Do prescribed burns yield enhanced carbon storage in salt marshes?: a comparative assessment in Delaware salt marshes with varied burn histories
Date
2023
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Publisher
University of Delaware
Abstract
Salt marshes and other tidal wetland ecosystems play crucial roles in the environment and provide a suite of ecosystem services including erosion control, providing breeding and nursery habitat for birds and fisheries species, and improvement of water quality. In the context of carbon cycling and climate change, tidal wetlands are of recent increased interest for a carbon sequestration ecosystem service. It has been estimated that these environments can sequester up to 244.7 ± 26.1 g C m-2 yr-1. Tidal marsh habitat has also been lost at alarming rates over recent centuries due in part to the invasion of Phragmites australis. Phragmites invasions have been associated with altered hydrology and reductions in native biodiversity in Delaware marshes and are frequent targets for restorations to reestablish native marsh habitat. Several tactics are commonly used by land managers for the removal of Phragmites such as mowing, application of herbicides, and prescribed burns. Phragmites may actually store organic carbon more efficiently than native species meaning its removal may unintentionally result in reduced carbon sequestration. Prescribed burns, however, are known to produce biochar, a recalcitrant form of organic carbon that is efficiently buried in soils and may recoup some of the lost carbon sequestration. With this thesis, I utilize core collections at three tidal marsh sites for the purpose of studying whether prescribed burns improve carbon storage in tidal Delaware marshes: a site with a known burn history (Roberts Farm), an adjacent site with no recent (<20 years) burn history (Rocks Tract), and a site with no history of burns (St. Jones). Cores (15 cm, n=10) were collected from each site and analyzed for total organic carbon (TOC) and Black Carbon (BC), the incompletely combusted remains of biomass burning. ☐ Two methodologies were utilized for BC analysis, the Benzene Polycarboxylic Acid (BPCA; BCBPCA) and Chemo-Thermal Oxidation at 375º (CTO-375; BCCTO) methods in order to assess the implications of methodology on BC quantification. The BPCA method generally reported a higher concentration of BC (mean = 4.97 ± 1.06 BCBPCA g-1 soil) compared to the CTO-375 method (mean = 1.84 ± 1.19 mg BCCTO g-1 soil) as has been found for terrestrial soil but in contrast to findings for marine sediments. Charring was observed in our organic-rich marsh soils using the CTO-375 method artificially inflating the BCCTO results in the St. Jones marsh in particular. A dilution test was conducted on a subset of samples using blank sand and found an average decrease in BCCTO of 1.6 ± 0.9 mg BCCTO g-1 soil. Future studies using BCCTO are encouraged to evaluate their data for potential charring by performing such dilution quality assurance procedures. ☐ The Roberts Farm marsh showed higher TOC (58.4 ± 14.4 mg TOC g-1 soil) and BCBPCA (5.80 ± 1.06 mg BCBPCA g-1 soil) concentrations than the adjacent unburned Rocks Tract marsh (47.2 ± 12.1 mg TOC g-1 soil; 4.76 ± 0.77 mg BCBPCA g-1 soil) suggesting that recent burns may have increased carbon storage in the Roberts Farm soils. Using the BCCTO method, however, Roberts Farm BCCTO concentrations (1.44 ± 0.26 mg BCCTO g-1 soil) were similar to Rocks Tract (1.31 ± 0.57 mg BCCTO g-1 soil) indicating that the soot-like portion of the BC pool to be similar at the two sites, and char materials may be contributing the differences observed in the BCBPCA data. TOC concentrations at the mesohaline St. Jones (81.2 ± 44.2 mg TOC g-1 soil) site were higher than at Roberts Farm and Rocks Tract demonstrating alternate controls on C storage than the presence or absence of burning. BCBPCA B6:B5 ratios were similar at the Roberts Farm and Rocks Tract marshes suggesting similar BC sources, and I speculate that the BC derived from the prescribed burns at Roberts Farm is transported atmospherically and hydrologically to the Rocks Tract marsh. All three marshes have measurable BC in their soils reflecting its ubiquitous delivery from sources other than the prescribed burns. ☐ This thesis demonstrates the utility of using multiple BC characterization methods for studying its quantities and characteristics and provides insight into the impacts of prescribed burns on soil C storage. The data suggest a potential C sequestration benefit from these prescribed burns. Additional studies are needed to assess the ubiquity of this benefit as well as to compare it quantitatively to C storage in Phragmites soils. Tidal marsh managers can use the C storage information found in this work within their salt marsh restoration decision-making strategies.
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Keywords
Black carbon, Carbon sequestration, Prescribed burns, Salt marsh, Carbon cycling