Phosphorus Sorption and Sedimentation in a Multipond System within a Headstream Agricultural Watershed

Abstract The movement, transformation, and assimilation of phosphorus (P) in a watershed are often related to the physicochemical characteristics of the sediments of the water storage structures in the watershed. Field and laboratory experiments were conducted to study P sorption by pond sediments in a multipond system in a watershed of 6.9 km2 in southeastern China. Being scattered in different ambient land uses, and receiving soil particles from various sources, the multipond system provides an excellent research example for P sorption as well as the relationship between pond properties and their location in a watershed. It was found that the pond sediments showed largely variable P sorption capacities in this watershed. The P sorption maximum (Smax) ranged from 390.0 to 729.2 mg/kg, and equilibrium P concentration (EPC0) ranged from 0.007 to 0.023 mg/L. The mean degree of P saturation (DPS) of the pond sediments was only 9.5%. Due to the large annual delivery of sediments to the ponds by soil erosion, the pond sediments never reached saturation. The location of the ponds could influence the physicochemical properties of sediments and their sorption characteristics. For Smax, the Hill pond value was significantly higher than that for the Village pond. For EPC0, the result was reverse. The oxalate-extractable Fe (Ox-Fe) and KCl-extractable P (KCl-P) were the key factors that influenced P sorption of the pond sediments. Ox-Fe could explain 84% of the variability in Smax and KCl-P could explain 82% of the variability in EPC0. The acidic and high Fe contents of laterite soil in southeastern China was the most important cause of the high P sorption capacity of the pond sediments.