Dissolved Organic Matter Sources from Soil Horizons with Varying Hydrology and Distance from Wetland Edge

Understanding hydrologic controls on carbon accumulation and export within geographically isolated wetlands (GIW) has implications for the success of wetland restoration efforts intended to produce carbon sinks. However, little is known about how hydrologic connectivity along the aquatic-terrestrial interface in GIW catchments influences carbon dynamics, particularly regarding dissolved organic matter (DOM) transport and transformation. The organic matter (carbon) that accumulates in wetland soils may be released into water, generating DOM. DOM is mobile and reactive, making it influential to aquatic metabolism and water quality. To understand the role of different soil horizons as potential sources of DOM, extractable soil organic matter (ESOM) was measured in soil horizons collected from upland to wetland transects at four Delmarva Bay GIWs on the Delmarva Peninsula in the eastern United States. ESOM quantity and quality were analyzed to provide insights to organic matter sources and chemical characteristics. Findings demonstrated that ESOM in shallow organic horizons had increased aromaticity, higher molecular weight, and plant-like signatures. ESOM from deeper, mineral horizons had lower aromaticity, lower molecular weights, and protein-like signatures. Organic soil horizons had the largest quantities of ESOM, and ESOM decreased with increasing soil depth. ESOM quantities also generally decreased from the upland to the wetland, suggesting that continuous soil saturation leads to a decreased quantity of ESOM. Despite wetland soils having lower ESOM, these horizons are thicker and continuously hydrologically connected to wetland surface water, leading to wetland soils representing the largest potential source of DOM to the Delmarva Bay wetland system. Knowledge of which soil horizons are most biogeochemically significant for DOM transport in Delmarva and other GIW systems will become increasingly important as climate change is expected to alter the hydrologic connectivity of wetland soils to the surface water-groundwater continuum and as wetlands are more frequently designed for carbon sequestration.