Effects of increased rainfall intensity on nitrogen and phosphorus removal in a restored wetland from an abandoned paddy field

Climate change has increased rainfall frequency and intensity, leading to greater nitrogen and phosphorus runoff from watersheds. This study evaluated nutrient removal efficiency ( RE ) in a restored wetland converted from an abandoned paddy field during nine rainfall events with varying rainfall amounts and intensities. High-frequency sampling was conducted at the wetland inlet and outlet to measure total nitrogen (TN), nitrate (NO₃–N), total phosphorus (TP), molybdate-reactive phosphorus (MRP), and flow rate. The responses of RE to rainfall characteristics and hydraulic loading rate (HLR) differed among nutrients. TN removal remained positive even during the largest rainfall event (237 mm rainfall; HLR, 7.1 cm h⁻¹), but the slightly negative removal of TP under these conditions suggested that the wetland’s capacity to remove phosphorus may have been exceeded. The RE s of NO₃–N and MRP decreased with increasing rainfall amount and HLR and were most strongly related to maximum HLR. These findings highlight the potential and limitations of restored wetlands for the sustainable utilization of abandoned agricultural land and as nature-based solutions (NbS) for maintaining water quality under climate change.