Assessment risk of phosphorus leaching from calcareous soils using soil test phosphorus

Accurate estimation of phosphorus (P) leaching is important because excess P may reduce surface and ground water quality. Little attention has been paid to estimate P leaching from soil tests in calcareous soils. The relation between different soil tests P (STP), P sorption index (PSI) and degree of P saturation (DPS) and leaching of P were examined for assessing the risk of P loss from calcareous soils. Columns leaching repacked with native soils were leached with either distilled water or 10 mM CaCl₂ solutions, separately. Four leaching events were performed at four days, and 28.7 mm of distilled water or 10 mM CaCl₂ solutions was applied at each leaching events. Compared with distilled water, CaCl₂ had a small ability to solubilize P from soils. Concentration of P in leachate in both leaching solutions was exceeding 0.1 mg l^-1 associated with eutrophication. Cumulative P leached P was ranged from 0.17 to 18.59 mg P kg^-1 and 0.21-8.16 mg P kg^-1, when distilled water and 10 mM CaCl₂ solutions were applied, respectively and it was higher in sandy clay loam soils compared with clay soils. Among evaluated environmental soil P tests, P_CaCl2-3h (P extracted by 10 mM CaCl₂ for 3 h), P_CaCl2-1h (P extracted by 10 mM CaCl₂ for 1 h) were more accurate than other soil P tests for predicting P concentration in the leachates in both leaching solutions and accounting for 83% and 72% of variation of P concentration, respectively. The water extractable P (WEP) (r = 0.771) and Olsen-P (P_Ols)(r = 0.739) were significantly related to the leached P concentration using distilled water solution in a split line model, with a change point of 27.4 mg P kg^-1 and 61.5 mg P kg^-1, respectively. Various DPS were calculated and related to the leached P concentration. Based on P extracted by Mehlich-3 (P_M3) and HCl (P_HCl) and PSI, the change point of the relationship between leached P concentration and DPS_M3-3 (P_M3(P_M3+PSI)×100) and DPS_HCl-2 (P_HCl(P_HCl+PSI)×100) for both leaching solutions was approximately the same, thus a mean value of 49% for DPS_M3-3 and 73% for DPS_HCl-2 was obtained. Soils were grouped into four categories of increasing P leaching potential based on WEP, P_Ols, and DPS_M3-3. The results indicated that 8.00%-25.50% of the soil grouped in no risk category whereas 8.00%-13.70% of the soils fell into the high risk category.