Dissolution And Transformation Characteristics Of Phosphate Rock And Its Bioavailability In Paddy Soils

Since the second national soil survey of China in early 1980's, P deficiency was paid high attention nationwide. The successive P fertilizer application has resulted in P accumulation in most paddy soils. Application of water-soluble phosphate on soil with high P status may result in the loss of P by leaching and surface runoff, and increasing the risk of eutrophication of waterbodies. For the development of sustainable, environmental-friendly agricultural production systems, the substitution of one slow-release PR for water soluble fertilizer in areas where P is required for maintenance situations need more major attention in research. Accordingly, laboratory incubation experiments and greenhouse pot experiments were conducted to investigate the physical-chemical processes of PR dissolution, and transformation of the dissolved-P in paddy soil, special emphasis was on the changes of soil P fractions with PR application, and the effects of P availability on plant growth and soil microbial community composition. The main conclusions obtained are as follows:(1) Influence of flooding time on characteristics of phosphate adsorption and desorption was investigated in three paddy soils with different P status under the constant temperature. The results showed that all phosphate adsorption isotherms could be well described by Langmuir equation (p < 0.01). From 7 day to 28 day, the P sorption capacity (Xm), maximum buffering capacity (MBC) and standard P requirement (SPR) of three paddy soils increased with the flooding time extended. At the same incubation time, Xm, MBC and SPR on soil NK and CK both with low P status were significantly higher than that on soil NPK with high P status. The resin capsule was used in this experiment to desorb P adsorbed on soil. It was found that amounts of P desorbed were significantly positively correlated with amounts of P adsorbed on three soils at all the incubation time. The average desorption rate (ADR) of three soils decreased with the flooding time extended. Under the same incubation time, ADR of soil NPK was significantly higher than that of soil NK and CK.(2) Dissolution and availability characteristics of two phosphate rocks namely Kaiyang phosphate rock (GPR) and Wucun phosphate rock (WPR) were investigated by incubation in the three flooded acid paddy soils described above. Results showed that the amounts of dissolution of phosphate rocks (PRs) in soils increased with the flooding time extended. More PRs dissolved in soil NK and CK than in soil NPK. More P of WPR dissolved in three soils than that of GPR. Soil available P increased following addition of PRs. SoilΔResin-P increased with the flooding time extended after WPR applied, and decreased with the flooding time extended after GPR applied. Compared with GPR, WPR can be more effective in increasing soil available P.(3) Two P fertilizer of varying solubility, namely, WPR and mono-calciumphosphate (MCP), were incubated with twelve flooded paddy soils to quantify factorsaffecting the dissolution and availability of PR. The amounts of WPR dissolution wassignificantly negatively correlated with soil pH (p < 0.01), exchangeable calcium andsoil silt content (p < 0.05), significantly positively correlated with soil clay content (p <0.05). Stepwise multiple regression indicated that soil acidity measured by KC1extractable pH was the soil property that most affected PR dissolution. The increase ofsoil available P (ΔResin-P) after WPR and MCP application were both significantlypositively correlated with soil Olsen-P (p < 0.01). Percentage ofΔResin-P byΔNaOH-Pafter MCP application was significantly positively correlated with soil Olsen-P.Percentage ofΔResin-P byΔNaOH-P after WPR application was significantlypositively correlated with soil Olsen-P (p < 0.01), negatively correlated with soil claycontent and Xm (p < 0.01). Stepwise multiple regression indicated that soil Olsen-P andXm were the soil properties that mainly affected percentage ofΔResin-P byΔNaOH-Pafter WPR application.(4) Pot experiments on three acid paddy soils with different P status were conducted to investigate the effects of P fertilizers on ryegrass growth and changes of soil P fractions after ryegrass planting. The results show that application of P fertilizers increased dry matter weight and P uptake of ryegrass on low-P status soils (NK and CK) by 32.7%-53.0% and 82.4%-151.9%, respectively; on high-P status soil (NPK) by 11.0%-14.0°/o and 51.1%-65.0%, respectively. Dry matter weight and P uptake of ryegrass on NPK soil were higher than that on NK and CK soil under the same fertilizer treatment. Average agronomic efficiency of P fertilizer (AEP) for KPR500, KPR1000 and SSP on three soils were 5.6, 2.5 and 124.2, respectively; average recovery efficiency of P fertilizer (REP) for KPR500, KPR₁₀₀₀ and SSP on three soils were 1.0, 0.7 and 21.1, respectively. Compared with the control (no P application), soil available P (Resin-P, NaHCO₃-Pi and NaHCO₃-Po) increased at different grade on the three soils after P fertilizers application. Soil NaOH-Pi also increased, while soil NaOH-Po was stable and less affected by addition of P fertilizers. Phosphate rock application increased soil HCl-P and Residual-P fractions substantially.(5) A pot experiment was conducted in two acid soils (pH 4.8) with similar properties except phosphorus (P) status to investigate the influence of different P fertilizer application [Kunyang phosphate rock (KPR), monocalcium phosphate (MCP)] on ryegrass growth and dissolution characteristics of phosphate rock in the two soils. Results showed that the application of P fertilizers produced significant increase of ryegrass growth compared with no P application on soil with low P status. Dry weight of ryegrass did not significantly increased after P application compared with no P application on soil with high status. Inorganic Resin-P, NaHCO₃-P and NaOH-P were significantly increased after MCP treatment compared with no P application. PR application only significantly increased Resin-P and HCl-P. Soil organic P fractions were stable and less affected by P application. After estimated as the methods of Tambunan et al., dissolution of PR on soil with high P status was slightly higher than soil with low P status under the same rate of phosphate rock application. Ryegrass growth was an important contributor to dissolution of PR on soil with high P status.(6) Phospholipid fatty acid (PLFA) analysis was used to investigate response of soil microbial community to different P fertilizer application and annual ryegrass. Results showed that ryegrass planting induced the symbiosis of vesicular arbuscular mycorrhizae indicated by PLFA 20:4. The total PLFA and bacterial PLFA were significantly higher in soil with KPR₂₅₀ and MCP₅₀ treatments compared to KPR₅₀ and control which were not statistically significant different from each other. The KPR₅₀, KPR₂₅₀, and MCP₅₀ treatments increased fungi PLFA by 72%, 106% and 71%, respectively. Both the principal components analysis and the cluster analysis of the PLFA data suggested that P treatments alter soil microbial community composition. P availability may be an important contributor to the soil microbial activity and community diversity.