Abstract
Application of phosphorus (P) fertilizers to P-deficient soils can also result in P accumulation. In this study, soil P status and P uptake by apple trees were investigated in 5-, 10-, and 15-year-old orchards in the semi-arid Loess Plateau, China, and subset soils with different soil P statuses (14-90 Olsen-P mg kg(-1)) were selected to evaluate the characteristic P adsorption. Due to the low P-use efficiency (4-6%), total soil P increased from 540 mg kg(-1) to 904 mg kg(-1), Olsen-P ranged from 3.4 mg kg(-1) to 30.7 mg kg(-1), and CaCl2-P increased from less than 0.1 mg kg(-1) to 0.66 mg kg(-1) under continuous P fertilization. The P sorption isotherms for each apple orchard were found to fit the Langmuir isotherm model (R2 = 0.91-0.98). K (binding energy) and Qm (P sorption maximum) decreased, whereas DPS (degree of phosphorus sorption) increased with increasing P concentration. CaCl2-P increased significantly with the increase of Olsen-P, especially above the change point of 46.1 mg kg(-1). Application of surplus P could result in P enrichment in P-deficient soil which has high P fixation capacity, thus posing a significant environmental risk.