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Abstract [Objectives] This study was conducted to explore whether the coastal wetlands in the Yellow River Delta have become secondary pollution sources.
[Methods] With a coastal reed wetland in the Yellow River Delta as a research object， the SMT method and DGT technology were combined to study the vertical distribution of different forms of phosphorus in the sediment and the distribution characteristics of phosphorus at the sedimentinterstitial/overlying water interface of the wetland to clarify the receiving and purification capabilities of wetlands in this region.
[Results] The total phosphorus content in the wetlands water intake area， dense reed area and water outlet area increased with the depth increasing; and the wetland phosphorus accumulated in the deep layer， and the total phosphorus content in the wetland sediments was as high as 936.30 mg/kg. The research on the phosphorus forms in the wetland sediments found that the content of Ca phosphorus was the highest in the wetland sediments， followed by the content of Fe/Albonded phosphorus， and the exchangeable phosphorus content was the lowest. The DGT sampling technology was used to investigate the distribution characteristics of phosphorus at wetland sedimentinterstitial/overlying water microinterface. It was found that the concentrations of phosphorus in the overlying water of the water intake area， dense reed area and water outlet area were significantly higher than that in the interstitial water. The wetland pollution level in this research area was high and had an obvious cumulative effect.
[Conclusions] The results show that the wetland sediments in this region have a strong ability to retain phosphorus， and can control the migration and transformation of endogenous phosphorus， and the wetland has not become a secondary pollution source.
Key words Wetland; P; Sediment; Overlying water
Chinas wetland area accounts for a large proportion of the worlds wetland area， and there are many types. The Yellow River Delta is the most complete and largest preserved new wetland in the warm temperate zone of China， with an area of 774 400 hm2， accounting for about 24.45% of the area in the region[1]. However， due to the drastic reduction in the amount of incoming water for the Yellow River， coastal erosion and industrial and agricultural production[2]， the area of natural wetlands in the Yellow River Delta has been shrinking， and a series of ecological and environmental problems such as wetland ecosystem structural and functional decline and loss of biodiversity have emerged[3-6]. At present， more research has been carried out on the degradation and restoration of wetlands in the Yellow River Delta from the causes and mechanisms of wetland degradation and wetland restoration measures， mainly focusing on wetland ecosystem restoration and conservation， wetland ecological security and ecological value evaluation，and a series of research results and engineering effects have been achieved[7-9]. The ability of wetland sediments to "sink" phosphorus is obviously related to the running time of wetlands， and the potential of phosphorus "sink" is very different due to different wetland soil characteristics in different regions[10-13]. For example， after two years of comprehensive measures such as sediment restoration， restoration and landscape construction on the degraded wetlands in the entrance of lakes and rivers in the northwestern Yunnan Plateau， the phosphorus removal capacity of the wetlands has been always good， as 91.7% of the phosphorus in the water body can be removed[14]. However， after just 2 years of running， the Maliaohe River wetland of Fuxian Lake has basically no phosphorus removal function[15]. It can be seen that the phosphorus removal capacities of wetlands in different regions will be different under the same running time. With the extension of the wetland running time， the phosphorus removal function of wetlands shows a decline trend， but the degree of decline varies greatly. For example， Carl et al.[16]reported that 5 years after the restoration of riparian wetlands， surface sediments were enriched with ionic bound phosphorus at 201 kg/hm2 every year， and the wetlands showed a clear "sink" ability. Similarly， in the absence of management， the severely degraded and reconstructed wetlands reduced their phosphorus retention rate from 60% to 10% after 15 years of running[17]. In addition， 12 wetlands were treated with highconcentration organic wastewater by a mixed treatment method. It was found that after 6 years of treatment， the removal rates of COD， BOD5 and TN in the wetlands remained at 90%， but the removal rate of TP decreased by 30%[18]. It can be seen that the ability of wetland substrates to retain phosphorus in different regions is quite different， and the wetland running time significantly affects the phosphorus sink capacity of wetlands.
The characteristics of phosphorus absorption and purification ability of degraded wetland sediments under wastewater remediation measures are still unclear. Based on this， in this study， with the degraded coastal wetland in the Yellow River Delta that had been restored with domestic wastewater for 3 years as a research object， the sedimentwater distribution law of phosphorus was investigated by a combination of field monitoring and the DGT technology， and the phosphorus release potential of the wetland was quantitatively evaluated. This study will provide an important theoretical basis for the use of domestic wastewater to restore degraded coastal wetlands in the Yellow River Delta. Materials and Methods
General situation of the research area
The research area is located in the severely degraded coastal salinealkali wetland in Zhanhua County， Binzhou， the hinterland of the Yellow River Delta. It had an area of 1 hm2， and its total watersoluble salt content and average pH were 0.9%-2.4% and 7.9， respectively. This area belongs to the East Asia and has a humid continental monsoon climate， with an average annual temperature of 12.5 ℃， an average annual precipitation of about 584 mmand an annual evaporation of 1 800-2 000 mm. At present， the emergent plants in the experimental area are mainly reeds. The density of reeds ranges from 30 to 350 plants/m2， with an average value of 243 plants/m2. The reed coverage had a minimum value of 18%， a maximum value of 90% and an average value of 75%. The irrigation wastewater came from domestic wastewater treated by a domestic sewage treatment plant in Shandong. The wastewater had a TP content of （1.02±0.13） mg/L， a pH value of （7.57±0.31）， a COD content of （93.26±3.08） mg/L， and a TN content of （8.59±0.16） mg/L.
Sample collection
Collection of sediment samples： In the wetland research areas， sediments of 0-5， 5-10， 10-15 and 15-20 cm depth were collected by diagonal sampling. The contents of different forms of phosphorus and TP in the sediments were analyzed， and the accumulated phosphorus contents in the wetland sediments that had been or not been restored and the vertical distribution characteristics of the profiles were compared and analyzed.
Monitoring of sedimentoverlying/interstitial water interface： Overlying water and sediment samples were collected from the wetlands. Sixteen sampling points were set in the research area， and the DGT technology was applied to obtain the profile concentration distribution of DRP with a resolution of up to 1 mm at the watersediment microinterface， and analyze the distribution law of DRP at the watersediment microinterface. The specific method was given as below. A platetype phosphate DGT device was marked at the edge of 1-2 cm below the top of the measurement window， and the probe was pushed with the push rod gently to the sediment until the mark was below the water interface of the sediment. When inserting， it was ensured that the probe penetrated the sediment vertically. The temperature and adsorption time was recorded until the adsorption time of 24 h. After drawing the gel probe out， it was washed with ultrapure water to ensure that there were no residual particles in the measurement window. The microscopic changes of DRP at the watersediment interface were studied. Results and Analysis
Profile distribution characteristics of total phosphorus in wetland sediments
Fig. 1 reflects the distribution laws of total phosphorus in different areas of the wetland. The variation trends of total phosphorus contents in the water intake area， dense reed area and water outlet area with increasing depth were generally the same. The total phosphorus content increased sharply at a depth of 0-15 cm， and reached its maximum in the range of 720-920 mg/kg at a depth of 10-20 cm， which was 1.5 times the total phosphorus content of 0-5 cm. It could be seen that phosphorus accumulated in the deep layer of wetlands.
Profile distribution characteristics of different forms of phosphorus in wetland sediments
Profile distribution characteristics of exchangeable phosphorus in sediments
The distribution law of exchangeable phosphorus was different in the sediments in different areas of the wetland （Fig. 2）. The overall contents were in the range of 56-106 mg/kg， and showed the same variation trend with the depth increasing. The exchangeable phosphorus contents in the dense reed area and water outlet area changed little with the increase of depth and were basically stable. Among them， the exchangeable phosphorus in the water outlet area showed a trend of increasing first and then decreasing with the depth increasing， and the maximum content was 97.27 mg/kg at a depth of 10-15 cm; and the exchangeable phosphorus in the dense reed area increased with the increase of depth， and reached a maximum of 111.87 mg/kg at 15-20 cm. The exchangeable phosphorus content of the water inlet area changed greatly with the depth increasing， showing a trend of increasing first and then decreasing， and the highest content was in the range of 95-106 mg/kg at the depth of 10-20 cm.
Profile distribution characteristics of Fe/Albonded phosphorus in sediments
The content of Fe/Albonded phosphorus in different areas of the wetland showed different distribution laws with the depth increasing （Fig. 3）， and fluctuated between 95 and 160 mg/kg. The Fe/Albonded phosphorus contents in the water intake area， dense reed area and water outlet area showed a trend of first increasing and then decreasing with the depth increasing， all with a maximum of 117-160 mg/kg at the depth of 0-10 cm; and below 10 cm， the Fe/Albonded phosphorus contents decreased sharply with the depth increasing. It can be seen that the Fe/Albonded phosphorus mainly accumulates in the surface layer of the sediments and has the potential of downward migration.
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Profile distribution characteristics of Ca phosphate in sediments
Ca phosphate in different depths of the wetland showed different distribution patterns in different areas （Fig. 4）. The overallcontents were in the range of 150-368 mg/kg， and it showed different trends with the depth increasing.
The Ca phosphate concentration near the water outlet increased with the depth increasing， and the maximum Ca phosphate deposition was 368 mg/kg at a depth of 15-20 cm， which was more than 2 times that of 5-15 cm. The downward migration of Ca phosphate was obvious. However， the concentration of Ca phosphate in the dense reed area was only higher in the surface layer， being 215.50 mg/kg. Due to the vigorous growth of reeds in this area， the large amount of Ca phosphate absorbed by the root system resulted in a lower level of Ca phosphate， while the reed root system had a large specific surface area and released some enzymes， which caused bottom microorganisms to be active and release Ca phosphate to the overlying water. The content of Ca phosphate in the water intake area gradually increased to a maximum of 363.70 mg/kg with the increase of depth. And the Ca phosphate gradually moved down to deposit with the increase of depth， while it was difficult for the surface sediment to release Ca phosphate outward.
Exchange characteristics of phosphorus at sediment overlying/interstitial water microinterface
The DGT technology was applied to study the distribution laws of phosphorus at the sedimentoverlying water interface in differentareas of the wetland. The phosphorus content at the sedimentoverlying water interface in the water inlet area of the wetland ranged from 0.34 to 0.39 mg/L， and the phosphorus content of the sediment was between 0.18 and 0.24 mg/L （Fig. 5）. Specifically， the phosphorus content reached a maximum of 0.39mg/Lat about 2 cm from the sediment upward， and a minimum of 0.18 mg/L at about 4 cm of the sediment， below which phosphorus increased and accumulated at the bottom of the sediment. On the whole， the phosphorus content in this area was higher than that in other areas， and generally consistent with the water inlet and dense reed areas in phosphorus distribution laws. It could be seen that the pollution level in this area was higher than in other areas.
The phosphorus content at the sedimentoverlying water interface in the dense reed area was in the range of 0.29-0.38 mg/L， while the phosphorus content in sediment ranged from 0.14to 0.23mg/L （Fig. 6）. The phosphorus content in the overlying water was higher than that inthe sediment. Specifically， the phosphorus content was up to 0.38 mg/L at about 2 cm from the sediment upward， and had a minimum of 0.18 mg/L at about 8 cm of the sediment， below which phosphorus increased and accumulated at the bottom of the sediment.
The phosphorus content of the overlying water in the wetland outlet area was significantly higher than that of the sediment. The phosphorus content of the overlying water was between 0.27 and 0.32 mg/L （Fig. 7）， and the phosphorus content in the sediment ranged from 0.13 to 0.18 mg/L. The phosphorus content of the overlyingwater was the highest at 2 cm from the sediment upward. The phosphorus content in the sediment showed a decreasing trend from 0 to 6 cm， then increased from 6 to 10 cm and finally accumulated at the bottom of the sediment. The phosphorus content higher in the overlying water than in the sediment indicates that the wetland has the ability to continue to purify sewage.
Conclusions
The total phosphorus accumulated at the bottom of the wetland was as high as 936.30 mg/kg at the bottom of the sediments. With the increase of the depth， the total phosphorus in the wetland sediments showed different trends in different areas.
The Ca phosphate in wetland sediments in different areas showed different variation trends， and its contents was high. The Ca phosphate changes in the water intake and dense reed areas had the same trend of first decreasing and then increasing. The content of Ca phosphate in wetland sediments was higher than those of other forms of phosphorus and changed greatly with the depth increasing.
The content of exchangeable phosphorus in the total phosphorus was relatively small. The exchangeable phosphorus at the water inlet increased first and then decreased with the depth increasing; the exchangeable phosphorus concentration in the dense reed area increased with the increase of depth; and the exchangeable phosphorus concentration at the water outlet did not change much with the depth increasing. The variation trends of Fe/Albonded phosphorus at different depths were the same， i.e.， first increasing and then decreasing.
The DGT technology was applied to explore the distribution of phosphorus at the sedimentoverlaying water interface of the wetland. The phosphorus contents in overlaying water in the dense reed， water inlet and water outlet areas were quite high， and higher than those in the sediments， indicating that the wetland has the ability to continue to purify sewage.
References
[1] TIAN JY， LI JL， SUN JK， et al. Domestic Wastewater Irrigation Wetland Restoration Technology in the Yellow River Delta[M]. Beijing： Chemical Industry Press， 2010. （in Chinese） [2] WANG HM. The analyse on the change of land use/cover and character of ecosystem in Yellow River Delta[D]. Hohhot： Inner Mongolia University， 2005， 8-10. （in Chinese）
[3] JEFFREY G. HOLMQUIST， JUTTA SG， et al. Efficacy of Low and High Complexity Vegetation Treatments for Restablishing Terrestrial Arthropod Assemblages during Montane Wetland Restoration[J]. Restoration Ecology， 2014， 22（5）： 649-656.
[4] HEIDARPOUR M， MOSTAFAZADEH FB，ABEDI KJ， et al. The effects of treated wastewater on soil chemical properties using subsurface and surface irrigation methods[J]. Agricultural Water Management， 2007， 90（1-2）： 87-94.
[5] GUAN B， YU JB， LU ZH， et al. The ecological effects of Suaeda salsa on repairing heavily degraded coastal salinealkaline wetland in the Yellow River Delta[J]. Acta Ecologica Sinica， 2011， 31（17）： 4835-4840. （in Chinese）
[6] DENG MY， XIE FJ， HOU CM. Restoration of vegetation and soil in degraded wetland of the Yellow River Delta[J]. Journal of Meteorology and Environment， 2012， 28（1）： 11-16. （in Chinese）
[7] CUI BS， HE Q， ZHAO XS. Researches on the ecological thresholds of Suaeda salsa to the environmental gradients of water table depth and soil salinity[J]. Acta Ecologica Sinica， 2008， 28（4）： 1408-1418. （in Chinese）
[8] CUI B， YANG Q， ZHANG K， et al. Responses of saltcedar （Tamarix chinensis） to water table depth and soil salinity in the Yellow River Delta， China[J]. Plant Ecology， 2010， 209（2）： 279 -290.
[9] YU J， WANG Y， LI Y， et al. Soil organic carbon storage changes in coastal wetlands of the modern Yellow River Delta from 2000 to 2009[J]. Biogeosciences， 2012， 9（6）： 2325-2331.
[10] MARCELO A， JENNIFER LM， MARTIN WD， et al. The Water Quality Consequences of Restoring Wetland Hydrology to a Large Agricultural Watershed in the Southeastern Coastal Plain[J]. Ecosystems， 2010， 13（7）： 1060-1078.
[11] ERIC DR， NHAN TN， SIBEL B， et al. Internal loading of phosphorus from sediments of Lake Pontchartrain （Louisiana，USA） with implications for eutrophication[J]. Hydrobiologia， 2012（684）： 69-82.
[12] LAUREN EK， JONATHAN O， STEPHEN KH. Reflooding a Historically Drained Wetland Leads to Rapid Sediment Phosphorus Release[J]. Ecosystems， 2014， 17（4）： 641-656.
[13] QIAN B， LIU L， XIAO X， et al. The effect of environmental microinterface on internal phosphorus release of the lake[J]. Journal of Hydraulic Engineering， 2013， 44（3）： 295-302. （in Chinese） [14] FU WC， TIAN K， XIAO DR， et al. The ecological restoration effort of degraded esturaine wetland in Northwest Yunnan Plateau， China[J]. Acta Ecologica Sinica， 2014， 34（9）：2187-2194. （in Chinese）
[15] ZHANG Q. Shift of phosphorus sinksource functions of constructed wetlands and a theoretical explanation[J]. Journal of Lake Sciences， 2007， 19（1）： 36-51. （in Chinese）
[16] CARL C H， LISA H， JOACHIM A， et al. Low phosphorus release but high nitrogen removal in two restored riparian wetlands in undated with agricultural drainage water[J]. Ecological Engineering， 2012（46）： 75-87.
[17] LUCCI GM， MCDOWELL RW， CONDRON LM. Evaluation of base solutions to determine quilibrium phosphorus concentrations （EPC0） in stream sediments[J]. International Agrophysics， 2010（24）： 157-163.
[18] JUNE H， TAKASHI I， KUNIHIKO K. Performance evaluation of hybrid treatment wetland for six years of operation in cold climate[J]. Environ. Sci. Pollut. Res.， 2015（22）： 12861-12869.
[Methods] With a coastal reed wetland in the Yellow River Delta as a research object， the SMT method and DGT technology were combined to study the vertical distribution of different forms of phosphorus in the sediment and the distribution characteristics of phosphorus at the sedimentinterstitial/overlying water interface of the wetland to clarify the receiving and purification capabilities of wetlands in this region.
[Results] The total phosphorus content in the wetlands water intake area， dense reed area and water outlet area increased with the depth increasing; and the wetland phosphorus accumulated in the deep layer， and the total phosphorus content in the wetland sediments was as high as 936.30 mg/kg. The research on the phosphorus forms in the wetland sediments found that the content of Ca phosphorus was the highest in the wetland sediments， followed by the content of Fe/Albonded phosphorus， and the exchangeable phosphorus content was the lowest. The DGT sampling technology was used to investigate the distribution characteristics of phosphorus at wetland sedimentinterstitial/overlying water microinterface. It was found that the concentrations of phosphorus in the overlying water of the water intake area， dense reed area and water outlet area were significantly higher than that in the interstitial water. The wetland pollution level in this research area was high and had an obvious cumulative effect.
[Conclusions] The results show that the wetland sediments in this region have a strong ability to retain phosphorus， and can control the migration and transformation of endogenous phosphorus， and the wetland has not become a secondary pollution source.
Key words Wetland; P; Sediment; Overlying water
Chinas wetland area accounts for a large proportion of the worlds wetland area， and there are many types. The Yellow River Delta is the most complete and largest preserved new wetland in the warm temperate zone of China， with an area of 774 400 hm2， accounting for about 24.45% of the area in the region[1]. However， due to the drastic reduction in the amount of incoming water for the Yellow River， coastal erosion and industrial and agricultural production[2]， the area of natural wetlands in the Yellow River Delta has been shrinking， and a series of ecological and environmental problems such as wetland ecosystem structural and functional decline and loss of biodiversity have emerged[3-6]. At present， more research has been carried out on the degradation and restoration of wetlands in the Yellow River Delta from the causes and mechanisms of wetland degradation and wetland restoration measures， mainly focusing on wetland ecosystem restoration and conservation， wetland ecological security and ecological value evaluation，and a series of research results and engineering effects have been achieved[7-9]. The ability of wetland sediments to "sink" phosphorus is obviously related to the running time of wetlands， and the potential of phosphorus "sink" is very different due to different wetland soil characteristics in different regions[10-13]. For example， after two years of comprehensive measures such as sediment restoration， restoration and landscape construction on the degraded wetlands in the entrance of lakes and rivers in the northwestern Yunnan Plateau， the phosphorus removal capacity of the wetlands has been always good， as 91.7% of the phosphorus in the water body can be removed[14]. However， after just 2 years of running， the Maliaohe River wetland of Fuxian Lake has basically no phosphorus removal function[15]. It can be seen that the phosphorus removal capacities of wetlands in different regions will be different under the same running time. With the extension of the wetland running time， the phosphorus removal function of wetlands shows a decline trend， but the degree of decline varies greatly. For example， Carl et al.[16]reported that 5 years after the restoration of riparian wetlands， surface sediments were enriched with ionic bound phosphorus at 201 kg/hm2 every year， and the wetlands showed a clear "sink" ability. Similarly， in the absence of management， the severely degraded and reconstructed wetlands reduced their phosphorus retention rate from 60% to 10% after 15 years of running[17]. In addition， 12 wetlands were treated with highconcentration organic wastewater by a mixed treatment method. It was found that after 6 years of treatment， the removal rates of COD， BOD5 and TN in the wetlands remained at 90%， but the removal rate of TP decreased by 30%[18]. It can be seen that the ability of wetland substrates to retain phosphorus in different regions is quite different， and the wetland running time significantly affects the phosphorus sink capacity of wetlands.
The characteristics of phosphorus absorption and purification ability of degraded wetland sediments under wastewater remediation measures are still unclear. Based on this， in this study， with the degraded coastal wetland in the Yellow River Delta that had been restored with domestic wastewater for 3 years as a research object， the sedimentwater distribution law of phosphorus was investigated by a combination of field monitoring and the DGT technology， and the phosphorus release potential of the wetland was quantitatively evaluated. This study will provide an important theoretical basis for the use of domestic wastewater to restore degraded coastal wetlands in the Yellow River Delta. Materials and Methods
General situation of the research area
The research area is located in the severely degraded coastal salinealkali wetland in Zhanhua County， Binzhou， the hinterland of the Yellow River Delta. It had an area of 1 hm2， and its total watersoluble salt content and average pH were 0.9%-2.4% and 7.9， respectively. This area belongs to the East Asia and has a humid continental monsoon climate， with an average annual temperature of 12.5 ℃， an average annual precipitation of about 584 mmand an annual evaporation of 1 800-2 000 mm. At present， the emergent plants in the experimental area are mainly reeds. The density of reeds ranges from 30 to 350 plants/m2， with an average value of 243 plants/m2. The reed coverage had a minimum value of 18%， a maximum value of 90% and an average value of 75%. The irrigation wastewater came from domestic wastewater treated by a domestic sewage treatment plant in Shandong. The wastewater had a TP content of （1.02±0.13） mg/L， a pH value of （7.57±0.31）， a COD content of （93.26±3.08） mg/L， and a TN content of （8.59±0.16） mg/L.
Sample collection
Collection of sediment samples： In the wetland research areas， sediments of 0-5， 5-10， 10-15 and 15-20 cm depth were collected by diagonal sampling. The contents of different forms of phosphorus and TP in the sediments were analyzed， and the accumulated phosphorus contents in the wetland sediments that had been or not been restored and the vertical distribution characteristics of the profiles were compared and analyzed.
Monitoring of sedimentoverlying/interstitial water interface： Overlying water and sediment samples were collected from the wetlands. Sixteen sampling points were set in the research area， and the DGT technology was applied to obtain the profile concentration distribution of DRP with a resolution of up to 1 mm at the watersediment microinterface， and analyze the distribution law of DRP at the watersediment microinterface. The specific method was given as below. A platetype phosphate DGT device was marked at the edge of 1-2 cm below the top of the measurement window， and the probe was pushed with the push rod gently to the sediment until the mark was below the water interface of the sediment. When inserting， it was ensured that the probe penetrated the sediment vertically. The temperature and adsorption time was recorded until the adsorption time of 24 h. After drawing the gel probe out， it was washed with ultrapure water to ensure that there were no residual particles in the measurement window. The microscopic changes of DRP at the watersediment interface were studied. Results and Analysis
Profile distribution characteristics of total phosphorus in wetland sediments
Fig. 1 reflects the distribution laws of total phosphorus in different areas of the wetland. The variation trends of total phosphorus contents in the water intake area， dense reed area and water outlet area with increasing depth were generally the same. The total phosphorus content increased sharply at a depth of 0-15 cm， and reached its maximum in the range of 720-920 mg/kg at a depth of 10-20 cm， which was 1.5 times the total phosphorus content of 0-5 cm. It could be seen that phosphorus accumulated in the deep layer of wetlands.
Profile distribution characteristics of different forms of phosphorus in wetland sediments
Profile distribution characteristics of exchangeable phosphorus in sediments
The distribution law of exchangeable phosphorus was different in the sediments in different areas of the wetland （Fig. 2）. The overall contents were in the range of 56-106 mg/kg， and showed the same variation trend with the depth increasing. The exchangeable phosphorus contents in the dense reed area and water outlet area changed little with the increase of depth and were basically stable. Among them， the exchangeable phosphorus in the water outlet area showed a trend of increasing first and then decreasing with the depth increasing， and the maximum content was 97.27 mg/kg at a depth of 10-15 cm; and the exchangeable phosphorus in the dense reed area increased with the increase of depth， and reached a maximum of 111.87 mg/kg at 15-20 cm. The exchangeable phosphorus content of the water inlet area changed greatly with the depth increasing， showing a trend of increasing first and then decreasing， and the highest content was in the range of 95-106 mg/kg at the depth of 10-20 cm.
Profile distribution characteristics of Fe/Albonded phosphorus in sediments
The content of Fe/Albonded phosphorus in different areas of the wetland showed different distribution laws with the depth increasing （Fig. 3）， and fluctuated between 95 and 160 mg/kg. The Fe/Albonded phosphorus contents in the water intake area， dense reed area and water outlet area showed a trend of first increasing and then decreasing with the depth increasing， all with a maximum of 117-160 mg/kg at the depth of 0-10 cm; and below 10 cm， the Fe/Albonded phosphorus contents decreased sharply with the depth increasing. It can be seen that the Fe/Albonded phosphorus mainly accumulates in the surface layer of the sediments and has the potential of downward migration.
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Profile distribution characteristics of Ca phosphate in sediments
Ca phosphate in different depths of the wetland showed different distribution patterns in different areas （Fig. 4）. The overallcontents were in the range of 150-368 mg/kg， and it showed different trends with the depth increasing.
The Ca phosphate concentration near the water outlet increased with the depth increasing， and the maximum Ca phosphate deposition was 368 mg/kg at a depth of 15-20 cm， which was more than 2 times that of 5-15 cm. The downward migration of Ca phosphate was obvious. However， the concentration of Ca phosphate in the dense reed area was only higher in the surface layer， being 215.50 mg/kg. Due to the vigorous growth of reeds in this area， the large amount of Ca phosphate absorbed by the root system resulted in a lower level of Ca phosphate， while the reed root system had a large specific surface area and released some enzymes， which caused bottom microorganisms to be active and release Ca phosphate to the overlying water. The content of Ca phosphate in the water intake area gradually increased to a maximum of 363.70 mg/kg with the increase of depth. And the Ca phosphate gradually moved down to deposit with the increase of depth， while it was difficult for the surface sediment to release Ca phosphate outward.
Exchange characteristics of phosphorus at sediment overlying/interstitial water microinterface
The DGT technology was applied to study the distribution laws of phosphorus at the sedimentoverlying water interface in differentareas of the wetland. The phosphorus content at the sedimentoverlying water interface in the water inlet area of the wetland ranged from 0.34 to 0.39 mg/L， and the phosphorus content of the sediment was between 0.18 and 0.24 mg/L （Fig. 5）. Specifically， the phosphorus content reached a maximum of 0.39mg/Lat about 2 cm from the sediment upward， and a minimum of 0.18 mg/L at about 4 cm of the sediment， below which phosphorus increased and accumulated at the bottom of the sediment. On the whole， the phosphorus content in this area was higher than that in other areas， and generally consistent with the water inlet and dense reed areas in phosphorus distribution laws. It could be seen that the pollution level in this area was higher than in other areas.
The phosphorus content at the sedimentoverlying water interface in the dense reed area was in the range of 0.29-0.38 mg/L， while the phosphorus content in sediment ranged from 0.14to 0.23mg/L （Fig. 6）. The phosphorus content in the overlying water was higher than that inthe sediment. Specifically， the phosphorus content was up to 0.38 mg/L at about 2 cm from the sediment upward， and had a minimum of 0.18 mg/L at about 8 cm of the sediment， below which phosphorus increased and accumulated at the bottom of the sediment.
The phosphorus content of the overlying water in the wetland outlet area was significantly higher than that of the sediment. The phosphorus content of the overlying water was between 0.27 and 0.32 mg/L （Fig. 7）， and the phosphorus content in the sediment ranged from 0.13 to 0.18 mg/L. The phosphorus content of the overlyingwater was the highest at 2 cm from the sediment upward. The phosphorus content in the sediment showed a decreasing trend from 0 to 6 cm， then increased from 6 to 10 cm and finally accumulated at the bottom of the sediment. The phosphorus content higher in the overlying water than in the sediment indicates that the wetland has the ability to continue to purify sewage.
Conclusions
The total phosphorus accumulated at the bottom of the wetland was as high as 936.30 mg/kg at the bottom of the sediments. With the increase of the depth， the total phosphorus in the wetland sediments showed different trends in different areas.
The Ca phosphate in wetland sediments in different areas showed different variation trends， and its contents was high. The Ca phosphate changes in the water intake and dense reed areas had the same trend of first decreasing and then increasing. The content of Ca phosphate in wetland sediments was higher than those of other forms of phosphorus and changed greatly with the depth increasing.
The content of exchangeable phosphorus in the total phosphorus was relatively small. The exchangeable phosphorus at the water inlet increased first and then decreased with the depth increasing; the exchangeable phosphorus concentration in the dense reed area increased with the increase of depth; and the exchangeable phosphorus concentration at the water outlet did not change much with the depth increasing. The variation trends of Fe/Albonded phosphorus at different depths were the same， i.e.， first increasing and then decreasing.
The DGT technology was applied to explore the distribution of phosphorus at the sedimentoverlaying water interface of the wetland. The phosphorus contents in overlaying water in the dense reed， water inlet and water outlet areas were quite high， and higher than those in the sediments， indicating that the wetland has the ability to continue to purify sewage.
References
[1] TIAN JY， LI JL， SUN JK， et al. Domestic Wastewater Irrigation Wetland Restoration Technology in the Yellow River Delta[M]. Beijing： Chemical Industry Press， 2010. （in Chinese） [2] WANG HM. The analyse on the change of land use/cover and character of ecosystem in Yellow River Delta[D]. Hohhot： Inner Mongolia University， 2005， 8-10. （in Chinese）
[3] JEFFREY G. HOLMQUIST， JUTTA SG， et al. Efficacy of Low and High Complexity Vegetation Treatments for Restablishing Terrestrial Arthropod Assemblages during Montane Wetland Restoration[J]. Restoration Ecology， 2014， 22（5）： 649-656.
[4] HEIDARPOUR M， MOSTAFAZADEH FB，ABEDI KJ， et al. The effects of treated wastewater on soil chemical properties using subsurface and surface irrigation methods[J]. Agricultural Water Management， 2007， 90（1-2）： 87-94.
[5] GUAN B， YU JB， LU ZH， et al. The ecological effects of Suaeda salsa on repairing heavily degraded coastal salinealkaline wetland in the Yellow River Delta[J]. Acta Ecologica Sinica， 2011， 31（17）： 4835-4840. （in Chinese）
[6] DENG MY， XIE FJ， HOU CM. Restoration of vegetation and soil in degraded wetland of the Yellow River Delta[J]. Journal of Meteorology and Environment， 2012， 28（1）： 11-16. （in Chinese）
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