Effects Of Mudflat Reclamation On Phosphorus Fractions Of Sediment And Its Releasing Risk Assessment

Mudflat reclamation is one of the main ways of tidal flat developments. The study on the effects of mudflat reclamation on sediment phosphorus ?P? fractions plays a vital role in developing our knowledge of changes in endogenous sediment P release and its potential pollution risk after natural wetlands convert to aquaculture ponds, and lays a basis for the sustainable use of littoral wetland resources. However, so far little is known about sediment P fractions in aquaculture ponds of mudflat reclamation and their influence factors. In this paper, Min River Estuary was selected as the study area. The sediment of aquaculture ponds at different years ?3 years and 5 years? and natural wetland ?Spartina alterniflora and nude beach wetlands? were sampled from the Min River estuary in November,2014. Sediment P fractions and physical and chemical properties were determined in the laboratory. Meanwhile, the effects of key environmental factors of aquaculture ponds such as flooded conditions, concentrates of phosphate (PO₄^3-) and aeration in overlying water and addition of bait on sediment P fractions were analyzed by incubation method. Moreover, Single factor standard index ?Si? and P release risk index ?ERI? were used to comprehensively analyze sediment P pollution levels and its potential release risk based on field investigation. The main results showed as follows:?1? The total P ?TP? contents ranged from 640?3250 and 670-835 mg·kg-1, inorganic P ?IP? contents ranged from 353-2318 and 395?512 mg·kg-1, organic P ?Org P? contents ranged from 212?1085 and 218?354 mg·kg-1, iron and aluminum-bound P ?Fe/Al-P? contents ranged from 171?1239 and 193?236 mg·kg-1, Calcium-bound P ?Ca-P? contents ranged from 158?757 and 158?209 mg·kg-1 in aquaculture ponds and natural wetland. IP dominated the TP fractions and it was mainly composed of Fe/Al-P in aquaculture ponds and natural wetland. In general, TP, Org P and IP did not change much after 3-year reclamation, while their contents, respectively, significantly increased by 175.27%, 126.29% and 205.58% after 15 years ?p<0.05?. They were more sensitive to the change of Fe/Al-P and Ca-P, significantly increasing by 40.08% and 15.33%, respectively, after 3-year reclamation. And the Fe/Al-P and Ca-P contents further increased with the extension of reclamation, with increased proportion as high as 262.10% and 129.70%, respectively. In addition, the proportion of Fe/Al-P occupied TP increased slightly, while the Org P/TP and Ca-P/TP declined after natural wetlands convert to aquaculture ponds.?2? Sediment water contents, total carbon ?TC?, total nitrogen ?TN?, ferrous iron (Fe²⁺) and the ratio of ferrous iron to ferric iron (Fe²⁺/Fe³⁺) had increased significantly on the whole, while bulk density and electrical conductivity ?EC? had significantly reduced, and there were no obvious changes in grain size and pH after natural wetlands convert to aquaculture ponds, especially for 15-year reclamation. The result of canonical correspondence analysis ?CCA? showed that water contents, bulk density, EC, TC, TN and-Fe²⁺ were considered to be important drivers of difference in P fractions in aquaculture ponds and natural wetlands. TP and IP were mainly affected by Fe³⁺, TC and TN, Fe/Al-P were mainly influenced by water contents, Fe²⁺?Fe and bulk density, Org P were significantly correlated with TC and TN, and Ca-P are more sensitive to EC, bulk density and water contents.?3? Sediment P fractions influenced by flooded condition, addition of bait, concentrates of PO₄^3-(PO₄^3--1?0.01 mg·L-1, PO₄^3--2?1.00 mg·L-1 and PO₄^3--3?5.00 mg·L-1) and aeration in overlying water were different. The concentrates of PO₄^3- in overlying water had a significant effect on TP, IP and Fe/Al-P contents, after 180-day incubation, which presented PO₄^3--1<PO₄^3--2< PO₄^3--3 ?p<0.05?. TP contents had a slight decline, while there was no obvious change in IP and Fe/Al-P contents in condition of PO₄^3--1 as compared with the beginning of the culture. TP and Fe/Al-P contents increased in condition of PO₄^3--2. And there were significantly increased 27.06%-39.48%,31.56%-81.84% and 71.35%-103.78% for TP, IP and Fe/Al-P contents in condition of PO₄^3--3, respectively ?p<0.05?. Aeration in overlying water promoted mineralization of Org P, whose contents significantly decreased by 22.49%-25.55%. Meanwhile, aeration promoted PO₄^3-in overlying water move into the sediment, as a result of which IP and Fe/Al-P contents significantly increased ?p<0.05?. Addition of bait had a deep impact on TP, IP and Fe/Al-P, after 180-day incubation. TP, IP and Fe/Al-P contents significantly increased by 41.61%,67.85% and 109.83%, respectively ?p<0.05?, while its not have an obvious effect on Org P and Ca-P.?4? There were significantly decreased 30.02% for Phosphorus sorption index ?PSI?, while there were significantly increased 49.51% for Phosphorus adsorption saturation ?DPS? after natural wetlands convert to aquaculture ponds, especially for 15-year reclamation. And these indicated that buffering power of the sediment to P decreased, and its potential release increased, while changes of DPS meant sediment phosphate adsorption capacity declined, and the sediment adsorb phosphate from overlying water would be limited. Si has increased from about 1.21 to 3.37, increasing by 178.51% over the natural wetlands, while ERI has increased from about 10.99% to 37.58%, increasing by 241.95% over the natural wetlands, and it means sediment P pollution degree was intensified and its potential pollution risk increased after reclamation.