Nitrogen cycle pattern variations during seawater-groundwater-river interactions enhance the nitrogen availability in the coastal earth critical zone

Hydrological interaction in highly vital and productive coastal earth critical zone (CECZ) significantly impacts the nitrogen cycle and availability, which is the essential foundation for agricultural production. The results of isotopes and microbial sequencing showed that nitrogen fixation and nitrite oxidation are enhanced in the palaeo-saltwater intrusion process. The weakening AmoCAB and Hao impedes the normal nitrogen cycling pathway, promoting the input of organic nitrogen, thus increasing nitrogen availability flux. In the river into the sea, nitrogen fixation is enhanced, while nitrite oxidation, denitrification, and DNRA are weakened. The whole intensity of the nitrogen cycle decreases. In addition, the oxidation of NH₄-N to NO₂-N in this process is inaccessible, which means that the nitrogen cycle flux is substantially reduced. In the river recharge to groundwater, nitrite oxidation, nitrogen fixation, and ammonia oxidation are enhanced. In the submarine groundwater discharge, the whole intensity of the nitrogen cycle gradually weakens and the nitrogen cycle flux greatly reduces. As the final pool of coastal groundwater and surface water, the significant reduction of the nitrogen cycle intensity and the absence of the pathway link allows nitrogen in seawater to flow out of the inorganic nitrogen cycle by the conversion to organic nitrogen, thus the net flux of nitrogen availability trapped in the hydrological systems increases. The hydrological interaction enhances the breadth of nitrogen sources, the diversity of pathways, and the flux of nitrogen in CECZ, thereby promoting more nitrogen being trapped in system and increasing nitrogen availability.