Increased Organic Carbon Burial in Northern Florida Mangrove‐Salt Marsh Transition Zones

Abstract

Blue carbon habitats like salt marshes and mangroves bury large amounts of carbon with limited area; however, they also are increasingly susceptible to current climate change. Combined effects of rising temperatures, decreasing freeze frequencies, and increasing sea level rise rates are resulting in mangrove replacement of salt marshes along the southern United States. Surface soils analyzed here from wetlands along northern Florida Atlantic and Gulf Coasts showed higher apparent sedimentation rates in mangrove-dominated sites (1.5-3.2 mm yr −1 ) and where mangroves are migrating into the marsh (termed transition sites, 2.3-3.8 mm yr −1 ). Average carbon burial rates were higher in transition sites for both coasts (27-47 g C·m −2 ·yr −1 ) compared to the respective mangrove (10-22 g C·m −2 ·yr −1 ) and salt marsh (4-7 g C·m −2 ·yr −1 ) sites. Lignin biomarker data (Λ-6, Λ-8, and C/V) indicated that mangrove and transition sites had higher lignin inputs from woody vascular plants compared to salt marsh sites, which may slow decadal-to centennial-scale decay. Higher amino acid concentrations in mangrove soils relative to mangrove biomass (1.8-2.3 mmol g C −1 vs. 0.2-0.9 mmol g C −1 ) and lower C/N indicated that these mangrove sites receive higher algal inputs than the transition and salt marsh sites, attributed to greater tidal inundation in the mangrove sites given their position near the shoreline. Overall, increased accretion, carbon burial, and lignin in mangrove transition sites indicate that this migration may increase carbon burial and increase the stability and residence time of buried soil carbon. Future studies on mangrove migration in northern Florida can verify this through replication and elevation analysis.

Blue Carbon Soil Stock Development and Estimates Within Northern Florida Wetlands

“…When comparing intraspecific differences within coasts, the transition cores contain the highest OC stocks for both sites (includes total and 1m stocks) while the mangrove cores have the lowest, suggesting site location may influence the amount of carbon buried and stored. This is similar to carbon burial results for these cores over the recent ∼120 years with higher carbon burial in the transition core associated with high biomass production and the location of the transition site being further inland than the mangrove site (Vaughn et al, 2020). The mangroves location along the shorelines meant there is likely more organic matter being exported with tidal and wave action, (e.g.…”

“…The spike in TOC ∼2,500cal yr. B.P. also corresponds to slight increases in total lignin (Λ-6 and Λ-8), similar to that seen in the transition core over the last ∼120 years with the recent mangrove expansion (Vaughn et al, 2020). This suggests the spike in TOC and lignin follows another shift in vegetation ∼2,500cal•yr.…”

“…for St. Augustine cores, go from 0.6 to 1.6mm•yr −1 at 50cm to 0.6mm•yr −1 for the entire core lengths). These 14 C derived rates are additionally lower compared to 210 Pb data at the surface of these cores, which revealed sedimentation rates of 0.4-3.8mm•yr −1 (Vaughn et al, 2020); however, they are in range for millennial-scale sedimentation rates in other salt marsh and mangrove environments (0.3-2.5mm•yr −1 ; Carter et al, 1993;Parkinson and White, 1994;Nydick et al, 1995;Saintilan and Wilton, 2001;Serandrei-Barbero et al, 2006). Besides an increase in degradation down-core, decreases in sedimentation rates, derived from 210 Pb data at the surface to 14 C data at the bottom of the cores, may suggest an increase in soil material added over time (or a combination of soil material increase up-core with degradation down-core).…”

“…Colhoun, 1986). This could explain the modern ages in the salt marsh core from Waccasassa Bay; however, as noted by the 210 Pb data for the surface soils of Waccasassa Bay (Vaughn et al, 2020), soil mixing in these wetlands is minimal. It thus seems unlikely bioturbation or re-depositional events contributed to the younger carbon at depth.…”

Carbon Balance in Salt Marsh and Mangrove Ecosystems: A Global Synthesis

“…For instance, sea-level rise will result in die-off of established plant communities with an increase in export and burial of plant debris; burial rates will concomitantly increase. It has already been observed that the encroachment of mangroves into salt marshes has resulted in an increase in storage of C ORG [238,239]. Increases in temperature and atmospheric carbon dioxide concentrations will likely result in increased mangrove and salt marsh productivity and respiration [240][241][242], altering the carbon balance of these ecosystems.…”