646 / 2024-09-18 19:22:44

Major processes controlling the nitrogen cycling and transport across the mangrove-salt marsh-estuarine continuum

plant invasion,nitrogen cycling,wetland sediments,eutrophication,source-sink

Nitrogen (N) cycling and the lateral transport across the mangrove-salt marsh-estuarine continuum are crucial for the functioning of both wetlands and marine ecosystems. However, the understanding of the key factors influencing nitrogen cycling and source-sink patterns at these interfaces is limited. This study investigated a tidal creek with a marked mangrove-salt marsh gradient in China using high-frequency time-series sampling of dissolved nutrients and observations of water exchange rate at the sediment-water interface and mangrove-estuarine interface. The multidisciplinary biogeochemical approaches were used to elucidate the variability in nitrogen concentrations. Annually, mangroves acted as sources of ammonium (NH₄-N) and dissolved organic nitrogen (DON) to the adjacent estuary while serving as a sink of nitrate (NO₃-N). Salt marshes served as a sink of NH₄-N and retained NH₄-N discharged from mangroves. Hydro-biogeochemical processes controlled nutrient fluxes across the mangrove-salt marsh-estuarine continuum. Driven by tidal pumping and surface water-groundwater exchange, mangroves groundwater (porewater) discharged NH₄-N and DON (net flux) to the estuary (source). In contrast, NO₃-N was infiltrated from surface water into groundwater with a net flux from estuary to mangroves (sink). Salt marshes had a lower groundwater discharge than surface water infiltration, serving as the nitrogen sink and reducing NH₄-N export to estuarine zone. Rainfall, temperature, and vegetation phenology were vital factors controlling the seasonal variations of NH₄-N and NO₃-N concentrations.