Degree

Doctor of Philosophy (PhD)

Department

Environmental Sciences

Document Type

Dissertation

Abstract

The impacts of sea-level rise and hydrologic manipulation are threatening the stability of coastal marshes throughout the world, thereby increasing the potential for re-mineralization of soil organic matter (SOM) in these systems. Such threats have prompted marsh restoration efforts, particularly in coastal Louisiana, yet it is unclear how the slowly decomposing (refractory) and quickly decomposing (labile) fractions of SOM may be differentially affected by different approaches to marsh restoration. Additionally, otherwise labile compounds may accumulate in the soil via a range of protective mechanisms, including rapid burial and association with organic compounds that are thought to enhance soil aggregation, such as autoclaved alkaline-extractable protein (AAEP)—a.k.a. glomalin-related soil protein. Here, I examined the upper 30 cm of soil across two chronosequences of coastal marshes—located in Sabine National Wildlife Refuge (Sabine) and in Wax Lake Delta (WLD)—representing the two principal approaches to coastal wetland restoration in Louisiana (dredge-spoil infilling and sediment diversion, respectively). By estimating the amounts of refractory organic carbon and nitrogen (ROC and RN), labile organic carbon and nitrogen (LOC and LN), and AAEP, I aimed to compare how these fractions accumulate within these two distinct systems and contribute to the total organic carbon and nitrogen (TOC and TN). Because methodological limitations have confounded previous efforts to accurately quantify the AAEP fraction, I applied a novel approach for estimating the quantity of AAEP in the studied soils based on amino acid analysis. Each fraction was highly positively correlated with TOC. Overall, the contributions of the fractions to TOC and TN were similar between the chronosequences and tended to increase with marsh age. AAEP was primarily co-purified with ROC, and was negatively correlated with LOC/TOC ratios, suggesting that it may not be involved in LOC preservation. Although a greater proportion of the new carbon accumulation in Sabine was refractory relative to WLD, the WLD marshes appeared to accumulate ROC more quickly—a paradox explainable by the characteristically faster accretion rates in WLD. The overall correlation between ROC and TOC did not differ between the two chronosequences and was remarkably similar to those previously observed elsewhere in coastal Louisiana.

Date

2-18-2020

Committee Chair

Laws, Edward A.

Available for download on Friday, March 17, 2023

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