Coastal Wetland Soil Carbon Storage at Mangrove Range Limits in Apalachicola Bay, FL: Observations and Expectations

Abstract

Globally, mangrove range limits are expanding, often at the cost of adjacent coastal ecosystems including saltmarshes, potentially leading to a change in ecosystem services such as organic carbon (OC) sequestration. Studies in the southeastern US have focused almost exclusively on Avicennia germinans range expansion, the most cold-tolerant mangroves in North America. The Apalachicola Bay region of north Florida represents the northern range limit of mangroves in the Gulf of Mexico, and uniquely also includes Rhizophora mangle. The objective of this research was to quantify soil OC density beneath both mangrove species and compare results to the soils beneath two contiguous native tidal saltmarsh species: Juncus roemerianus and Spartina alterniflora in a barrier island setting. Dominant plant taxa were not a significant predictor of soil OC density, highlighting the relative importance of site-specific environmental attributes as controls on soil properties. Soil profile δ13C compositions included a range of values reflective of C3 and C4 plant inputs, suggesting that shifts in plant taxa, both from marsh to mangroves and between marsh species, have been occurring at all sites in this study. These findings support much of the literature on mangrove encroachment, which indicates mangrove soil OC concentrations, densities, or stocks are less than or equal to that of co-located tidal marsh habitats. Through a systematic review, the potential of several proposed explanatory variables (climate, environmental setting, plant physiology and productivity, and duration of encroachment) were identified to evaluate how soil OC density in mangrove habitats might increase over time, which is critical to forecasting how continued mangrove expansion might affect blue C storage as these habitats evolve.

Linking temperature sensitivity of mangrove communities, populations and individuals across a tropical‐temperate transitional zone

“…Higher mangrove aboveground biomass (Doughty et al., 2016; Osland et al., 2022; Simpson et al., 2021) may also increase blue carbon storage. However, comparative studies regarding changes to belowground carbon storage under mangrove encroachment into saltmarsh have generated mixed results (Osland et al., 2022; Steinmuller et al., 2022), and mangrove encroachment into saltmarsh has not yet resulted in differences in soil carbon storage in Apalachicola Bay (Steinmuller et al., 2022). These findings highlight the role that local environmental settings play in soil carbon cycling and storage, reinforcing the need for further long‐term investigation into how mangrove poleward expansion affects soil carbon cycling.…”

Do global change variables alter mangrove decomposition? A systematic review

“…Biomass quality is likely altered by mangrove species identity, as is suggested in Table 2 and Figure 6, and decay will depend on species‐specific responses or biotic interactions among species to global change drivers. However, our knowledge of chemical and physical controls on SOM development and decomposition in mangrove ecosystems is very limited (but see Breithaupt et al (2020) and Steinmuller et al (2022) who effectively used both radio and stable isotopes to assess mangrove SOM development), and these conclusions need to be further substantiated with experimental work that explicitly manipulates global change variables in decomposition and SOM processing studies.…”

“…Studies should consider pairing above‐ and belowground decomposition bags, which will give a holistic understanding of the factors driving decomposition in the ecosystem. Furthermore, SOM is more important than fresh litter in driving blue C storage and yet we have limited knowledge of the drivers of SOM formation in coastal wetlands (Breithaupt et al, 2020; Spivak et al, 2019; Steinmuller et al, 2022). Examinations of the biogeochemistry of mangrove SOM, and its sensitivity to decomposition during environmental perturbations such as drought, fire, or hurricanes, is an important future direction of research. Salinity studies .…”