| Coastal salt marshes are located at the boundary between ocean and land. Both tides and waves can significantly influence the exchange of chemical materials between the marsh and the adjacent ecosystems. The biogeochemical process in the estuarine salt marsh is very complicated and the combined effects of the physical, chemical and biological processes can markedly influence the signature of the organic materials in the salt marsh. As one of the important issues of the global carbon cycle, the biogeochemistry studies of organic materials in coastal salt marshes have been an absorbing world-wide topic. However, the organic geochemistry study in the salt marsh is still limited up to the present. For the salt marsh in the Changjiang Estuary, one of the most important coastal salt marshes in the world, the source, distribution, chemical transformation and transportation in the system remains largely unknown.Surface sediments, three sediment cores, plants, benthic invertebrates and suspended particulate matter (SPM) were collected from the salt marsh of the Changjiang Estuary. Elemental (TOC, TN, TP and TS) and stable carbon and nitrogen isotopic (δ13C, δ15N) compositions, as well as lipids, including n-alkanes, fatty acids, PAHs and sterols were measured. Two surface sediment samples collected from high marsh and bare flat were size-fractionated into four primary size fractions: >63, 32-63, 4-32 and <4 μm. Litterbag experiment was carried out for both roots and stems of Scirpus triqueter, the dominant plant in the study area, during a 391-day period. The purpose of this study is to characterize the sources of organic matter in sediments, to elucidate the factors influencing the distribution of organic matter in sediments, as well as to understand the degradation processes of organic matter in the environment. The role of the salt marsh in carbon exchange for the adjacent ecosystems (i.e. the Changjiang Estuary and the atmosphere) is also evaluated.The TOC concentration in surface sediments is quite low, ranging from 0.1 to 0.7%, with the highest value in the high marsh. C/N ratios of surface sediments are from 6 to 11, indicating the predominately riverine source of organic matter in the marsh with minor contributions from marsh vegetation. A similar trend of general landward increases in TOC, TN, TS and clay percentage in surface sediments is observed, reflecting the increased sorption of more organic matter to fine particles. The distribution of TP, however, shows less correlation with grain size, probably dueto the large abundance of Ca-P that can be found in all grain size classes. Despite the influence of grain size, TOC and TN content in some core sections, i.e., middle layers of core sediments in high and low marsh, can be significantly affected by the contribution of vegetation, with 22-55% of TOC (averaged 37%) and 0.6-35% of TN (averaged 16%) in the sediments from the impact of vegetation.The in situ contribution of marsh plants carbon to sediment organic matter is clearest in the high marsh, where the low δ13C of the plants (-28.0‰) is reflected by a sediment δ13C (-24.3-23.8‰) lower than values found for the low marsh and bare flat sediments (-24.2-22.8‰ and -23.1—-21.9‰, respectively). The effect of grain size on the spatial difference of isotope composition in the marsh sediments is insignificant, based on the observation that similar isotope values are found in different size particles, both for δ13C and δ15N. Relative contributions of plant-derived and river-carried organic matter to the sedimentary organic pool can be calculated based on a two end-member mixing model. The result indicates that river input comprises 49-84% of the organic matter present in the core section (with average of 71%). The high accumulation of plant-derived organic matter is observed in the mid-depth of the core, consistent with the former finding by TOC distribution. Nutrient utilization by plant assimilation, however, shows great impact on the surface sediment δ15N composition, due to the isot...
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