Abstract：
There is no doubt that hypoxia and seawater mixture are profoundly affecting the global nitrogen (N) cycle. However, their mechanisms for altering N cycling patterns in shallow coastal groundwater are largely unknown. Here, we examined shallow groundwater N transformation characteristics (dissolved inorganic N and related chemical properties) in the coastal area of east and west Shenzhen City. Results showed that common hypoxic conditions exist in this study area. Ions/Cl- ratios indicated varying levels of saltwater mixture and sulfide formation across this study area. Dissolved oxygen (DO) affects the N cycle process by controlling the conditions of nitrification and the formation of sulfides. Salinity affects nitrification and denitrification processes by physiological effects, while sulfide impacts nitrification, denitrification, and dissimilatory nitrate reduction to ammonium (DNRA) processes through its own toxicity mechanism and the provision of electron donors for DNRA organisms. Redundancy analysis (RDA) results indicate that the influence magnitude is in the following order: DO > sulfide > salinity. Seawater mixture weakened the nitrification and denitrification of groundwater by changing salinity, while hypoxia and its controlled sulfide formation not only weaken nitrification and denitrification but also stimulated the DNRA process and promotes N regeneration. In this study area, hypoxia is considered to exert greater impacts on N cycling in the coastal shallow groundwater than seawater mixture. These findings greatly improve our understanding of the consequences of hypoxia and seawater mixture on coastal groundwater N cycling.
摘要：
毫无疑问，缺氧和海水混合物正在深刻影响全球氮（N）循环。然而,它们改变浅层沿海地下水氮循环模式的机制尚不清楚。这里,我们研究了深圳市东部和西部沿海地区的浅层地下水氮转化特征（溶解性无机氮和相关化学性质）。结果表明，该研究区域存在常见的缺氧条件。离子/Cl-比表明整个研究区域的盐水混合物和硫化物形成水平不同。溶解氧（DO）通过控制硝化条件和硫化物的形成来影响N循环过程。盐度通过生理效应影响硝化和反硝化过程，虽然硫化物影响硝化，反硝化,以及通过其自身的毒性机制和为DNRA生物提供电子供体，将硝酸盐异化还原为铵（DNRA）过程。冗余分析（RDA）结果表明，影响大小依次为：DO>硫化物>盐度。海水混合物通过改变盐度削弱了地下水的硝化和反硝化作用，而低氧及其受控的硫化物形成不仅削弱了硝化和反硝化，而且刺激了DNRA过程并促进了N的再生。在这个研究领域，与海水混合物相比，缺氧被认为对沿海浅层地下水中氮循环的影响更大。这些发现极大地改善了我们对缺氧和海水混合物对沿海地下水氮循环的影响的理解。
