Abstract:

The Kaihui River watershed was selected as study area, where 12 ponds were sorted into 3 types: Type I,ponds excavated out of farmland in recent years; Type II,, ponds at the foot of hills under intense human disturbance; Type III, ponds at the foot of hills under little human disturbance. Water quality, physic-chemical properties of sediment and phosphorous adsorption behavior in sediment of the ponds were analyzed relative to type of pond. Results show that the three types of ponds followed an order of TypeⅠ﹥Ⅱ﹥Ⅲ in concentration of total P, oxalate extracted P, inorganic P of all other forms (excepted NH₄Cl-P), and bio-available P, which corresponded to the order of the ponds in water pollution degree. Among the inorganic forms of phosphorous, an order was found of metal oxide bound P (NaOH-P)﹥calcium bound P (HCl-P)﹥reducible P (BD-P)﹥loosely sorbed P (NH₄Cl-P) in terms of concentration. NaOH-P was the dominant form, accounting for 68.51%. Concentrations of BD-P and HCl-P were significantly and positively related to Fe_ox (P<0.01), while the concentrations of NH₄Cl-P and NaOH-P were to Al_ox (P<0.05). Fitting of P adsorption with the Langmuir equation found that the equilibrium P concentration (C_0EP), maximum sorption (S_max), and adsorption constant (K_c) in the sediments of the 12 ponds ranged from 0.02 - 0.12 mg·L^-1, 526.32 - 826.45 mg·kg^-1, and 0.31 - 1.11 L·mg^-1, respectively. The sediment in Pond 6 (TypeⅠ) was the highest in C_0EP and the lowest in S_max and K_c, so tha, it was the highest in potential P releasing risk. Pond 10 and Pond 11 (TypeⅡ) were quite high in K_c and S_max and relatively low in C_0EP. Pond 3 varied between Type I and Type II in P adsorption capacity. It is, therefore, quite obvious that the input of extraneous pollutants through human activities has a certain impact on P content and P adsorption behavior in the sediments of the ponds in the study area, and that in controlling P pollution in headwater regions of agricultural watersheds, it is essential to put P input load and P adsorption capacity of pond sediments under consideration.

Key words: agricultural watershed, pond, sediment, phosphorus form, adsorption