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, PO), 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, CaCO, AlO, 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 PO > PA > nHAP. A high fraction of increased clay and mineral particle-associated P (P-E) was observed for PO and PA; while significant Ca-associated P (P-Ca) for nHAP. Under the experimental conditions, both CaCO and GT were the most influential factors for PO, PA, and nHAP retention. Goethite strongly inhibited PO transport due to its high PO adsorption capacity, while CaCO strongly inhibited PA transport due to its strong association with PA under alkaline conditions. Both CaCO and GT can severely inhibit nHAP transport due to the favorable electrostatic conditions as well as the Ca bridging effect. These results indicated that CaCO 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 CaCO.