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Abstract

Benthic nitrogen transformation pathways were investigated in the sediment of the East China Sea (ECS) in June of 2010 using the N-15 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 N-15 labeling fraction of NO3-, and a part of the (NH4+)-N-15 derived from DNRA also formed N-30(2) 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 (NO3-)-N-15 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 (NH4+)-N-15+(NO3-)-N-14, anammox rates were not significantly influenced by DNRA due to the high background of (NH4+)-N-15 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 N-2 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.