Abstract. Benthic nitrogen transformation pathways were investigated in the sediment of the East China Sea (ECS) in June of 2010 using the 15N isotope pairing technique. Slurry incubations indicated that denitrification, anammox and dissimilatory nitrate reduction to ammonium (DNRA) as well as intracellular nitrate release occurred in the ECS sediments. These four processes did not exist independently, nitrate release therefore diluted the 15N labeling fraction of NO3−, and a part of the 15NH4+ derived from DNRA also formed 30N2 via anammox. Therefore, current methods of rate calculations led to over and underestimations of anammox and denitrification respectively. Following the procedure outlined in Thamdrup and Dalsgaard (2002), denitrification rates were slightly underestimated by an average 6% without regard to the effect of nitrate release, while this underestimation could be counteracted by the presence of DNRA. On the contrary, anammox rates calculated from 15NO3− experiment were significantly overestimated by 42% without considering nitrate release. In our study, this overestimation could only be compensated 14% by taking DNRA into consideration. In a parallel experiment amended with 15NH4++14NO3−, anammox rates were not significantly influenced by DNRA due to the high background of 15NH4+ addition. The significant correlation between potential denitrification rate and sediment organic matter content (r = 0.68, p < 0.001, Pearson) indicated that denitrification was regulated by organic matter, while, no such correlations were found for anammox and DNRA. The relative contribution of anammox to the total N-loss increased from 13% at the shallowest site near the Changjiang estuary to 50% at the deepest site on the outer shelf, implying the significant role of anammox in benthic nitrogen cycling in the ECS sediments, especially on the outer shelf. N-loss as N2 was the main pathway, while DNRA was also an important pathway accounting for 20–31% of benthic nitrate reduction in the ECS. Our study demonstrates the complicated interactions among different benthic nitrogen transformations and the importance of considering denitrification, DNRA, anammox and nitrate release together when designing and interpreting future studies.

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The 15 N isotope pairing technique is widely used to quantify anammox, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) in sediments. However, the effects of DNRA on anammox and denitrification in slurry incubations and on genuine N 2 production in intact core incubations have not been fully explored. We developed a mathematical model describing these effects of DNRA, and tested this model in field and computational studies. Our model calculations revealed that for slurry incubations the presence of DNRA tends to under -and overestimate the actual anammox and denitrification rates, respectively, when calculated according to Thamdrup and Dalsgaard (2002). We found this underestimate of anammox to be proportional to the 15 NH 1 4 mole fraction (F A ), and the overestimate of denitrification to be related to both F A and the relative contribution of anammox to the total N 2 production (ra). We propose three alternative procedures to better quantify anammox and denitrification rates in slurry incubations and two procedures for intact core incubations. Our model calculations also revealed that for intact core incubations, the presence of DNRA leads to an overestimate of the genuine N 2 production rate when calculated according to Risgaard-Petersen et al. (2003). This overestimate depends on the ra and the mole fractions of 15 NH 1 4 and 15 NO 2 3 in intact core incubations. The results of our field experiments and numerical modeling indicated that the overestimate of the genuine N 2 production was less than 1% at two sites in the East China Sea.

The Kuroshio intrusion from the West Philippine Sea (WPS) and mesoscale eddies are important hydrological features in the northern South China Sea (SCS). In this study, absorption and fluorescence of dissolved organic matter (CDOM and FDOM) were determined to assess the impact of these hydrological features on DOM dynamics in the SCS. DOM in the upper 100 m of the northern SCS had higher absorption, fluorescence, and degree of humification than in the Kuroshio Current of the WPS. The results of an isopycnal mixing model showed that CDOM and humic‐like FDOM inventories in the upper 100 m of the SCS were modulated by the Kuroshio intrusion. However, protein‐like FDOM was influenced by in situ processes. This basic trend was modified by mesoscale eddies, three of which were encountered during the fieldwork (one warm eddy and two cold eddies). DOM optical properties inside the warm eddy resembled those of DOM in the WPS, indicating that warm eddies could derive from the Kuroshio Current through Luzon Strait. DOM at the center of cold eddies was enriched in humic‐like fluorescence and had lower spectral slopes than in eddy‐free waters, suggesting inputs of humic‐rich DOM from upwelling and enhanced productivity inside the eddy. Excess CDOM and FDOM in northern SCS intermediate water led to export to the Pacific Ocean interior, potentially delivering refractory carbon to the deep ocean. This study demonstrated that DOM optical properties are promising tools to study active marginal sea‐open ocean interactions.