| Nitrogen is one of essential elements for biological growth on earth. It has always been studied and utilized by human being for ecological development and food supply. In recent decades, due to the effects of intense human activities, substantial reactive nitrogen has been transported into estuarine and coastal seas, as a result leading to serious eutrophication and frequent occurrence of harmful algae blooms. Therefore, it is of significant to study the biogeochemical cycle of nitrogen for understanding the occurrence of eutrophication in estuarine ecosystems.At present, anaerobic ammonium oxidation (anammox) has been identified as a significant pathway of nitrogen removal in natural ecosystems. In the early 1990's the first direct evidence of the anaerobic oxidation of NH4+ came from a waste water treatment facility in Delft, the Netherlands. A few years after the initial discovery of anammox, the anaerobic oxidation of ammonium coupled to nitrite reduction with N2 as the end product, is a newly discovered microbial transformation pathway in the marine ecosystems, estuaries and freshwater lakes. The ubiquitous presence of anammox in marine ecosystems has changed our knowledge of the global nitrogen cycle.The estuarine and coastal tidal flat, as a typical transitional zone between land and ocean, is a multifunctional and complex ecosystem with special ecological values and potential resources. The study area is situated on the Yangtze Estuary and adjacent coastal tidal flat areas. In the present study, the eight sampling sites were selected along the Yangtze estuarine and coastal lines, including Xupu (XP), Liuhekou (LHK), Wusongkou (WSK), Bailonggong (BLG), Luchaogang (LC), Daxinang (DXG), Yinyang (YY) and Chongming (CM). Based on the technique of nitrogen-isotopic trace, the temporal and spatial changes of the anammox process occurring were investigated in the intertidal sediments of the Yangtze Estuary. The mainly findings are as follows:1. There were obviously temporal and spatial changes of the anammox rates in the tidal sediments of the Yangtze Estuary. The anammox rates ranges from 18.82 to 52.04μmol N-29*kg-1*d-1,24.92 to 153.73μmol N-29*kg-1*d-1,34.52 to 133.41μmol N-29*kg-1*d-1 and 7.09 to 24.33μmol N-29*kg-1*d-1 in spring, autumn, fall and winter, respectively. In general, the anammox rates in spring and winter were lower than in summer and autumn. Also, the anammox rates in fresh water areas were higher than in brackish areas.2. Significantly temporal and spatial variations of the anammox rates were observed in the Chongming eastern tidal flat. Generally, the anammox rates in dry season were lower than in flood season. Also, the maximum rates of anammox appeared in the high tidal flat, followed by the middle and low tidal flats.3. Under the periodical changes of exposure-immersion, the spatial and temporal distributions of the anammox rates were controlled probably by salinity and sediment temperature. H2S, OC and C/N rate can also affect the anammox rate in intertidal surface sediments of the Yangtze Estuary. However, the anammox rate had no correlation with Fe and Mn contents.4. The results showed that the total rates of nitrogen removal were from 101.29 to 787.31μmol N-30*kg-1*d-1. In contrast, the anammox rates ranged from 7.09-153.73μmol N-29*kg-1*d-1 at the study area. This comparison shows that about 5 to 23% of N2 production in sediments of the Yangtze Estuary may be attributed to the activities of anammox bacteria. Therefore, the anammox process plays a significant role in reactive nitrogen removal at the study area.
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