Waterfowl's presence enhanced N2O emissions from restored coastal wetlands: Trade-offs between biodiversity and climate.

Restoration of coastal earthen aquaculture ponds to wetlands revived the local biodiversity including avian species, but dense congregation of waterfowl especially during the migrating seasons may lead to unintended ecological consequences. A three-year study was conducted in restored coastal wetlands in southeastern China, where a seawall separated two neighboring open-water areas with and without waterfowl, allowing a natural experiment to investigate the effects of waterfowl on nitrogen dynamics including N2O production and emissions. Migratory species were estimated to account for 79-85 % of the observed waterfowl. Relative to the control plots without waterfowl, plots frequented by waterfowl had significantly lower dissolved oxygen and sediment pH, higher levels of NO3--N, NH4+-N, total dissolved nitrogen, microbial biomass nitrogen, abundances of denitrification-related genes (nirS, nirK) and sediment N2O production potential. N2O emissions were elevated by about 121 % in waterfowl-influenced plots, with most of the excess emissions occurring in months when migratory waterfowl abundance peaked. The between-habitat differences were consistent over the three years. Our findings reveal a hidden ecological cost of waterfowl recovery, i.e., avian-derived nutrient deposition and associated greenhouse gas emissions may partially offset the climate benefits of restored wetlands. These results have direct implications for coastal wetland management and blue carbon accounting, suggesting that biodiversity conservation and greenhouse gas mitigation do not always align.