Daily inundation induced seasonal variation in the vertical distribution of soil water salinity in an estuarine mangrove forest under a tropical monsoon climate

Previously, we revealed opposing patterns of the vertical distribution of soil water salinity between the dry and rainy seasons in an estuarine mangrove forest under a tropical monsoon climate. This study clarifies the causes of such seasonal variation in soil water salinity distribution. We investigated the salinity of inundating water, and diurnal changes in soil water salinity and water table levels. Freshwater inundated the study plot in the rainy season, while saline water inundated it in the dry season. At theSonneratia,AvicenniaandRhizophorasampling sites, soil water salinity measured in experimental pits fluctuated diurnally. Mean salinity significantly decreased in the dry season but tended to increase in the rainy season. The effect of evaporation on salinity was negligible. Water table levels at all sites primarily fluctuated within 0-10 cm from the surface and were generally not synchronized with tidal movements. We divided the soil profile into a thin tide-sensitive layer and an underlying aquifer layer based on soil water movement pattern. The inundating water saturated the tide-sensitive layer and then gradually infiltrated into the aquifer layer. The water in the tide-sensitive layer was partially replaced by subsequent inundation. The water in the aquifer layer was gradually desalinized or salinized, depending on the salinity level of the inundating water, although soil water from the former season remained still in deep soil horizons. Thus, seasonal variation of the vertical distribution of soil water salinity was induced by daily inundation and subsequent infiltration processes in the mangrove soil.