Anaerobic ammonium oxidation, denitrification and dissimilatory nitrate reduction to ammonium in the East China Sea sediment.

Benthic nitrogen transformation pathways were investigated in the sediment of the East China Sea 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 nitrate release by nitrate storing organisms occurred in the East China Sea sediments. These four processes did not exist independently, the nitrate release therefore diluted the ¹⁵N labeling fraction of NO₃^-, 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 Thampdrup and Dalsgaard (2002), denitrification rates were slightly underestimated by on 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^{+ 14}NO₃^-, anammox rates were not significantly influenced by DNRA due to the high background of ¹⁵NH₄⁺ addition. Excluding measurements in which bioirrigation was present, integrated denitrification rates decreased from 10 to 4mmolNm^-2d^-1 with water depth, while integrated anammox rates increased from 1.5 to 4.0 mmolNm^-2d^-1. Consequently, 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. This study represents the first time in which anammox has been demonstrated to play a significant role in benthic nitrogen cycling in the East China Sea sediment, 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 nitrogen transformation in the East China Sea. 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. [ABSTRACT FROM AUTHOR]