Effects Of Long-term Fertilization On Releasing Characteristics And Fractions Of Phosphorus From Greenhouse Soils

Long-term application of phosphorus with animal manure or fertilizer in amounts exceeding removal with crops lead to buildup of phosphorus in greenhouse soils, and increased risks of phosphorus losses to ground and surface water where it could seriously degrade water quality through the stimulation of eutrophication. The objective of this research was to investigate the effects of long-term fertilization on releasing characteristics and fractions of phosphorus from greenhouse soils, and to find out affecting factors and mechanisms for phosphorus release. The results are summarized as follows:1. The relative amounts of phosphorus fractions in total phosphorus (TP) from long-term fertilized greenhouse soils were in the order of HCl-P > Residual-P > NaOH-P_i/NaHCO₃-P_i> H₂O-P > Organic phosphorus (NaOH-P_o/NaHCO₃-P_o)。H₂O-P, NaHCO₃-P_i, NaOH-P_i and NaHCO₃-P_o showed significantly positive correlations with olsen-P, while NaOH-P_o and Residual-P showed significantly negative correlations with olsen-P. Inorganic phosphorus, especially NaHCO₃-P_i, NaOH-P_i and HCl-P, played more important role in plant-available phosphorus than organic phosphorus. Phosphorus adsorption isotherm was successfully described by Langmuir and Freundlich equations, and Freundlich equation was better than Langmuir equation.2. The equations of Elovich, power function and parabolic diffusion were successfully used to describe the kinetics of phosphorus release from greenhouse soils receiving long-term fertilization, the most appropriate of which was Elovich equation followed by power function equation. The application of manure and fertilizer increased releasing amounts and percentages of phosphorus from greenhouse soils, and increased releasing rate parameters such as "α, DR_in, DR_f "of Elovich equation and "a" of power function equation while it decreased "β" of Elovich equation.3. The organic matter (OM), cation exchange capacity (CEC), total phosphorus and olsen-P showed significantly positive relationships with releasing amounts as well as with releasing rate parameters of phosphorus from long-term fertilized greenhouse soils. However, pH value, clay content, available Fe and Al showed insignificantly positive relationships with them. Furthermore, H₂O-P, NaHCO₃-P_i, NaHCO₃-P_o and NaOH-P_i showed significantly positive relationships with releasing amounts as well as with releasing rate parameters of phosphorus from long-term fertilized greenhouse soils, while NaOH-P_o and Residual-P showed significantly negative relationships with them.4. Organic acids promoted phosphorus release from greenhouse soils receiving long-term fertilization, and citric acid induced more amount of phosphorus release than malic acid did. The amounts and percentages of phosphorus released from greenhouse soils were increased with increasing additions of organic acids. In generally, phosphorus release from greenhouse soils receiving long-term fertilization was rapid at first few hours (â… step) and more slowly (â…¡step) until an apprarent equilibrium was approached (â…¢step) after organic acids addiction.5. Greatest phosphorus was released from NaOH-P_i among phosphorus fractions, and increased with increasing additions of organic acids. When citric acid and malic acid were at high concentrations (≥0.5 mmol L^-1), the releases of phosphorus fractions was in the order of Fe-oxide- and Al-oxide- associated (NaOH-P_i) > Ca-associated (HCl-P) > plant-available inorganic P (NaHCO₃-P_i) > H₂O-P. Fe-oxide- and Al-oxide- associated inorganic P (NaOH-P_i) played an important role in phosphorus release from greenhouse soils receiving long-term fertilization.6. Organic acids induced phosphorus release and Fe dissolution from different iron oxides, and phosphorus could be readsorbed to iron oxides.7. Six organic acids (citric, malic, malonic, oxalic, succinic, and tartaric) promoted phosphorus mobilization from iron oxides in varying degree, and greater phosphorus were released from ferrihytrite than from goethite. There was a consistent trend across both oxides and all organic ligands that phosphorus release increased with decreasing pH. Dissolution was the most important mechanism for phosphorus release from iron oxides.