Research On The Phosphorus Fractions And Releasing Risk Assessment In Wetland Sediments Of Qi’ao Island Of Pearl River

The fraction and spatial and temporal distribution characteristic of phosphorus in the sediments of threetypical wetlands(intertidal region, mangrove and phragmites region) in Pearl River estuary Qi’ao island werestudied in this paper based on field sampling and laboratory simulation experiments. The characteristics ofadsorption and desorption of phosphorus in the sediments of different types of wetlands were compared, andthe effects of overlying water salinity variation on adsorption and desorption actions of phosphorus in thesediments were investigated emphatically. and the phosphorus release risk in estuary sediments were evaluated,which benefit to further discussion of pollution source, distribution characteristics and risk of eutrophication ofphosphorus in the sediments of typical wetlands in Pearl River estuary.Results indicated that the total phosphorus (TP) contents in surface sediments of different types ofwetlands in Pearl River estuary Qi’ao island rangend from687to1068mg/kg, which average content was882.6mg/kg. Inorganic phosphorus (IP) was the major fraction of total phosphorus (TP) and its average contentwas533.8mg/kg, which content was about60%of TP, while organ phosphorus content was about33%of TPcontent. Among the IP, the main fractions was phosphorus bound to Al, Fe(Fe/Al–P) which accounting for63%of IP. The results of temporal distribution characteristic of different forms of phosphorus showed thatmaximum value of inorganic phosphorus content in mangrove and phragmites region appeared in winter andautumn respectively, while minimum value appeared in spring and summer respectively. Total phosphoruscontent in intertidal region maintained in relatively high level that was less varied with seasons changing. Thecontent of surface sediments of wetlands in mangrove region changed obviously with seasonal fluctuation.Maximum and minimum value of organic phosphorus content appeared in winter and autumn respectively.Contents of Fe/Al-P and Ca-P of surface sediments in all regions in spring and winter were generally higherthan that in summer and autumn. The content of Fe/Al-P was the main factor influencing content of TP and IPof surface sediments in all regions.Adsorption of surface sediments of different types of wetlands accorded with Langmuir adsorptionisotherms equations. Absorption mainly occurred in the first half hour, adsorption rate was between276.2to299.7mg kg-1h-1. The adsorption of phosphorus of sediments in the vegetated region was higher than that inintertidal region, and adsorption in summer was obviously higher than that in winter. The adsorption ofphosphorus of sediments in mangrove region was high and less affected by seasonal variation, and adsorptionmaximum value is between400.0to476.2mg/kg, the adsorption of phosphorous maximum value changesobviously with seasonal fluctuation in phragmites and intertidal region, the adsorption of phosphorousmaximum value in summer is1.35times and1.5times of that in winter. The adsorption of phosphorousminimum value in intertidal region in winter is256.41mg/kg. The adsorption of phosphorous of sediments hadsignificant positive correlation with content of organic matter and active iron of sediments. When adsorption is saturated, desorption of phosphorous in sediments of different types of wetlands presented the same rule.Desorption was accomplished in2hours, the desorption content of phosphorous in mangrove, phragmites andintertidal regions were varied between7.78to13.3mg/kg,9.3to11.73mg/kg and5.42~10.9mg/kg, respectively.The content of desorption in vegetated region was a little higher than that in intertidal region, and that in springwas a little higher than that in other seasons.With the increase of salinity, adsorption of phosphorous showed aslight decrease but desorption showed significant increase. Salinity increased from10‰to30‰, the averagedesorption content in mangrove, phragmites and intertidal region was respectively increased2.8,2.7and3.2times. The effect on adsorption and desorption of phosphorous in sediments of salinity variation in vegetatedregion is obviously lower than that in intertidal region, in which the effect in intertidal region in winter is themost significant.Mangrove and phragmites regions had higher PSI and lower DPS, which meant that the vegetated regionhad more powerful buffer capacity to phosphorus compared with the intertidal region, especially in autumn.The potential risk of eutrophication in the research area is high. Reducing terrestrial phosphorus discharge andpreventing the sediment Fe/Al-P release to the interstitial water are the possible solutions to reduce the risk ofeutrophication in estuary wetlands.