Variability of organic carbon accumulation on a tidal wetland coast

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University of Delaware
Organic carbon burial in tidal wetlands has been proposed to be an important means to mitigate the impacts of greenhouse gas emissions on climate change, given that coastal marshes and mangroves are a major sink for atmospheric carbon dioxide globally. Among the different types of wetland ecosystems, carbon accumulation in tidal marshes (tidal fresh, brackish, and salt marshes) is particularly high with a global average of 244.7 g C m-2 yr-1 (Ouyang and Lee, 2014). In these ecosystems high primary production rates coupled with slow rates of decomposition cause organic matter to accumulate in sediments and soils. It is crucial to understand the processes governing carbon burial to address how marsh ecosystems will respond to climate change. If marsh accretion and landward migration keeps pace with sea level rise, then marshes may continue to function as a carbon sink. This study investigated the spatial variability of carbon accumulation on the tidal wetland coast of western Delaware Estuary. The following three specific objectives were addressed: (1) patterns and rates of marsh carbon accumulation along the estuarine salinity gradient; (2) the time-dependence of carbon accumulation averaged over timescales ranging from several decades to millennia; and (3) variations in carbon accumulation associated with different soil organic matter size fractions. While it is generally known that organic carbon accumulates in marsh soils through a combination of plant primary production and inputs of allochthonous particulate organic matter and mineral sediment, there are many unanswered questions concerning the mechanisms of soil formation and carbon burial. Understanding variations in carbon accumulation rates both spatially and temporally in Delaware Estuary tidal wetlands will provide important geological context for assessments of carbon sequestration regionally. Radionuclide chronometry (137Cs, 210Pb, 14C) for 24 short cores and five long cores was used in this study along with measurements of organic carbon concentration from core subsamples to compute carbon accumulation rates. Total organic carbon and total nitrogen concentration was determined for a total of 613 soil samples. A soil fractionation procedure was developed to isolate the specific contributions of plant biomass, particulate organic and inorganic (mineral sediment) sedimentary materials. Using a combination of sieving and centrifugation, four soil carbon size fractions were isolated and analyzed for organic carbon and nitrogen content: >125μm, 63–125μm, 4–63μm (silt size), and <4μm (clay size). Results indicated significant spatial variation in carbon accumulation rates along the tidal wetland coast. Short term rates (~50-year average) determined using 137Cs or 210Pb chronology were generally higher (246.4 + 101.8 g C m-2 yr-1) for the brackish marshes than the salt marshes sampled for this study (147.8 + 66 g C m-2 yr- 1). The higher rates of carbon accumulation can be explained by higher rates of mineral sediment accumulation in the brackish marshes, perhaps because of the closer proximity of the brackish marshes than the saltmarshes to the estuarine turbidity maximum zone of Delaware Estuary. As a whole, rates of carbon accumulation determined for this study fall within the range of rates reported for a wide range of tidal marshes dominated by Spartina vegetation. As expected, short-term rates of marsh carbon accumulation (overall mean=172.4 + 85.9 g C m-2 yr-1) were higher than longer-term rates (72.8 + 24.3 g C m-2 yr-1) determined using radiocarbon dating (200–1500 year averages). Carbon accumulation rates varied inversely with the period of averaging even though soil carbon concentrations did not exhibit decreases over the same time span. This suggests that non-steady-state sediment accumulation of the soil column exerts a control on long-term burial of marsh carbon. A major finding of this research is that fine-grained particulate organic matter and (or) mineral-associated carbon delivered by the tides comprises a significant (>40%) proportion of the total soil carbon buried in Delaware Estuary marsh soils. Although plant biomass carbon (>125μm and 63–125μm fractions) had the highest organic carbon concentrations among the four size fractions, the silt- and clay-sized fractions were large and dominated the soil carbon inventory in four out of the five marshes investigated. The silt- and clay-sized carbon samples were compositionally distinct from plant biomass carbon accumulating at the same soil depth. Specifically, C/N ratios for the fine-grained carbon fractions were suggestive of degraded C-4 plant matter or a mixture of C-3 plant matter and terrestrial soil organic matter. The source of this fine-grained, allochthonous carbon is presumably aged marsh deposits, including particulate organic matter and mineral-associated carbon, reintroduced to the surface by marsh edge erosion and perhaps particle bioturbation. Identifying the composition, provenance, and mineral associations of this fine-grained carbon will require further research.