Effects of microtopography on soil development of the conserved area in the Isahaya Bay polder

After reclamation of the Isahaya Bay polder, a conserved area is covered with naturally developed vegetation under high groundwater level conditions. This study investigated how alluvial deposits transform into soil without utilizing artificial water drainage systems for agricultural land uses. Soils were surveyed under four vegetation compositions, including those dominated by goldenrods (GR), occupied by high-grown reeds (HRD), covered with low-grown reeds (LRD), or a mixture of goldenrods and reeds (MIX). The microtopography of the polder differentiated the research site with a higher elevation at the GR location, followed by a MIX, HRD, and a lower elevation at the LRD location. The soils were characterized by their physicochemical properties and ionic composition of the soil water. Electrical conductivity (EC), which can be an indicator of residual seawater, was lowest in the GR, followed by the MIX, HRD, and LRD. The soil moisture regimes affected by microtopography led to different frequencies of wet-dry cycles in the soil, resulting in a developing sequence of soil structure from LRD to GR with decreasing EC. The ionic composition of soil water varied with elevation and soil depth. The residual seawater still influenced the ionic composition of deeper soil horizons at lower elevations, while specifically high rates of Ca2+ and SO42- were observed in GR. The oxidation of pyrite is stimulated by soil aeration with lowering water level, resulting in soil acidification and the simultaneous dissolution of calcium carbonate from seashells. The sequential changes in soil properties with elevation indicated the effects of microtopography on soil moisture dynamics, which led to variations in soil formation and vegetation in the conserved area.