Abstract
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.
Highlights
The East China Sea (ECS) is one of the most expansive continental shelf seas, bounded on the west by mainland China and on the east by the western Pacific Ocean island chain (Fig. 1)
Nutrient enriched water is restricted to the west inner shelf, where it is influenced by the Changjiang Diluted Water (CDW), while the outer shelf is dominated by the oligotrophic Kuroshio Surface Water (KSW)
Intracellular nitrate storage has been observed in diverse environments (An and Gardner, 2002; Risgaard-Petersen et al, 2006; Thamdrup, 2012), where it was carried out by a diverse range of benthic organisms, such as sulfur oxidizing bacteria (Fossing et al, 1995; Schulz et al, 1999; Sweerts et al, 1990), benthic foraminifera (Glud et al, 2009; RisgaardPetersen et al, 2006) and diatoms
Summary
The East China Sea (ECS) is one of the most expansive continental shelf seas, bounded on the west by mainland China and on the east by the western Pacific Ocean island chain (Fig. 1). DNRA was demon- 2.1 Sample collection and preparation strated to be the dominant pathway of the benthic nitrate reduction in the tropical estuarine sediment (Dong et al, 2011), Sediment was collected at five sites from the Changjiang it has been demonstrated that DNRA can be performed estuary to the outer shelf of the ECS during a cruise on by fermentative bacteria (Tiedje, 1988). It has the R/V Kexue No 3 from 8 to 22 June, 2010 (Fig. 1 been shown that both nitrate storing bacteria (Preisler and Table 1). Exetainer vials containing the subsamples were sealed and stored at room temperature upside down until subsequent N2 isotope ratio analysis
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