Release of phosphate in a wetland by changes in hydrological regime

Phosphate can be removed from the water column in wetlands by adsorption, sedimentation, or biological uptake. However, phosphate removal efficiency in wetlands is variable because phosphate can also be re-released from wetland sediment to water body under certain conditions. This study was conducted to investigate mechanism of changes in removal efficiency of phosphate under different hydrological regime. For this, we constructed wetland microcosms, which were exposed to consecutive drying and rewetting periods. At the initial period of wetland operation, phosphate removal efficiency was high (60.1–100%) probably due to high adsorption capacity and low phosphatase activity. However, a large amount of phosphate was released in a rewetting period followed by a drying period. This result can be explained by two mechanisms. Firstly, rewetting of soil followed by drying released phosphate by desorption of previously adsorbed phosphate, which was clearly demonstrated in our supplementary isotherm experiment. Secondly, we observed a huge shift of phosphatase activity by drying of soil (1.4 nmol min−1 g−1 in the initial period and 12.4 nmol min−1 g−1 in a drying period), which lasted even in the rewetting period (14.2 nmol min−1 g−1). These results suggest that repeated drying and rewetting events in wetlands can cause substantial release of phosphate by chemical changes and activation of P-mineralizing enzyme, which may last even when the water table is returned to the original level.