Physical Protection in Aggregates and Organo-Mineral Associations Contribute to Carbon Stabilization at the Transition Zone of Seasonally Saturated Wetlands

Abstract

Wetlands store significant soil organic carbon (SOC) globally due to anoxic conditions that suppress SOC loss, yet this SOC is sensitive to climate and land use change. Seasonally saturated wetlands experience fluctuating hydrologic conditions that may also promote mechanisms known to control SOC stabilization in upland soils; these wetlands are therefore likely to be important for SOC storage at the landscape-scale. We investigated the role of physicochemical mechanisms of SOC stabilization in five seasonally saturated wetlands to test the hypothesis that these mechanisms are present, particularly in the transition between wetland and upland where soil saturation is most variable. At each wetland, we monitored water level and collected soil samples at five points along a transect from frequently saturated basin edge to rarely saturated upland. We quantified physical protection of SOC in aggregates and organo-mineral associations in mineral horizons to 0.5 m depth. As expected, SOC decreased from basin edge to upland. In the basin edge and transition zone, the majority of SOC was physically protected in macroaggregates. By contrast, overall organo-mineral associations were low, with the highest Fe concentrations (5 mg Fe g -1 soil) in the transition zone. While both stabilization mechanisms were present in the transition zone, physical protection is more likely to influence SOC stabilization during dry periods in seasonally saturated wetlands.As future climate scenarios predict changes in wetland wet and dry cycles, understanding the mechanisms by which SOC is stabilized in wetland soils is critical for predicting the vulnerability of SOC to future change.

Water‐Soluble Organic Matter From Soils at the Terrestrial‐Aquatic Interface in Wetland‐Dominated Landscapes

“…Due to the relatively small spatial scale and low-relief landscape setting of Delmarva Bays, water table elevation along the transects was estimated assuming a linear groundwater table in-between the two monitoring wells. Kottkamp et al (2022) validated this approach at the wetlands used in this study, finding an R 2 of 0.87 between measured and estimated water levels along the upland to wetland gradient. Water level data were used to calculate minimum, mean, and maximum water level relative to the ground surface at each transect point.…”

“…Mean bulk densities from Kottkamp et al. (2022), O horizon thickness, saturation duration from 1 October 2019 to date of winter sampling in 2020, and resulting predicted WSOM release during this saturation period are also presented. Results are presented as mean ± standard error.…”

“…As climate change is expected to alter the hydrologic regime of wetlands (e.g., rapid water level rise during more intense precipitation events), portions of the landscape may shift from carbon sinks to sources, making it necessary to understand how hydrology drives DOM release from soils (S. E. Tank et al., 2018). While past work has measured soil carbon storage, methane‐cycling microbial communities, greenhouse gas emissions, and seasonal DOM patterns in wetland‐dominated landscapes (e.g., Badiou et al., 2011; Chow et al., 2013; Hondula et al., 2021; Hosen et al., 2018; Kottkamp et al., 2022; Maietta et al., 2020), we lack information on the DOM sources contributing to surface water DOM dynamics. Water‐soluble organic matter (WSOM) quantifies potential DOM release from soils (Gabor et al., 2015; Jones & Willett, 2006; Rennert et al., 2007).…”

Setting a reference for wetland carbon: the importance of accounting for hydrology, topography, and natural variability

“…Further, inundation does not always slow decomposition [142]. Decomposition rates also depend on factors such as organic matter chemistry, physicochemical stabilization, microbial communities, and the availability of alternative electron acceptors [145][146][147]. Overall, though hydrology is a 'master variable' in wetland function, its breadth of biogeochemical influence makes it difficult to quantify its role in specific processes.…”

Estimation of Soil Organic Carbon Content in Coastal Wetlands with Measured VIS-NIR Spectroscopy Using Optimized Support Vector Machines and Random Forests

“…Soils are known as the largest terrestrial carbon reservoirs and are rich in organic carbon content [1]. Wetlands contain profuse amounts of carbon stocks, and wetland soils are a major component of the terrestrial carbon cycle [2], with coastal wetland soil organic carbon (CW-SOC), also known as "blue carbon" [3], playing an important role in maintaining the global carbon cycle balance. Located at the intersection of land and sea, ref.…”