| Phosphorus (P) is one of the necessary elements for the plant, which is mainly from the soil. Phosphate fertilizer is often applied to meet the gap between the need of plant and the presence in soil. While those applied P was not completely utilized, a large portion of them was fixed into the bulk of P in soil. Too much of P in soil could be lost with the runoff in the farmyard, which could lead to the eutrophication. So trying to improve the utilizing efficiency of P in soil has become a hot topic. In this paper, how the ryegrass, after the low molecular weight organic ligands was applied into soil, and the plants, which could secrete the low molecular weight organic acids (LMWOA) under the condition of P-deficiency to utilize the different fractions of inorganic P (Pi) in soil was investigated, and the effect of drying course on the measurement of Pi was also explored.The whole thesis was separated into three parts: (1) Studying how the low molecular weight organic ligands to mobilize Pi in soil. Four low molecular weight organic ligands were added into soil where ryegrass was planted to investigate the utilization of P in soil. (2) Exploring the utilization of different fraction of P by different plants which could secrete different LMWOA under the condition of P-deficiency. (3) Investigating the effect of drying course on the measurement of Pi.(1) The contribution of different fractions of Pi in various soils to the plant was different. For the yellow brown soil, Al-P and Fe-P were the main forms. For the paddy soil, Fe-P and O-P were the main ones. Different low molecular weight organic ligands had the different mobilizing-capability for the same form of Pi in the same soil. In the yellow brown soil, citrate, tratrate and malate mainly mobilized Al-P, Oxalate mainly mobilized Ca-P. hi the paddy soil, oxalate and tratrate mainly mobilized Fe-P. Organic ligands increased the release of the total Pi, but the changes in different fractions were not the same, some increased, some decreased.(2) Pi was the main source for the plant and organic P (Po) also played animportant role. Pi was the main form absorbed by the plants in the yellow brown soil; In the paddy soil, the situation also hold the same true except in the soil of planting the buckwheat. In the red soil with the low level of P, Po was the main form for the uptake of plant.During the growing period, Pi and Po could change into each other. In the yellow-brown soil, part of Pi changed into Po in all treatments except in the Treatment V. In the soil of planting ryegrass, the increase of Po amounted nearly to 50% of Pi. The direction of change between Pi and Po had the relation with the different plants in the paddy soil. Po in the red soil decreased, while the content of Pi increased.The rates between the release of different Pi fractions to the ones of total Pi in one certain soil were also various. Gemerally speaking, the order of such rates in the yellow brown soil was Al-P > Fe-P > Ca-P, O-P; The one in the paddy soil was Al-P > Fe-P, Ca-P > O-P. but the orders in different treatments were not definitely the same.The rates between the release of the same form of Pi to the total one of Pi in different soils were also various. The rates of Al-P in the yellow brown soil and the paddy soil were high, while the rates of Fe-P in both of the soils differed widely. The rate of Fe-P in the yellow brown soil with the white lupin was the highest, while that in the paddy soil was the lowest. Such rates might have the relation with the character of soil and the plants. Most of the rates of O-P were comparatively lower. The rates of Ca-P in the brown soil were less than 10%; the one in the brown soil with white lupin was 91.47%, the one in the treatment of amaranth was 22.41%, the rates of other treatments were within this range.(3) The content of different fractions of Pi in soil changed during the drying course. The content of Al-P decreased, while the ones of other forms of Pi increased in the yellow soil. The increase rate of Ca-P (60.09%...
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