Effect of a novel constructed drainage ditch on the phosphorus sorption capacity of ditch soils in an agricultural headwater catchment in subtropical central China

Plant vegetation has been reported to be one of the best management practices to control the transport of nutrient pollutants in agricultural drainage. In this study, soil samples were collected from a novel constructed drainage ditch (ND) vegetated with strips of Indica canna, Hydrocotyle vulgaris, Sparganium stoloniferum, Myriophyllum aquaticum, and Juncus effusus, from an upstream ordinary agricultural drainage ditch (OD), and from an adjacent paddy field (PD), at the 0-5 and 5-15 cm layers, to compare phosphorus (P) sorption capacity. In the ND, there was a great difference in the properties of ditch soils between at the 0-5 cm and 5-15 cm layers. For instance, total organic carbon (TOC) at the 0-5 cm layer increased rapidly with the growth of Indica canna, Hydrocotyle vulgaris, and Myriophyllum aquaticum in the ND for approximately 3 years. Correspondingly, those three plants elevated the P sorption maximums (Smax) of the ditch soil, which were determined to be 521, 612, and 552 mg kg−1, respectively. The equilibrium phosphorus concentration (EPC0) and the degree of P saturation (DSP) of the ditch soils in the ND were also shown to be lower than those in the OD, which may be attributed to efficient ditch clearance in the ND. The Pearson's correlation analysis revealed significant correlations of TOC with the key parameters of the P sorption isothermal curves, such as the Freundlich adsorption constant (KF), Smax, EPC0, and DSP. Furthermore, the multivariate regression analysis showed that TOC explained more than 50% of the variability of Smax and DPS, suggesting that the vegetated drainage ditch played an important role in regulating the P adsorption capacity of the ditch soils. Our collective results indicated that plant vegetation and ditch clearance can be optimized to enhance the P retention capacity of ditch soils via robust and re-vegetated drainage ditch systems, such as the ND, and have a great potential to reduce the P loss from agricultural headwater catchments.