Coastal embankments impact seasonal plant-soil nitrogen dynamics in a temperate intertidal Phragmites australis salt marsh

Coastal embankments are extensively employed in coastal salt marshes; however, knowledge is still lacking regarding the seasonal effects of embankments on nitrogen (N) storage in Phragmites australis salt marshes. Therefore, we surveyed live and dead aboveground N (AGN) storage, and belowground N (BGN) storage of plant system. Further, the soil total, recalcitrant, labile, and dissolved organic N (STON, SRON, SLON and SDON, respectively), ammonia (NH4+-N), nitrate (NO3 -N), and microbial biomass N (MBN) in soil system of embanked and neighbouring natural P. australis salt marshes were examined during the initial stage and vigorous stage of plant growth. Additionally, the soil physicochemical properties were quantified, as well as the microbial ammonization (RA) and nitrification (RN) rates. The embankment significantly strengthened the BGN storage during the initial (582.6%) and vigorous stage of plant growth (228.1%), and markedly decreased the dead AGN storage during the initial (70.3%) and vigorous stage of plant growth (65.5%). It also significantly amplified STON (51.9%) and SLON (58.1%) during the initial stage of plant growth, and STON (51.9%) and SRON (55.9%) during the vigorous stage of plant growth. However, the embankment had negligible effects on soil soluble N (e. g., SDON, NH4+-N and NO3  -N). Moreover, the microbial capacity of N immobilization (e.g., MBN) and mineralization (e.g., RN) were remarkably stimulated during the vigorous stage of plant growth due to the embankment. Our results implied that changes in physicochemical properties (e.g., soil moisture, electrical conductivity) due to the embankment were likely more suitable for the growth and survivability of P. australis during both the initial and vigorous stage of plant growth. Decreased N inputs from aboveground P. australis residues were offset by the increased N inputs from its immense complement of belowground tissues; thus, soil organic N storage was enhanced due to the embankment. Furthermore, soil soluble organic N was primarily derived from the AGN inputs of P. australis residues, which resisted decomposition, and the inorganic N that was mostly derived from soluble organic N. This meant that the soil soluble N was stabilized and not influenced by the embankment. Finally, due to weakening N demands by vegetation and additional soil N that microorganisms could utilize during the vigorous stage of plant growth, the microbial N immobilization and mineralization capacity strengthened. This study provides further comprehensive knowledge to elucidate the effects of coastal embankments on the N pool in P. australis salt marshes.

Automated summary

This study investigated the seasonal effects of coastal embankments on nitrogen storage in Phragmites australis salt marshes. The results showed that the embankment significantly enhanced belowground nitrogen storage and decreased dead aboveground nitrogen storage during both the initial and vigorous stages of plant growth. Furthermore, the embankment increased soil total and labile organic nitrogen during the initial stage and total and recalcitrant organic nitrogen during the vigorous stage of plant growth. However, the embankment had little effect on soil soluble nitrogen and greatly stimulated microbial immobilization and mineralization capacity during the vigorous stage of plant growth.