|Other Abstract||The loss of phosphorus (P) of soils in China is one of the main reasons for the water eutrophication. As a special ecosystem connecting the land and the ocean, coastal wetlands have a strong ability to filter and settle external pollutants. Wetlands are considered as the source, sink, and transformer of P. The P storage capacity of soil (SPSC), the threshold for P of soil reaching or exceeding the critical value of P environmental risk, describes the remaining P storage or release capacity in the soil, which can be used to evaluate P storage capacity and potential release risks in the wetland. In this study, different land use types in the Yellow River Delta and restored wetlands in the Yellow River Delta were taken as the research object, and the method combining the isothermal adsorption of P was used to fit the maximum adsorption amount (Qmax) of P in the soil of Yellow River Delta, and the degree of P saturation (DPS) and the key control factor of P storage capacity (SPSC) of soil in the Yellow River Delta was clarified. The P storage capacity of different land-use types in the transition zone from the land to the sea in the Yellow River Delta was studied. The critical value of loss of P in soil and the SPSC were explored in wetlands in the Yellow River Delta. The effect of salinity, vegetation conditions, time changes, and profile depth on SPSC was discussed. Furthermore, the preservation mechanism of P in restored wetlands was explored through indoor cultivation experiments. The results were as followed:
1. Analysis experiments on soil in different land-use types in the transition zone from the land to the sea in the Yellow River Delta show that the critical value of the degree of P saturation (DPS) is 14.41% in alkaline soil of the Yellow River Delta, which indicate that when the DPS in the soil exceeds 14.41%, the concentration of water-soluble P increases significantly in the runoff fluid of the soil, that is, the P of soil has a high loss Risk, and when the DPS is less than 14.41%, the risk of loss of P in soil is low. On the basis of the critical value of soil loss, the calculation formula of the P storage capacity (SPSC) of soil in the Yellow River Delta is: SPSC = (14.41%-DPS) × (0.007 CaM3 + 0.153 MgM3 + 0.133 Feox + 0.106 Alox). The order of the SPSC of different soil use types in the Yellow River Delta is listed as followed: restored wetland (90.4 ± 20.2mg/kg) > tidal flat wetland (46.6 ± 14.0 mg/kg) ≈wasteland（42.4 ± 22.0 mg/kg) ≈Cotton field (36.6 ± 26.5 mg/kg) ≈Orchard (31.6 ± 25.8 mg/kg) ≈Grain field (16.2 ± 38.0 mg/kg) > Vegetable field (-47.6 ± 88.2 mg/kg). The data of SPSC shows that the P storage capacity of soil gradually increases from vegetable field of the inland to the restored wetland of near-shore, and the risk of P release gradually decreases. Therefore, the restored wetlands is the “highland” of P storage on the transition zone from the land to the sea in the Yellow River Delta, and the P storage function of the restored wetlands should be fully utilized.
2. The P storage capacity of soil in the restored wetland is affected by many factors. Wetland restoration reduces the salinity of soil and significantly improves P storage capacity of soil (SPSC in the restored wetlands ranges from 86.3-154.0mg/kg, SPSC in the unrestored wetlands ranges from 57.4-111.8mg/kg). And the longer the wetland recovery time, the higher the SPSC value: 2002 restored wetland (R2002)> 2006 restored wetland (R2006) ≈unrestored wetland (R0).that the SPSC of restored wetlands is significantly and negative correlated with pH (p<0.01), and is significantly and positive correlated with Feox, Alox, TOC, MgM3, and clay content (p<0.01), indicating that the main matrix for P storage is Feox, MgM3 and Alox in restored wetlands in the Yellow River Delta, and the clay, TOC content and pH significantly affect the P storage. Indoor cultivation experiments show that the restored wetlands in the Yellow River delta store P mainly in the form of adsorbed P, calcium-magnesium-bound P and iron-aluminum-bound P, while the unrestored wetlands store P mainly in the form of adsorbed P and calcium-magnesium-bound P. Therefore, the storage mechanism of P in soil before and after wetland restoration was significantly different, that is, the contribution of iron and aluminum to P preservation was significantly increased after wetland restoration.Therefore, the storage of P in restored wetlands in the Yellow River Delta is mainly related to the accumulation of organic matter and the activation of P binding ions such as calcium, magnesium, iron and aluminum during the restoration of wetlands.
In summary, in the transitional zone from the land to the sea in the Yellow River Delta, the P storage capacity of soil in the restored wetland is maximum, which can serve as a natural "ecological barrier" against land-based P from entering the ocean. Wetland restoration significantly improves the P storage capacity of soil. The P storage capacity of soil in the restored wetlands is mainly related to the accumulation of organic matter and the activation of calcium, magnesium, iron and aluminum plasma in the process of wetland restoration. The research results of this study can provide basic data for wetland protection and management, and also provide a scientific basis for achieving P balance in the river basin and environmental pollution control in the Bohai Sea.|