Dynamics of phosphorus fractions in surface soils of different flooding wetlands before and after flow-sediment regulation in the Yellow River Estuary, China

To investigate the dynamics of phosphorus fractions and their influencing factors in the surface soils of estuarine wetlands experiencing different hydrological conditions before and after flow-sediment regulation, soil samples were collected in wetlands (tidal flooding wetlands (TFW), fresh-water restoration wetlands (FRW) and freshwater flooding wetlands (FFW) of the Yellow River Estuary in each month from April to October of 2012. Our results showed that the average contents of organic phosphorus (OP) occurred in the following order: FRW soils (60.05 mg/kg) > TFW soils (38.72 mg/kg) > FFW soils (27.56 mg/kg), and accounted for less than 12% of total phosphorus (TP). In contrast to the pattern for OP, FRW soils contained lower inorganic phosphorus (IP) levels than TFW and FFW soils from April to August (p < 0.05). After the flow-sediment regulation, the TP, OP, moderately labile OP (ML-OP) ferrous/aluminum-bound IP (Fe/Al-P), and occluded IP (Oc-P) in the three wetlands decreased. The soluble and loosely bound IP (S/L-P) contents in TFW decreased, while the S/L-P contents in FRW soils increased. The levels of phosphorus fractions were affected by water and salt conditions, soil texture, exchangeable mineral element contents, and nutrient status. The Fe/Al-P and Oc-P in the three types of wetland soils were released, and the increase in S/L-P in FRW soils after the flow-sediment regulation might increase the risk of eutrophication in the coastal waters. The findings of this study could contribute to providing basic data regarding phosphorus fractions in different flooding estuarine wetlands of the Yellow River Estuary and guiding flow-sediment regulations and freshwater restoration to enhance the ecological functions of estuarine wetlands.