Retardation factors in controlling the transport of inorganic, organic, and particulate phosphorus in fluvo-aquic soil

Excessive application of fertilizers has caused a high load of phosphorus (P) in the North China Plain. The fate of P and its effects on aquatic ecosystems depend on its chemical speciation in soils. However, few studies systematically investigated the transport and retardation of different P species in the fluvo-aquic soil. In this study, the transport of inorganic P (orthophosphate, PO₄), organic P (phytic acid, PA) and particulate P (hydroxyapatite nanoparticles, nHAP) in the fluvo-aquic soil were investigated by column experiments, and their retardation from major soil components such as kaolin, CaCO₃, Al₂O₃, and goethite (GT) was also investigated by monitoring breakthrough curves and fitting transport models. The transport of P species in fluvo-aquic soil followed the order of PO₄ > PA > nHAP. A high fraction of increased clay and mineral particle-associated P (P-E) was observed for PO₄ and PA; while significant Ca-associated P (P-Ca) for nHAP. Under the experimental conditions, both CaCO₃ and GT were the most influential factors for PO₄, PA, and nHAP retention. Goethite strongly inhibited PO₄ transport due to its high PO₄ adsorption capacity, while CaCO₃ strongly inhibited PA transport due to its strong association with PA under alkaline conditions. Both CaCO₃ and GT can severely inhibit nHAP transport due to the favorable electrostatic conditions as well as the Ca²⁺ bridging effect. These results indicated that CaCO₃ played a key role in regulating the retention of organic P and particulate P in the calcareous soil, and also suggested the important role of Fe (hydr)oxides in controlling the transport of inorganic P, which could out-compete that of CaCO₃.