Salt marsh vegetation is an important contributor of dissolved organic matter (DOM) to coastal waters. The dynamics of DOM leaching from different marsh plants, however, have not been well studied or compared.

In this study, we conducted laboratory experiments to investigate the processes of DOM leaching from three common marsh plants (Phragmites australis, Suaeda salsa and Aeluropus littoralis) collected from the Yellow River Delta (YRD) salt marsh in October 2016. The YRD is one of the largest and most well-protected coastal ecosystems on the east coast of China.

We found that the plant leaves released DOM at much higher concentrations than the plant roots or stems, as measured by the dissolved organic carbon (DOC) and dissolved nitrogen (DN). On average, 15% of the biomass C and 30% of the biomass N were released from the plant leaves as DOC and DN during the 27-day incubation period. The DOM released from the plants was very labile, and 92.4%–98.1% of the DOC and 88.0%–94.6% of the DN released from the plants were consumed by bacteria during the 27-day incubation period. The fluorescence characteristics of the plant-released DOM indicated that chromophoric dissolved organic matter was a major fraction of the DOM and that protein-like components were the primary organic fractions released from the plants. Bacterial degradation altered both the fluorescence properties and the chemical composition of the DOM. The results of the laboratory experiments were well supported by the field investigation, which indicated that a large amount of DOM was outwelled from the YRD salt marshes in late autumn. Our study suggests that the DOM released from the biomass of salt marsh plants provides an important source of both DOC and DN for marsh and coastal waters. The highly labile DOC and DN provide essential food sources to support microbial communities in the YRD salt marsh and adjacent coastal waters.