Composition of organic matter in soils from tidal marshes around the Chesapeake Bay, USA, as revealed by lipid biomarkers and stable carbon and nitrogen isotopes

Abstract

The ability of tidal marshes to maintain their elevation despite rising sea levels depends on the accumulation of organic matter (OM) from estuarine particulate matter and in situ marsh plant production. Although previous studies have examined carbon in marshes, few have used the source composition of OM in marsh soils to provide insight into the processes controlling the delivery, transformation, and fate of carbon within marshes. This study used multiple geochemical tools (i.e., n-alkanes, fatty acids (FA), sterols, and stable carbon and nitrogen isotopes) to characterize the sources of OM in surface (0–10 cm) and subsurface (30–40 cm) soils collected along transects from the marsh edge to the low marsh/high marsh transition in tidal marshes around the Chesapeake Bay. Four study sites were selected for their different physical settings and soil types, allowing us to characterize the dominant OM sources in the soils and identify factors that contributed to differences in marsh OM composition. Contributions of OM from estuarine (i.e., short-chain FA, brassicasterol, cholesterol), marsh plant (i.e., long-chain FA, mid- and long-chain n-alkanes, sitosterol, taraxerol), and microbial (i.e., branched fatty acids, ergosterol) sources were identified in all soils. Results from a stable isotope mixing model allowed us to quantify the relative contributions of these OM sources to the marsh soils and indicated that soil OM was comprised of 29.0 ± 9.0% estuarine POM, 22.7 ± 5.5% riverine POM, 22.1 ± 11.3% C3 marsh plant OM, and 37.9 ± 13.8% C4 marsh plant OM. Relative contributions from these sources varied depending on location in the marsh with estuarine OM contributing a greater fraction of OM near the marsh edge and to surface soils while marsh plants contributed larger amounts of OM to soils in the marsh interior and in the sub-surface. Our results suggest that long-term carbon sequestration in marsh soils in controlled by the marsh plant community, and that changes to a marsh's physical setting or plant community in response to changing climate or human activity could alter the sequestration of carbon in marshes.