Effect of nitrogen pollution input on carbon cycling in mangrove constructed wetlands

Nitrogen pollution can disrupt nitrogen cycling processes in mangrove wetlands, its effects on carbon (C) cycling are poorly understood. This study investigated the effects of varying nitrogen pollution on C input in Kandelia obovata, and disentangled the distinct impacts of microbes versus K. obovata - microbe interactions on C metabolism. Under high N pollution, K. obovata enhanced photosynthesis but suppressed root exudation (R_DOC) and altered exudate composition, which inhibited microbial gluconeogenesis and TCA cycle activity, accompanied by higher greenhouse gas (GHG, including CO₂ and N₂O) emissions and reduced soil C pool stability. Notably, independent microbial treatment maintained a more stable C pool compared with K. obovata - microbe interactions. Conversely, low nitrogen pollution decreased photosynthetic C input but stimulated the release of R_DOC and dissolved organic C, and this pollution might suppress anaplerotic reactions, TCA cycle, and gluconeogenesis of K. obovata - microbe. Furthermore, soil C pool was particularly influenced by interactions between K. obovata and microbes, despite resulting in lower GHG emissions. Medium N pollution provided an appropriate environment characterized by moderate photosynthesis and CO₂ release, along with the lowest N₂O emissions. Moreover, compared to the effects of independent microbial, the influence of K. obovata - microbe contributed more significantly to maintaining the stability of soil C pools, which related to a decreased intensity of anaplerotic reactions, the TCA cycle, and gluconeogenesis. This study can provide a theoretical reference for local authorities to mitigate GHG emissions and preserve C sink functions of mangrove wetlands under N pollution stress.