[Effects of drying-rewetting alternation on nitrogen, dynamics in a typical coastal wetland: a simulation study]

The coastal wetland, a key transitional zone between land and ocean, is a complex and sensitive ecosystem with special environmental processes and functional values. Sediment and sea water samples were taken from Chongming east intertidal flat, a typical coastal wetland in China. Sediment cores were incubated under simulated spring tides (approximately twice a month) and semi-diurnal tides (two almost equal high tides and two low tides in a lunar day), to define the responses of nitrogen to periodic alternation of drying and re-flooding. The contents of NO3(-) -N, NO2(-) -N, NH4(+) -N, dissolved organic N (DON), and the activities of nitrate reductase (Nar), nitrite reductase (Nir), and hydroxylamine reductase (Hyr) in sediments were determined during the incubation. Nitrification stood in a dominant position during the drying period with sediment moisture decreasing from 35% to 5% -7% in simulated spring tides. The NO3(-) -N and NO2(-) -N were found to be substantially converted into DON following a further exsiccation (the sediment moisture decreased from 5%-7% to 0%-3%). However, the conversion rates decreased with the increase of drying and rewetting cycles. Following the rewetting of dried sediments (from 0% -3% to 37% -45%), the contents of NO3(-) -N, NO2(-) -N, NH4(+) -N and DON increased to 1 to 3 times that of the dry sediments. Nar and Nir activities in sediments rapidly increased after the rewetting, indicating that the reduction of NO3(-) -N and NO2(-) -N were substantially enhanced. Significant positive correlations were found between Nar and Nir activities, and Hyr activities and NH4(+) -N contents during the simulated spring tides. Moreover, the decreases of NO3(-) -N and NO2(-) -N contents were observed to be significantly correlated to the increases of NH4(+) -N contents. Combined with the fact that the sediment had a high organic carbon content but a very low NO3(-) -N content, it can be concluded that the reduction of NO3(-) -N and NO2(-) -N following the rewetting was dominated by nitrate ammonification. In contrast, NO3(-) -N, NO2(-) -N, NH4(+) -N and DON contents in sediments were relatively stable during semi-diurnal tides, which were (3.0 +/- 0.3), (1.2 +/- 0.1), (133.3 +/- 4.3) and (41.1 +/- 10.6) mg x kg(-1), respectively. This indicated that semi-diurnal tides slightly affected the variations of nitrogen contents in the wetlands.