Phosphorus Stress Trees Secretion Of Organic Acids And The Activation Of Soil Phosphorus

In this paper, the kinds and amount of exudative organic acids from roots of 3 provenances of mason pine (Pinus massoniana ) and 2 provenances of Chinese fir (Cunninghamia lanceoata) were studied under P stress by the method of solution culture. The mechanism of phosphorus mobilization from acidic forest soils under Chinese fir and masson pine by low molecular weight (LMW) organic acid has also been studied with chemical extraction and soil inorganic phosphorus fractionation methods. The main results were as follows: Low P stress intensively stimulate mason seedling and fir seedling to add the release of Low molecular weight organic acids by root. Acetic acid, tartaric acid and malic acid were the major component of root exudates for mason species, Acetic acid and tartaric acid were the ones for fir species under P deficiency. And the lower of phosphorus supply in the medium, the more of LMW organic acids excreted from roots of Chinese firs and masson pines. The release of acidic forest soil phosphorus was significantly stimulated by organic acids for all of 3 soils tested. For three acidic soils, among citric acid, mixed acid, oxalic acid and tartaric acid that were studied, the ability of citric acid in mobilizing soil phosphorus was the highest, the following is oxalic acid or mixed acid, and tartaric acid is the lowest, which is in accordance with the order of logKAl of the organic acids. Moreover, to some concentration, with the increasing of organic acid concentration, the activation of organic acid become more available to mechaniz soil phosphorus. The ability that the organic acid activates forest soil phosphorus included action of H+ ionized from organic acid, action of organic anion and interaction of H+ and organic anion. Three actions improved phosphorus release from Ca-P, Al-P, Fe-P and adsorptive phosphorus on group minerals by acid-soluting, complexing and competing adsorption sites methods and so on. For yellow brown soil under Chinese fir, the efficiency of interaction of H+ and organic anion was higher than the other two action. For red soil under Chinese fir in Fujian, the efficiency of three actions was equivalent. Organic acids inhibited adsorption of acidic forest soil to phosphorus mainly by competing adsorption sites, competitive complexing etc. But for different soil, the ability of organic acid inhibiting phosphorus adsorption followed the order: yellow brown soil under Chinese fir> yellow brown soil under mason pine> red soil under Chinese fir in Fujian. For yellow brown soil and red soil under Chinese fir, the effect of humic acid inhibiting phosphorus adsorption was better than that of tartaric acid. But for yellow brown soil under masson pine, the effect of two organic acids was on the contrary to that for soil under Chinese fir.the ability of organic acids inhibiteding phosphorus adsorption is positively correlated with acids concentration. Organic acids had an obvious desorption to phosphorus which was absorbed by the acid soil, the style of desorption were mainly acid-soluting, ion exchange and co-ordination etc. For 3 soils, the ability that organic acid desorb adsorbed phosphorus followed the order: yellow brown soil under Chinese fir> yellow brown soil under mason pine> red soil under Chinese fir in Fujian. Comparing humic acid to tartaric acid, the former desorption was high for each of 3 soils. With the increasing of organic acid concentration, the desorption of humic acid increase first and then fall, the desorption of tartaric acid grew gradually. The effect of organic acids on differentiation of acidic forest soil inorganic phosphorus fraction was obvious, and the sequence of 3 organic acids affecting inorganic phosphorus fraction of 3 soils was oxalic acid>humic acid>tartaric acid. After treatment with the organic acids, the content of Fe-P and Ca-P decreased significantly, while the content of Al-P and O-P increased abvously. Organic acids stimulated the dessolution of soil sparingly soluble Phosphate hugely in order to advance availability of soil phosphorus.