Carbon and nitrogen composition and stable isotope as potential indicators of source and fate of organic matter in the salt marsh of the Changjiang Estuary, China

Elemental (TOC, TN, C/N) and stable carbon and nitrogen isotopic (δ¹³C, δ¹⁵N) compositions were measured for surface sediments, three sediment vibrocores, plants, and suspended particulate matter (SPM) collected from salt marsh of the Changjiang Estuary. The purpose of this study is to characterize the sources of organic matter in sediments and to further elucidate the factors influencing the isotope signature in the salt marsh. Our results indicate that organic matter preserved in the sediments is predominantly controlled by the particulate organic matter in the Changjiang Estuary. 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 (-28.1‰) is reflected by a sediment δ¹³C (-24.7‰) lower than values found for the low marsh and bare flat sediments (-23.4‰ and -23.0‰, 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. Nutrient utilization by plant assimilation, however, shows great impact on the surface sediment δ¹⁵N composition, due to the isotope fractionation. With extensive plant coverage and the consequent low surface water nitrate concentration, δ¹⁵N values of the high marsh surface sediments show ¹⁵N enrichment.