Organic Acids Affecting On Phosphorous Release Of Dark Brown Forest Soil

Dark Brown Forest Soil, which is a maximum zonal soil in area, was acted as studying subject by way of extractions (one time and many times repeated sequential extraction) and leaching. The effect of insoluble phosphorus nutrient caused by oxalic acid and citric acid, high contents eluted from forest litters, was conducted when they were used as extracting reagents. It was found that organic acid anions of different concentrations had different effects on phosphorus determination by coloration reactions in some degree. So P coloration time sequences about organic acids' kind and concentrations were studied in different coloration time, at the same time, the effect of soil coloration method and water coloration one on phosphorus release was compared.The results as following:(1) The effect on phosphorus coloration was growing with the increasing concentration of organic acid anion added. In detail, when the concentration of the organic acid reached the number(citric acid 7.2mmol/L and oxalic acid 3.2mmol/L), they had strong impacts on phosphorous standard solutions coloration. And exceeding the concentrations above, prolonging P coloration time is ineffective; within the range of lower concentrations (citric acid<0.6mmol/L, oxalic acid <0.4mmol/L), they have no effect on phosphorous coloration; between the two number concentrations, prolonging coloration time may reduce availability of coloration. A horizon soil was extracted by citric acid, oxalic acid and corresponding citrate and oxalate solutions, and P was determined respectively with water and soil coloration methods, then the coloration time sequences were done. Two methods of the determining P had certain differences. In a word, the water coloration was better than the soil coloration. And 10mmol/L organic acid and corresponding salt solutions had the most conformable extracting effect, so they were the best extracting reagents; Ranging between 0 and 10mmol/L, soil P release number was rising with the increasing of the extracting reagents' concentration. When 20mmol/L organic acid and corresponding salt solutions were used, the coloration reaction didn't take place at first, however after a period of time coloration appeared and gradually rose with the prolonging coloration time.(2) Drawing a conclusion from citric acid and oxalic acid extracting soil experiments: the effect of citric acid encouraging phosphorus release was better than oxalic acid, that is to say citric acid advancing phosphorus release was more obviously than oxalic acid. This was closely connected with the chemical structure of the organic acid, citric acid is tri-acid and could form five number ring's or six number ring's stable structure with some metals, but oxalic acid is binary acid. By comparison, citric acid had stronger complexation than oxalic acid, and citrate had better effect on P release than oxalate.(3) Both repeated sequential extraction procedures and leaching procedures could fully show that, considerable P could be released from Dark Brown Forest Soil while extracted by organic acids.