Date of Degree

10-2014

Document Type

Dissertation

Degree Name

Ph.D.

Program

Earth & Environmental Sciences

Advisor(s)

Jeffrey A. Bird

Subject Categories

Biogeochemistry | Environmental Sciences | Organic Chemistry

Keywords

atmospheric black carbon, decomposition, fine roots, nitrogen, pyrogenic carbon, stable isotopes

Abstract

Soil organic matter (SOM) is the dominant reservoir of organic carbon (OC) in terrestrial ecosystems, storing approximately three times the size of the C pool in the atmosphere. In temperate forests, a major fraction of the SOM consists of slowly decaying soil organic C (SOC) pools. While slowly cycling C pools constitute a large reservoir of stable C in soils, the dominant environmental factors controlling this C pool remain unresolved. This research investigates two significant, but poorly characterized slowly decaying C pools: fine root litter (< 2mm) and thermally altered plant biomass (pyrogenic organic matter, PyOM). Specifically, I used compound-specific stable isotope analysis (13C and 15N) as my main methodological approach to examine the (1) decomposition of root litter and PyOM in temperate forest soils, and (2) the factors (soil type, nitrogen addition, and SOM) that affected the stability of these two SOC pools. This was accomplished by integrating the results of: a 180-d incubation study on PyOM decomposition, a study on the molecular composition and physicochemical structure of PyOM, δ13C measurements of PyOM molecular markers (13C-benzene polycarboxylic acids) in soils, measurements of atmospheric PyOM-C deposition, and a 2-yr field study on root litter decay. PyOM-C at 450oC had a centennial mean residence time (MRT) in temperate forest soils, and the mineralization of PyOM-N was affected by reactive mineral surfaces. Future tracing experiment studies will greatly benefit from the use of 13C-BPCA approach to quantify PyOM turnover rates in soils. In northern Michigan, PyOM-C deposition fluxes from the atmosphere to soils were low, but provided background data relevant for future assessments of atmospheric PyOM-C concentration. Lastly, maple roots decomposed faster than those reported by previous studies in temperate ecosystems, suggesting that root litter C is not a stable SOM pool in northern temperate forest soils. To improve long-term predictions of the impact of climate change on SOC fluxes, ecosystem scale C models should consider root detritus as a fast-cycling C pool in northern forest soils, incorporate the effect of soil mineral assemblage on the stability of SOM, and no longer assume that PyOM has a millennial MRT in soils.

 
 

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