Phosphorus sorption capacity in soils from freshwater restored coastal wetlands increased with restoration age

Abstract

• P sorption capacity in soils increased with wetland restoration age. • Elevated P sorption related to the improvement of soil properties in restored wetland. • The P loss potential was less affected by the wetland restoration age. • The restored wetland was identified as the P storage hotspot at the landscape scale. Coastal wetlands, located at the intersection of land and sea, are considered to be a phosphorus (P) sink, source, and reactor in the global P cycle. However, the variation in P sorption capacity with wetland age and the related sorption mechanisms are not well understood. In this study, we collected the surface soil samples (0–20 cm) from 10 coastal freshwater wetlands that had been restored for 0–44 years surrounding China’s Bohai Rim. Through a series of batch experiments, we studied the factors controlling P sorption and release in these soils. The results showed that the maximum P sorption capacity (Q max ) in these wetlands was 294–2447 mg kg −1 , with an average of 1472 mg kg −1 . The Q max generally increased with increasing wetland restoration age. The degree of P saturation (DPS) and the P eutrophication risk index (ERI) for coastal wetlands were 0.5–9.5% and 0.9–21.1%, respectively. The DPS and ERI values did not vary significantly with wetland age. These data suggest that freshwater restoration significantly increases P sorption capacity, while not greatly affecting the P release potential in the coastal wetlands surrounding Bohai Rim. These observations may be related to the soil desalination and organic matter and amorphous Fe and/or Al accumulation that occur during the freshwater restoration of degraded wetlands. Coastal wetlands (especially restored wetlands) were suggested as P storage hotspots among upland, wetland, and offshore along the coastal watersheds of Bohai. These findings indicate that the freshwater restoration significantly improved the soil properties in the degraded coastal wetlands, and therefore increased P sorption capacity in the restored wetlands.