Effects of recirculation of seawater enriched in inorganic nitrogen on dissolved organic carbon processing in sandy seepage face sediments

• DOC production rates covaried with porewater flow rates in our experiments.
• Benthic organic matter decomposition increase the refractory nature of the resulting DOC fluxes in our experiments.
• High DIN concentrations and high seepage velocities act to catalyze the microbial breakdown of refractory organic material.
• The availability of DIN in the experiments increases the release of labile DOC.
• Our results suggest that increasing DIN availability in coastal permeable sediments enhances organic C processing within the sediment.

Human activities are now the main source of bioavailable Nitrogen to Earth's ecosystems. Disruption of the N cycle can promote changes to the biogeochemical cycling of other elements in particular to the oxygen and carbon cycles. Nevertheless, information on how increasing Dissolved Inorganic Nitrogen (DIN) availability might affect the benthic processing of organic matter in coastal sandy sediments is currently unavailable. Here, we present a series of flow-through reactor experiments conducted with undisturbed sediment cores collected at an intertidal seepage face (Ria Formosa lagoon, SW Iberian Peninsula) in order to evaluate the effect of increasing DIN availability on benthic dissolved organic C (DOC) reactivity. Our results show that the metabolic activity of the benthic community can be a significant source of DOM to the porewater. Furthermore, in the absence of DIN, increasing porewater flow rates accelerated benthic DOM production and increased the recalcitrant nature of DOC transported across the sediment-water interface. In contrast, the co-occurrence of high DIN concentrations and high seepage velocities increased the microbial breakdown of organic material. Our results suggest that the availability of DIN in permeable seepage faces increases the net production of labile DOC, by enhancing the ability of the benthic microbial community to process refractory organic matter. Hence, we suggest that increasing DIN availability in coastal permeable sediments accelerates benthic organic C processing, promoting the release of more labile DOC to receiving water bodies.