Hypoxia exerts greater impacts on shallow groundwater nitrogen cycling than seawater mixture in coastal zone.

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.