| The agricultural headwater stream directly receives non-point source caused by nitrogen and phosphorus that derived from the catchment area, and which would affect water quality of the downstream water by progressive transmission. Residual nitrogen and phosphorus in the bottom sediment can release back to the overlying water body through water-sediment exchange. Therefore, agricultural headwater stream is generally as source and sink of nitrogen and phosphorus. In the study, a typical water system of agricultural headwater stream in Chaohu Lake basin was investigated. The water stream system was divided into four styles of stream morphology, including ponds, branches, main channel and mainstream deep pools. To estimate the risk of phosphorus release four styles of stream morphology, we analyzed the forms and space-time variations of phosphorus in the sediment, resolved the phosphorus adsorption characteristic of sediment by multivariate statistical analyses, and quantified the phosphorus release risk via calculating the phosphorus sorption index. The main results were outlined as follows:（1） On the main channel, the water environment of upstream is significantly different from downstream, the average concentrations of NH+and PO43- in downstream was about 10 times than those in upstream, outperforming the national water V class standard. TDS and EC of the upstream water are lower than those of downstream. The ORP of water is greater than 0 in both upstream and downstream, the water had the oxidation characteristics.（2） The contents of TP in the surface sediments ranged from 137.517 to 1709.229 mg·kg-1 with an average value of 532.245 mg·kg-1, and the order of the average contents of phosphorus forms was IP(350.347mg·kg-1)>OP(167.333mg·kg-1)>Fe/Al-P (78.869mg·kg-1)>Ca-P(56.343 mg·kg-1)>Ex-P(6.609 mg-kg-1); The contents of various phosphorus forms had a same trend in all the four stream styles, deep pool>main channel>branch>pond; Of TP, bioavailability of phosphorus and generalized bioavailability of phosphorus account for 13.64-22.18% and 41.72-64.26%, respectively. Overall, the proportions of the bioavailability and TP of main channel and deep pool in Autumn were higher than those in Spring, while the proportions of the bioavailability and TP of branch and pond remain unchanged between Spring and Autumn.（3） Correlation analysis showed that TP of the sediments had a significant correlation with other various forms of phosphorus, such as Ex-P, Fe/Al-P, Ca-P, OP and IP, but OM had no significant correlation with any index. Clustering analysis showed that in autumn, the deep pool and main channel were clustered in one class, while the pond and branch were clustered together in a class. In spring, branch, main channel and deep pool were clustered at the same class, Variance analysis showed that the differences among the four stream styles were larger in Autumn than those in Spring.（4） Adsorption isotherms of sediments to phosphorus indicated that isotherms for phosphorus sorption were well fitted by Henry linear equation. According to the equation, adsorption-desorption equilibrium concentrations EPCo of different sediments to phosphorus ranged from 0.012 mg·L-1 to 0.171 mg·L-1; The adsorption of sediments to phosphorus had same tendency in the four style streams, it was fast in the first 3h, and reached balance after 9h. The adsorption behavior of phosphorus was in line with pseudo-second adsorption kinetics model feature.（5） The PSI of the surface sediments ranged from 24.49 to 69.94（mgL） （100g.μmol）-1, the averages PSI were 59.95,34.56（mgL） · （100g.μmol）-1 in 2014 autumn and 2015 spring, respectively. These results indicated that risk of phosphorus was higher in spring than that in autumn. |
