The contents and vertical distribution of phosphorus fractions were analysed comparatively for core sediment samples taken from north sampling site for Core AB1 and south sampling site for Core AB2 of Aobao Lake in Lianhuan Lake using the SMT sequential extraction method. With the dating approach

the evolution characteristics and polluted history of phosphorus and the risk of phosphorus release in lake sediments were discussed. The results showed that the contents and vertical distribution of phosphorus fractions in Core AB1 and Core AB2 varied and the contents of phosphorus fractions in Core AB1 were higher than those in Core AB2

with fluctuations significantly with the depth. During the mid—1940s and 1981

the contents of phosphorus fractions and OM in Core AB1 showed large fluctuations; the contents of Ca-P

IP

and TP in Core AB2 first increased and then decreased

while the contents of Fe/Al-P

OP

and OM relatively constant. During 1981 to 2009

the contents of Ca-P

IP

and OM changed less and the contents of Fe/Al-P

OP

and TP showed relatively large fluctuation in Core AB1; the contents of Ca-P

Fe/Al-P

and TP first increased and then decreased to different degrees and the contents of IP

OP

and OM tended to increase in Core AB2. The sources of phosphorus in core sediments of Aobao Lake were mainly from industrial wastewater

sewage

and the loss of pesticides and fertilizers. The content of phosphorus in the north of lake was higher than that in the south

which was closely related with human activity. There were several villages in the northern part of Aobao Lake but none in the southern part. By using single single factor standard index method

the assessment of ecological risk of phosphorus in surface sediments showed that the content of total phosphorus in the northern part of lake exceeded the standard and showed a certain safety risk. Meanwhile

active phosphorus (Fe/Al-P and OP) in the upper part of sediment showed a greater increase than that at the bottom of Core AB1

and active phosphorus (Fe/Al-P and OP) were the dominant fractions of the increasing phosphorus sedimentation in Core AB1. It was indicated that phosphorus in the sediment in Aobao Lake would release more easily to the overlying water

and the effect on the eutrophication of lake water should be given attention.