Effect Of Organic Acids On Black Soil Phosphorus Fractionations And Phosphorus Availability

The effect of organic acids on soil phosphorus fractionations and phosphorus availability was study in typical black soil region. Firstly, the distribution characteristics of soil phosphorus fractionations in the soil depth of 0-190cm of soils from natural secondary forest without reclamation interference were studied by the method of Sui in typical black soil region. Then, the adsorption capacity of low molecular weight organic acids was studied by the analysis of sorption properties of different organic acids. On the basis of those studies, the effect of organic acids on soil inorganic phosphorus fractionations was studied through the add of low molecular weight organic acids different in types, concentrations and pH, and high molecular weight organic acids of humic acid different in types, concentrations by the method of Sui and Zhang shou jing’s sequential extraction technique. We also investigated the phosphorus sorption and desorption properties after humic acid added by measuring, calculating and analyzing the maximum adsorption capacity, phosphorus sorption index and desorption rate and so on.The results showed that: soil phosphorus content of NaOH-Po was the largest proportion, accounted for 50.78% of total phosphorus soil, while the labile phosphorus contents and ratio to the total P of water soluble phosphorus were lower than other P fractionations in soil profiles; In the vertical space, the contents of various forms of phosphorus in the surface layer (0-10 cm depth) were highest. Except NaOH-Po, other soil phosphorus fractionations showed the trend of decreased at depths of 0 to 70cm, then increased, and finally decreased along the soil profile. Correlation analysis and path analysis showed that NaHCO3-Pi, NaOH-Pi and NaOH-Po are more contributive forms to available phosphorus.The content and rate of P release in the studied soils was initially rapid before 480h, followed by a slower release that lasted up to 2160 h. P fractions were all increased significantly with amended P. Phosphorus in control and amended soils were fractionated by the methods described of Sui and Jiangbofan’s sequential extraction technique before and after 2160h incubation. NaHCO3-Pi were larger than other inorganic fractionations in the proportions of inorganic phosphorus changes amounts within 24h by sequential extraction technique of Sui, which the proportions of NaHCO3-Pi and NaOH-Pi were 33.72% and 24.87% after 2160h respectively. Ca2-P was larger than other inorganic fractionations in the proportions of inorganic phosphorus changes amounts within 24h by the Jiangbofan-Guyichu’s sequential extraction technique, which the proportions of Al-P and Fe-P were 30.70% and 20.66% after 2160h, respectively. But the contents of Res-P and O-P changed to a lesser extent. This indicates that the water-soluble fertilizer transformed into unstable component relatively in the soil, retentioning in the moderately labile inorganic phosphorus fractionations.The adsorption capacity of low molecular weight organic acids (oxalic acid, citric acid and tartaric acid) increased with the initial concentration of low molecular weight organic acids increasing within initial organic acids concentration the range of 1-20mmol/L in the black soil, and the adsorption amount was different with the different type and initial organic acids concentration of organic acids. The maximum adsorption amount of citric acid by Langmuir equation calculated was largest, while tartaric acid lowest. The maximum adsorption ratio of oxalic, citric and malic acids could reach 57.15% ,29.26% and 8.42%, respectively. Under the same conditions of the initial concentration, the adsorption ratio of oxalic was higher than citric within initial organic acids concentration the range of 1-10 mmol/L, and that of citric was higher when the initial organic acids concentration was 20 mmol/L, While the adsorption ratio of tartaric acid was lowest. The adsorption capacity of oxalic acid increased with pH value rising when the initial concentration of oxalic acid was 1 mmol/L, and then showed increased firstly, and then decreased with the further increase of initial concentrations.The release rate of phosphorus from soil was increased with the increase additions of oxalic acid and citric acid, When their concentrations were≤1.0 mmol/L, oxalic acid was lower than citric acid, and when≥ 1.5mmol/L, oxalic acid was higher in the capability (p<0.05). Low-molecular-weight organic acids could promote the release of NaHCO3-Pi and NaOH-P; by the methods described of Sui, but when their concentrations were≥ 1.0mmol/L, soil HCl-Pi was released. Al-P, Fe-P, and Ca-P were released by Zhang shoujing et al. The P-activating capability of the organic acids declined with rising pH. It is suggested that P release from phosphates in soils induced by organic acid should be attributed to the joint efect of acidity effect of proton and complex effect of organic anion. Path analysis of inorganic phosphorus fractions to soil phosphorus extracted by organic acids showed that NaHCO3-Pi, NaOH-P; and HCl-Pi by Sui and Al-P by Zhang shoujing et al are more contributive forms to phosphorus activating.The content of available phosphorus showed increasing trend in both fertilization and unfertilized soil phosphorus with the increase of humic acid content, which 20.0% of humic acid added were increased by 1.14 and 2.20 times. The analysis of Sui found that both NaOH-Pi and HCl-Pi expressed as a growing trend and Res-P in contrast. Zhang shou j ing’s phosphorus fractionations showed that the content of Al-P showed increasing trend with the increase of humic acid content in both fertilization and unfertilized soil phosphorus, and the content of Ca-P showed an increase or no significant difference, but the content of O-P reduced with increasing humic acid gradually. This indicated that humic acid can improve the utilization of fertilizer to some extent, and promote the transformation of insoluble to soluble inorganic phosphorus forms.Langmuir equations can be used to simulate soil P adsorption isotherms of different humic acid content at the significant levels(p<0.05).The maximum P adsorption capacity (Xm) increased with increasing humic acid content, but constant of P bonding energy (K) and maximum buffering capacities (MBC) increasing first and the decreased trend. While the humic acid content of 11.2% the lowest. When soil humic acid content was 11.2%, both the maximum P desorption capacity (Dm) and desorption rate (Dr) were the lowest, showing higher soil P bioavailability. To coordinate soil phosphorus nutritions controlled by changing the content of humic acid scientifically and rationally had important theoretical and practical significance for the rational use of soil fertility and utilized of phosphate fertilizer for farmland.Soil inorganic phosphorus fractionations release from black soil with different organic content changed with humic acid before and after the extraction of oxalic acid, citric acid and and malic acid. Results showed that the total inorganic phosphorus release decreased with the increase content of humic acid after the extraction of organic acids. The soil inorganic phosphorus release amounts of NaHCO3-Pi, NaOH-Pi, HCl-Pi decreased with the increase content of humic acid after the extraction of organic acids following the methods described by Sui, while the release of Al-P, Fe-P, Ca-P from Zhang shou jing’s inorganic phosphorus fractionations decreased too, except for 9.96% of organic matters. For the same kind soil of humic acid added, the ability to activate of oxalic acid was the highest, and malic acid was the lowest than both oxalic acid and citric acid extraordinarily, regardless of NaHCO3-Pi, NaOH-P; and HCl-Pi from Sui and Al-P, Fe-P, Ca-P from Zhang shou jing’s inorganic phosphorus fractionations. The contents of soil Res-P and O-P were lower after the extraction of organic acid, but it was no different among soils of different organic matter contents.