Numerical modeling of ammonium and nitrate nitrogen transport in estuarine wetland sediments: a case study of the Minjiang Estuary

The transport and transformation of ammonium nitrogen (NH4+-N) and nitrate nitrogen (NO3–-N) in estuarine wetland sediments are influenced by tidal fluctuations. However, current research on this topic primarily relies on field experiments, which are time-consuming and often lack continuity, particularly regarding the dynamic changes in nitrogen (N) transformation during tidal cycles. In this study, the Minjiang Estuary wetland in China was selected as the research area to investigate the transport and transformation of NH4+-N and NO3−-N in sediments under tidal influence. A numerical model based on HYDRUS was developed, and its simulation accuracy was within acceptable limits. Transformation parameters for N at different tidal flats and months were successfully obtained. Results suggested that nitrification, mineralization, and denitrification coefficients tended to be higher in summer (August) than in autumn (November), although the seasonal pattern varied across tidal flat positions and soil depths. Spatially, nitrification and mineralization often decreased with depth, whereas denitrification tended to increase. With greater inundation depth, denitrification and mineralization often showed an increasing trend. Model simulations indicated that sediment inundation depth and solute concentration were key factors controlling NO3−-N leaching, which increased with tidal level but with a certain degree of lag, especially during spring tides, while neap tides showed greater variability at intermediate tidal flat. This study provides theoretical insights into N transport parameters in estuarine wetland sediments and offers a modeling approach exemplified by the Minjiang Estuary, contributing to the sustainable management of estuarine wetland ecosystems.