EFFECT OF CITRIC ACID, GLUCOSE AND ORGANIC MATTER ON PLANT P UPTAKE AND SIIL P FRACTIONATIONS IN A HIGHLY WEATHERED ULTISOL
Han Xing-guo(Institute of Botany, Chinese Academy of Sciences, Nan Xin Cun 20, Beijing 100093)
The effect of citric acid and glucose on P uptake by different species of cultivated and wild plants, and P fractionations of the soil was investigated in an acidUltisol. Experiments were conducted in the greenhouse, using continuous additionof citric acid and glucose solutions, and a complete budget study designed to examine the changes of soil phosphorus in various fractions employing a P fractionation procedure. Without inorganic P additions, citric acid increased P uptake bysoybean (Glycine max (L.) Merr.) and sorghum (Sorghum bicolor), indicating that citric acid solubilized the P bound by Al and Fe oxides. The addition of P (20 mg P kg-1 soil)in combination of a continuous addition of eitric acid solution resulted in higher P uptake by soybean and sorghum. The addition of glucose, with and without addition of P,did not change the P uptake by plants. However, the addition of glucose increased tEe organic P in NaOH fraction, and a decrease in inorganic NaOH fractions.A negative microbial P was obtained for all the treatments, especially when glucose was added. This suggests that the method described by Hedley is not suitable for determination of microbial P in very acid soils having little organic matter content. The increase of P uptake by sorghum and a decrease of P in the NaOH fractions in the soil with the addition of inorganic P and citric acid suggests that organic ligands can prevent P from being fixed by soil particles.The addition of oak (Quercus spp.) and red clover (Trifolium pratense) litter to a similar acid soil resulted in a higher P uptake by pokeweed (Phytolacca americana). After a period of 7 months, the resin-P and inorganic bicarbonate P decreased substantially compared with the P at the beginning of experiment. This can be attributable to the P uptake by plant, P taken up by microbial organisms, an increase in organic P, and an increase in HCl fraction. The results also showed that both clover and oak litter did not increase the inorganic P in NaOH fractions.This indieates that products of organic matter decomposition might have prevented the mineralized P from being immobilized by clay particles and Fe- and Al-oxides.An attempt was also made to elucidate the mechanisms involved in the P solubilization by the products of organic matter decomposition, and the resuction of P adsorption by Al- and Fe-compounds in the presence of organic ligands.
【CateGory Index】： Q948.113