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, yet this SOC is sensitive to climate and land use change. Seasonally saturated wetlands experience fluctuating hydrologic conditions that may promote the physicochemical 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 at the transition zone between wetland and upland where saturation in the upper soil profile 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 be a dominant mechanism of SOC stabilization 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

“…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.…”

“…For example, Wetland soils were saturated 72% of the 2020 water year on average. The Wetland points have little aquatic vegetation present and the constant saturation favors methanogen microbial communities, lower decomposition rates, and macroaggregate carbon stabilization (Kottkamp et al., 2022; Maietta et al., 2020). In contrast, the Upland points were rarely saturated in the upper 50 cm of the soil profile by the groundwater table during the 2020 water year and therefore only received moisture via throughfall that infiltrated into the soil profile.…”

“…Kottkamp et al. (2022) reported soil organic matter stocks ranging from 4,000 to 12,000 g C/m 2 in shallow Delmarva Bay wetland soil horizons. This indicates that WSOM, which typically ranged from 1 to 7 g WSOC/m 2 , represents a very small proportion of Delmarva Bay soil organic matter stocks.…”

“…Continuous water level data, averaged to a daily time step, were collected over the 2020 water year (1 October 2019 to 30 September 2020) using HOBO pressure transducers (Model No. U20‐001‐04) in the previously established wetland and upland monitoring wells (Hondula et al., 2021; Kottkamp et al., 2022; Maietta et al., 2020). 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.…”