Nitrogen Input Alters Root Exudation of Kandelia obovata and Nitrogen Cycling in Constructed Mangrove Wetlands.

The role of mangrove root exudates in mediating the nitrogen cycle, particularly under high dissolved inorganic nitrogen (DIN) input, in coastal ecosystems remains unclear. This research investigated variation in the root exudates, and nitrogen transformation and output, in constructed mangrove wetlands planted with Kandelia obovata under high, moderate, and low nitrogen-input levels (PCWs-H, PCWs-M, and PCWs-L, respectively). PCWs-H promoted increased root density and biomass accumulation, enhancing soil nitrogen sequestration, whereas PCWs-L induced greater specific root length, specific root surface area, and number of root tips. These changes directly influenced denitrification efficiency. Hydroxymethoxyphenylcarboxylic acid-O-sulfate and Arg-Ser released in root exudates under PCWs-H might act as potential denitrification inhibitors, thereby suppressing denitrifiers and impairing dissolved nitrogen purification. Elevated nitrogen loading predominantly limited denitrification, resulting in relative NO3--N removal rates of PCWs-H < PCWs-M < PCWs-L (p < 0.05). Compared with PCWs-H and PCWs-L, the enhanced soil organic nitrogen storage under PCWs-M was associated with flavonoids in root exudates. Metagenomic analysis showed that denitrification was the dominant nitrogen removal pathway. Nitrogen loading influenced the effects of root exudates on the microbial community. Under PCWs-H, triterpenoids promoted norBC and nirK/S abundance but depressed amoABC abundance. Sterols and flavonoids in exudates under PCWs-L depressed nosZ abundance, instead activating dissimilatory nitrate reduction to ammonium. Compared with PCWs-H and PCWs-L, N2O emissions were minimal under PCWs-M. This study revealed that mangrove root exudates mediate the nitrogen cycle in mangrove wetlands, providing a theoretical basis for local authorities to manage DIN inputs and mitigate N2O emissions.