Phosphorus retention capacity of agricultural headwater ditch sediments under alkaline condition in purple soils area, China

Phosphorus (P) retention by headwater ditch sediments adsorption plays a pivotal ecological role in P buffering in freshwater ecosystems. Previous studies focused on headwater ditch sediment adsorption and its P retention capacity in acid conditions, but little information is available for headwater ditches under alkaline condition. In this study, adsorption behavior of phosphorus was investigated in headwater ditch sediments under alkaline condition using a batch equilibrium technique, thus determining phosphorus retention capacity of headwater ditch sediments collected at 11 sites at base-flow on 2 March 2006 in purple soils area of China. Results showed that headwater ditch sediments had elevated phosphorus sorption maximum (Smax) values (122.72–293.23 mg P kg−1) and P binding energy (K) values (1.64–8.65 L mg−1), while they had low equilibrium phosphorus concentration (EPC0) (0.001–0.108 mg L−1) and degree of phosphorus saturation (DSP) (1.93–10.19%). Analysis of EPC0 and soluble P concentration indicated that sediments acted as a sink for P across all headwater ditches. Therefore, there were high intrinsic P retention capacities of headwater ditch sediments. Positive correlations of both K and Smax with oxalate-extractable Fe (r of 0.93 and 0.81, p < 0.05) and total carbon (TC) (r of 0.89 and 0.74, p < 0.05) were found, thus suggesting that organic matter and amorphous or poorly crystalline Fe would play dominant roles in P adsorption in the headwater ditch sediments under alkaline condition. Since neither Smax nor K were correlated with CCE (CaCO3) (r of 0.15 and −0.06, p > 0.05), the high-energy sorptive surfaces of Fe oxides were more important than CaCO3 in P sorption of sediment under alkaline condition. At the same time, these poor correlations between CCE and K and Smax imply a non-linear relationship between P retention and the content of carbonate. The negative correlations of both K and Smax with pH (r of–0.73, and–0.58, p < 0.05) revealed that an increase in pH would not improve sediment retention capacity under alkaline conditions.