地下河口（STE）是在海陆交界面形成的地下淡水输入与海水入侵的混合地带，由于其氧化还原条件的快速变化，成为生物地球化学反应的活跃区。前期研究结果表明，在STE中富含铁、锰和硫化物的无氧层，虽然没有检测到硝态氮，但却有过量氮气产生，推测存在一种厌氧硝化作用（anNTR），为厌氧氨氧化（Anammox）和反硝化（DNF）过程的发生提供了有效的底物，形成anNTR-DNF/Anammox耦合脱氮过程。本项目拟运用地球化学、同位素示踪和分子生态学的综合技术方法，深入研究这种耦合脱氮过程的发生机制以及与环境因素之间的相互作用关系，分析这种耦合氮循环过程的反应热点，并通过原位实验估算其对近海氮通量减少的贡献。研究结果将会提高目前对无氧环境中氮素生物地球化学循环过程的认知，阐明无氧环境中微生物脱氮的一种新机制，揭示人类活动影响下的陆-海界面STE系统在氮素循环平衡中的生态功能。

Humans are modifying global nitrogen (N) cycle at an alarming rate. The release of reactive nitrogen（Nr）into the environment drives eutrophication and hypoxia, which are the greatest threats to coastal ecosystems worldwide. Significant Nr contamination of coastal aquifers may arise from agricultural runoff, septic tank discharge and landfill leachate. Transport of contaminated groundwater via submarine subterranean estuary (STE) is a major source of Nr to the coastal zone and an overlooked driver of eutrophication. STE, a region of the coastal aquifer where mixing between discharging terrestrial water and marine water, has critical roles in the biogeochemical processes due to steep gradient and rapid variation of redox states driven by tide. Recent studies of the biogeochemical processes have suggested the STE as a hotspot of removing Nr associated with permeable nature of sandy sediments..Preliminary results from a STE in the Pear River Estuary showed that there was substantial N2 production in ferruginous and sulfidic zones where neither nitrite nor nitrate was detectable, suggesting anoxic nitrification (anNTR) occurred by oxidation of ammonium in the absence of oxygen using other common chemical oxidants such as metal oxides (namely, Fe and Mn) or sulfate. anNTR could provide nitrite for anammox and denitrification, serve as a vital link to N2 production, and attenuate N loads discharging from a subterranean estuary (STE). Therefore, coupled anoxic nitrification to anammox or denitrification in coastal groundwater may be a major unrecognized sink for fixed nitrogen at the land-sea interface. In addition to coastal groundwater, the coupled N cycle has potential importance in anoxic environments. This proposal focuses on the STE because geochemical conditions there appear optimal for the proposed reactions to occur, and our preliminary data show strong evidence for the coupled processes of nitrogen removal. The proposed work uses a combined geochemical, 15N isotope tracer and microbiological approach to evaluate environmental controls on this new tpye N cycle as well as to estimate its contribution to reduction of fixed N fluxes to the coastal ocean. Four approaches are proposed: (1) Field characterization of anoxic nitrification reactions and associated microbial communities in a subterranean estuary; (2) Laboratory incubation experiments to identify hotspots of the cryptic N cycle; (3) Controlled microcosm experiments to determine geochemical controls on anoxic nitrification; and (4) in situ assessment of anoxic nitrification to estimate the importance of the cryptic N cycle in a coastal aquifer. The proposed project would change the current recognition of the biogeochemical nitrogen cycle in anoxic environments and proved a pattern of microbial nitrogen loss in anoxic environments. At same time, this study will allow us to better understand interactions among the nitrogen, metals, and sulfur cycles in coastal environments under the ongoing human modification.