Dissolved Organic Matter Sources from Soil Horizons with Varying Hydrology and Distance from Wetland Edge
Wardinski, Katherine Mary
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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.
General Audience Abstract
Wetlands store carbon in their plant biomass and soils, which helps to mitigate the effects of climate change by keeping carbon out of the atmosphere. Carbon builds up in wetland soils because the continuously wet conditions slow down the microbial processes that would otherwise break down the organic matter (carbon) and release it to the atmosphere via greenhouse gas emissions. However, the organic matter that accumulates in wetland soils may be released into water, generating dissolved organic matter (DOM). This DOM has the potential to flow out of the wetland, providing a source of energy to aquatic organisms or impacting downstream water quality. Not all wetlands are continuously connected to other water bodies. Geographically Isolated Wetlands (GIW) are wetlands that you could walk all the way around and keep your feet dry. Despite lack of continuous surface water connections, GIWs may still influence downstream water quality via groundwater flow paths or seasonal surface water connections. This variable connectivity makes GIWs a unique setting to study carbon storage and fluxes in wetland soils. The potential for soil-derived DOM generation was studied by extracting organic matter from soils along a wet to dry gradient in Delmarva Bay GIWs. Shallow soils had the largest quantities of extractable soil organic matter (ESOM) and this organic matter is likely sourced from plant inputs to the soil. ESOM from deeper soils was more similar to the microbes that consume and alter the organic matter as it cycles deeper into the soil. Soils located in the wetland basin had less ESOM because continuous saturation depleted the pool of ESOM. Despite lower values of ESOM, wetland soils are very thick and continuously saturated, making these soils the largest potential contributor of soil-derived DOM to Delmarva Bay GIWs. This work furthers our understanding of how hydrology drives carbon cycling in GIWs and will inform wetland restoration efforts designed to create carbon sinks.
- Masters Theses