the effects on ecosystem C and N dynamics are poorly understood. To examine whether differences in dominant vegetation affect the quantity and quality of plant organic matter inputs to soil, ecosystem C and N pools and rates of plant turnover were compared among historically grazed Wyoming big sagebrush, introduced perennial crested wheatgrass, and invasive annual cheatgrass communities. Since low soil moisture during the summer may inhibit the microbial colonization of plant detrital inputs and result in C-limitations to microbial growth, soils were treated with an in situ pulse of plant detritus prior to the onset of the summer dry-season, and rates of soil C and gross N cycling were compared between treated and untreated soils. Finally, because plant detritus is the dominant form of labile C input to soil microbes over a large portion of the year, the decomposition of 13C-labeled annual grass detritus was used to determine the importance of plant detritus versus soil organic matter as microbial substrate. Results revealed large differences in ecosystem C and N pools, and in the quantity of plant C and N inputs to soil among vegetation types, but differences in soil C and N cycling rates were more subtle. Plant biomass pools were greatest for sagebrush stands, but plant C and N inputs to soil were greatest in cheatgrass communities, such that rates of plant C and N turnover appeared to be accelerated in disturbed ecosystems. Earlier release of plant biomass to soil detrital pools stimulated N availability to a greater extent than C availability relative to untreated soils, and this effect could not be predicted from the C:N stoichiometry of plant detritus. Finally, in situ decomposition of cheatgrass detritus was rapid; however, there was no clear evidence of a time-lag during summer in microbial colonization of recently released plant detritus, and microbial consumption of plant detritus did not result in N-limitations to microbial growth.

Comments

Embargoed at author's request, with department's permission

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