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The impacts of coastal embankments on nitrogen (N) dynamics are of great importance in salt marshes. However, effects of embankments on seasonal N dynamics in the plant-soil systems of Suaeda salsa salt marshes are still not well understood. Therefore, the edaphic moisture and salinity, biomass, N concentrations and storage in different plant tissues; the extent of organic and inorganic N subpools in soil subsystems; as well as N immobilization and mineralization by microorganisms in embanked and neighboring natural S. salsa salt marshes through all four seasons were investigated in Yancheng, Eastern China. Our results revealed that embankments increased the biomass and N storage of dead tissues (e.g., litter), while they decreased the biomass and N storage of live tissues (e.g., living aboveground tissues and roots) of S. salsa through decreased edaphic moisture and salinity. Furthermore, under the influence of embankments edaphic water-dissolvable N, e.g.,edaphic dissolved organic N (EDON) and nitrate and nitrite N (TNN), significantly increased with higher N inputs from S. salsa litter. This water-dissolvable N was an important source of N for plants, which translated to higher N concentrations in live plant tissues. However, this excess absorbed N appeared not to facilitate the growth of plants, which would ultimately return to the soil from plant residues. Our results suggested that the decreased edaphic soil moisture and salinity due to embankments had negative impacts on the biomass and N storage of S. salsa plant tissues, which were likely due to shifts in the edaphic osmotic potential. Subsequently, a cascading butterfly effect may be initiated due to the impaired growth of S. salsa , to which more attention should be paid. This study represents a step forward by providing valuable insights and an improved elucidation of the seasonal plant-soil N dynamics of S. salsa salt marshes. • The embankment reduced the N storage of living plant tissues by decreasing the edaphic moisture and salinity. • Increased litter N storage due to the decreasing edaphic moisture and salinity by the embankments. • Enhancing N inputs from litter by the embankment lead to the edaphic water-dissolvable N remarkably increased. • The increased N concentrations of microbes and plants caused by lowering edaphic moisture and salinity due to the embankments. [ABSTRACT FROM AUTHOR]

Recently, coastal land reclamation through the use of embankments has increased worldwide, and its impacts on ecosystem processes and functionality have been widely reported. However, the ecosystem nitrogen (N) cycling turnover processes following the establishment of coastal embankments remain unknown. For this study, we investigated stocks of various types of N in soils, N storage in plant subsystems, the microbial immobilization and mineralization of N, as well as soil physiochemical properties in embanked and adjacent unembanked Spartina alterniflora , Suaeda salsa , and Phragmites australis saltmarshes in the coastal wetlands of China. Coastal embankments in a S. alterniflora saltmarsh significantly decreased the total plant N storage by 50.24%, concentrations of total soil organic N (SON) by 55.16%, ammonia nitrogen (NH 4 -N) by 32.33%, nitrite nitrogen (NO 2 -N) by 41.23%, nitrate nitrogen (NO 3 -N) by 13.38%, and the value of soil net ammonification (R A) by 83.49%. However, in P. australis saltmarshes they significantly increased the total plant N storage by 160.24%, concentrations of SON by 57.28%, NH 4 -N by 7.89%, NO 2 -N by 101.76%, NO 3 -N by 32.05%, and the value of R A by 392.95%. Nevertheless, there were no significant differences in N cycling following the establishment of coastal embankments in S. salsa saltmarshes. Significant changes in the organic and inorganic soil N pools of S. alterniflora and P. australis saltmarshes were driven by plant residue inputs, which were significantly affected by decreasing soil salinity in these coastal wetlands. Furthermore, our results indicated that coastal embankments altered the immobilization of soil N and mineralization of microorganisms by influencing the growth and activities of soil microbes, which were primarily associated with changes in soil organic and inorganic N pools following the development of coastal embankments. In summary, alterations in the organic N inputs of plants were initiated by the dramatically decreased soil salinity, which was the main determinant that drove N cycling in the soil subsystems of coastal wetlands subsequent to the establishment of coastal embankments. • Coastal embankments decreased plant and soil N pools in Spartina alterniflora saltmarsh. • Coastal embankment increased plant and soil N pools in Phragmites australis saltmarsh. • Embankment had negligible effect on plant and soil N pools in Suaeda salsa saltmarsh. • Variations of plant and soil N pools were mainly driven by decreased soil salinity following coastal embankments. [ABSTRACT FROM AUTHOR]