Nitrogen cycling in plant and soil subsystems is driven by changes in soil salinity following coastal embankment in typical coastal saltmarsh ecosystems of Eastern China

Abstract 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 (NH4-N) by 32.33%, nitrite nitrogen (NO2-N) by 41.23%, nitrate nitrogen (NO3-N) by 13.38%, and the value of soil net ammonification (RA) 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%, NH4-N by 7.89%, NO2-N by 101.76%, NO3-N by 32.05%, and the value of RA 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.