Factors controlling concentration and decomposition of dissolved organic matter in pore water on the shelf of the East China Sea

Understanding of microbial communities and carbon cycling in marine sediment fills in the gap of biogeochemical cycle of carbon and nutrients in the largest carbon reservoir. Nevertheless, information on what factors controlling concentration and decomposition of dissolved organic matter (DOM) and their interaction with the nutrient cycles in shelf sediments is limited studied. Here, we determined variation of dissolved organic carbon (DOC) and nutrients and DOM quality by excitation–emission matrix spectroscopy combined with parallel factor analysis (EEM–PARAFAC) in two contrasting core sediments’ pore water systems (C10 vs N6 station). By utilizing Miseq illumine sequencing V4-V5 regions of 16S rRNA gene, we found the distinct bacterial community structures between two core samples. C10 harbor abundant denitrifies, whereas bacterial groups participated in degrading high molecular weighted organic matters in N6. Our results show that the metabolic activity of the benthic community can be a significant factor controlling the dynamic of DOM and nutrients in the pore water. Furthermore, in the absence of nitrate, high DOM accelerated ammonium production and increased the recalcitrant nature of DOC accumulated in the core. In contrast, the co-occurrence of high nitrate concentrations and low bioavailability of DOM enhanced the denitrification process in C10 station. The optical properties of DOM, however, also changed successively along the increasing depth in N6 but limited variation in C10 core. Our results suggest that both original composition of DOM and microbial community are important drivers controlling DOM and nutrients dynamics in marine sediment. The biogeochemical processes controlling the DOM composition are complex and merit further investigation.