Carbon burial in soils of the Great Marsh, DE: evaluating accumulation rates and organic matter composition
Date
2022
Authors
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Publisher
University of Delaware
Abstract
Salt marshes have been reported to bury organic carbon (OC) up to 50x faster than temperate terrestrial forests (244.7 ± 26.1 g m-2 yr-1, 5.1 ± 1.0 g m-2 y-1). Estimates vary widely (18-1713 g C m-2 y-1) though, and studies that estimate OC accumulation rates for terrestrial forests use much longer timescales (70-15,000 years) than those reported for salt marshes (<150 years). Furthermore, the physical and biogeochemical controls on C storage in salt marsh soils are not fully understood, and there is a need for studies that relate organic matter (OM) composition to C storage estimates. In this study, OC accumulation rates were calculated using both 137Cs and 210Pb dating for four salt marsh soil cores collected in October 2020 and June 2021 in the Great Marsh, DE. 210Pb-derived (144 ± 15 years) and 137Cs-derived (57.5 ± 0.5 years) dating methods produced OC accumulation rates of 65.7 ± 13.3 g C m-2 yr-1 and 144 ± 22 g C m-2 yr-1, respectively. OC accumulation rates were additionally shown to be more variable when calculated over shorter, 137Cs-based timescales. C accumulation rates calculated over short timescales (<60 years) are inflated due to a greater contribution of labile, seasonally cycling OC in the root zone. The relative standard deviation of OC accumulation rates between the four cores decreased as timescale increased, indicating increased accuracy of estimates with increasing timescale. OC accumulation rates reported in scientific literature are only comparable when using the same or similar timescales. Only the 137Cs-based C accumulation estimate from this study was comparable to regionally (NW Atlantic 134.0 ± 12.8 g m-2 yr-1) and globally (244.7 ± 26.1 g m-2 yr-1) reported rates, likely because most studies use 137Cs or marker horizons to estimate blue C burial rates rather than 210Pb or 14C that date soils over longer timescales (210Pb ≤ 200 years; 14C ≤ 60,000 years). The blue C community may thus be currently reporting inflated OC burial rates. These rates need to be recontextualized within their associated timescales. In addition to timescale, depth is also of major importance when calculating C accumulation rates. A balance of any seasonal, labile OC inputs with remineralization-induced outputs is necessary to attain an accurate C accumulation rate, independent of vegetative or seasonal influence. Based on the soil OC profiles from this study and a handful of other published works, I suggest collecting cores of at least 50 cm in order to achieve this balance. However, if the research question at hand requires the use of shorter timescales, this recommendation would become irrelevant. ☐ The organic matter (OM) composition at selected depth ranges was also characterized to evaluate the forms of OC that accumulate over time to better understand the forms of C being stored in marsh soils. Soil and humic acid (HA) molecular properties were characterized at discrete depths via elemental analysis, 13C NMR, UV/Vis and EEMs spectroscopy. Overall, the variability in soil and humic acid characteristics decreased with depth, suggesting increasing homogeneity in salt marsh soil OM properties over the course of burial and early diagenetic processes below the root zone. Soil OC concentrations were highly variable at the surface but decreased and converged to similar values at depth. Similarly, HA C/S ratios were high at the surface but decreased and converged over depth, potentially indicating sulfurization and selective retention of organic sulfur compounds. HAs from all four cores showed increasing structural similarity over depth with marsh aromatic C contributions increasing with core depth. The increasing structural similarity with depth suggests that apparent vegetative, seasonal, and possibly tidal influences on HA properties observed at the surface dissipate during early diagenesis. Future molecular level characterization is needed to evaluate whether certain compounds or compound classes (i.e., sulfurization products) may be selectively preserved and/or which potential mechanisms (e.g., selective preservation, mineral interactions, dilution, environmental conditions) are most important for burying C in salt marsh soils.
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Keywords
Organic matter, Salt marshes, Accumulation rate, Burial rate, Great Marsh, Delaware