Dissolution And Transformation Characteristics Of Phosphate Rock In Latosols And Rubber Tree Seedling Rhizosphere

Many studies have suggested that phosphate rock (PR) could be as effective as water soluble phosphate fertilizer in increasing plant yield and improving soil P status in many tropic regions. Water soluble P is the mainly P fertilizer applied for improving nutrition status of rubber tree, the rational use of PR has not gotten enough attention at present. The application of PR has a good prospect in crops of tropical regions particularly in plantation crops and organic agriculture in China. Accordingly, the objectives of this study were to investigate inorganic and organic P fractions of fifteen Latosols with different parent materials in rubber plantations of Hainan Province by using Hedley et al’s method, and dissolution and transformation characteristics of phosphate rock in latosols and rubber tree rhizosphere. To achieve these objectives, laboratory incubation experiments and greenhouse pot experiments were conducted. Results of this study can provide useful assistance for the rational application of PR on crops of tropical regions in China. The main conclusions obtained are as follows:1. Hedley’s sequential extraction method was used to investigate inorganic and organic P fractions of fifteen Latosols with different parent materials in rubber plantations of Hainan Province. Results showed that:there were no significant difference among Resin-P of Latosols, NaHCO3-Pi of Latosols, NaOH-Pi of Latosols, HCl-P of Latosols developed from sand shale, granite, gneiss, neritic sediment and basalt. All soils studied were dominated by Residual-P, Residual-P and percentage of Residual-P/total P of Latosols developed from basalt were significant higher than that developed from the other four parent materials (p<0.05). Total organic P content of soils developed from neritic sediment were significant lower than that developed from the other four parent materials (p<0.05), organic phosphorus in all soils studied represented an average of14.67%of the total soil P. Except soils from neritic sediment, soil organic P fraction of the soils from sand shale, granite, gneiss and basalt were mainly occurred in NaOH-Po, which represented an average of83.68%of total soil organic P. Resin-P, NaHC03-Pi, NaOH-Pi, HC1-P were positive correlated with each other (p<0.05), which were all positive correlated with soil Bray2-P (p<0.05). NaHCO3-Po was positive correlated with NaOH-Po (p<0.01), NaHCO3-Po and soil total organic P were both positive correlated soil organic matter (p<0.05).2. Dissolution and availability characteristics of two PRs namely Kunyang phosphate rock (KPR) sampled from Yunnan Province and Wucun phosphate rock (WPR) sampled from Jiangxi Province were investigated by laboratory incubation in ten Latosols with different parent materials under rubber plantations. PR was added to obtain rates of0mg(P)/kg (control) and500mg(P)/kg soil. The soil and PR mixture was placed in plastic container and incubated with a moisture level near60%of the soil water-holding capacity at25℃for35days. Results showed that the dissolution of two PRs in all soils increased with the incubation extended. Dissolution kinetics of two PRs in ten Latosols were best described by Elovich equation, followed by Langmuir equation, Mitscherlich equation was not suitable for describing dissolution of PRs in Latosols. Average potential maximum dissolution of two PRs on Latosols developed from basalt was2.16,1.73,2.49and2.39times of Latosols developed from sand shale, granite, gneiss and neritic sediment, respectively. Maximum dissolution of two PRs on Latosols were positive correlated with soil organic matter, soil hydrolytic total acidity, soil free iron oxide content and soil cation exchange capacity (CEC). Dissolution rate of two PRs on Latosols were positive correlated with soil hydrolytic total acidity, soil free iron oxide content and soil CEC. Stepwise multiple regression indicated that soil free iron oxide content was the soil factor that most controlled maximum dissolution of two PRs, soil adsorption parameter K and CEC were the soil factors that most affected dissolution rate of KPR and WPR on Latosols, respectively. Soil available P increased after PRs application, but the increment of soil available P was different for all soils and sampling periods throughout the incubation. This study indicated that better effect could be obtained when PR is applied first on Latosols developed from basalt; compared with KPR, WPR with low grade may be same effective in increasing soil available P on Latosols.3. A pot experiment was conducted on latosols developed from basalt to investigate the influence of different P fertilizer application (KPR and superphosphate) on rubber tree seedlings phosphorus content and characteristics of phosphate rock in latosols. The P treatments were KPR at0mg P/kg (Control),100mg P/kg (PR100), and250mg P/kg (PR250) and superphosphate at100mg P/kg (SSP100). Results showed that rhizosphere NaHCO3-Po content of Control and SSP100were significant higher than its rhizosphere. There were no significant difference between each P fraction of rhizosphere and non-rhizosphere under PR application. Compared with Control, PR250application significant increased soil available P (sum of Resin-P, NaHCO3-Pi and NaHCO3-Po). Compared with SSP100, the effect of PR application on improving P status of rubber tree seedlings was weak. PR application also significantly increased HCl-P and Residual-P. The significant increase of Residual-P after PR application may due to phosphate rock was coated by iron, aluminum and manganese oxides in latosols developed from basalt, which can not extracted by1mol/L HCl solution.