| The Three Gorges Reservoir is a super large reservoir, and the water environment security is a major environmental issue at home and abroad. Moreover, the water eutrophication is a major threat to the security of water environment in the reservoir area. Phosphorus is the main limiting factor of eutrophication, the increase of phosphorus concentration in the water body directly increases the risk of eutrophication. In the Three Gorges Reservoir area, the soil of the fluctuating zone has a cyclical characteristic of dry-wet alternation, and the cyclical characteristic will have an important effects on the soil phosphorus activity and release properties. During the dry period, the rainfall is concentrated, agricultural runoff flows from neighboring small watershed fluctuation band, thus, the particulate phosphorus entrained may be deposited on the soil of hydro-fluctuation belt, and the soluble phosphorus can be absorbed by the soil. At the same time, the seasonal utilization of the hydro-fluctuation belt will also increase the soil phosphorus load. During the flooding, with the drastic changes of the property of fluctuating soil, phosphorus accumulated in the soil can be released into the overlying water and increase the concentration of phosphorus, therefore, the soil of hydro-fluctuation belt becomes an important source of phosphorus to water. It is very important to study the activity change, release rule and effect factors of the exogenous phosphorus in the reservoir area for preventing the eutrophication of the water body. Therefore, the object of the experiment are purple alluvial and gray-brown purple soil which are widely distributed in Three Gorges Reservoir area. The indoor cultivation experiment is to explore the characteristics of the activity and the morphological transformation of phosphorus with different phosphorus saturation in the soils mentioned above. Influences of montmorillonite, kaolin, humic acid, fulvic acid and goethite(collectively referred to as activity modulators) on phosphorus forms, activivity and release were analyzed. The main results are as follows:1. Phosphorus activity(Olsen-P) was mainly affected by the amount of exogenous phosphorus, the effect of the application time and the type of soil. After adding 0-100% sorption saturation(0-100% Qm) of exogenous phosphorus, characterization of soil phosphorus activity Olsen-P content(Ct) decay exponentially, the available equation Ct=ae-kt+b was fitted, and the fitting degree was about 94%. Olsen-P content in cultured 15d are the largest decline, then gradually leveled off and remained stable, the final equilibrium concentration(b) increased with the increase of the phosphorus. Affected by soil type, Olsen-P decline rate was faster in purple alluvial, and the Olsen-P content of purple soil in the same phosphorus saturation was lower than that of gray-brown purple soil. The Olsen-P content of soils with exogenous phosphorus and active control agent was mainly affected by the content of exogenous phosphorus, the nature of the active control agent and the type of soil. Under the conditions of 50% Qm exogenous phosphorus(purple soil is 500 mg/kg, 454.5 mg/kg), in compared with only adding exogenous phosphorus soil, montmorillonite and kaolin in two kinds of soil would reduce soil Olsen-P content, the effect of passivation of montmorillonite on soil P activity was more obvious than the kaolin, and the decline in purple alluvial is larger than that in gray-brown purple soil. In purple alluvial, the content of Olsen-P was decreased from 18.75% to 14.14%, 17.88% by montmorillonite and kaolin, while the content of gray-brown purple soil was decreased from 40.9% to 36.65%(montmorillonite), 39.39%( kaolin) in the gray-brown purple soil. Humic acid, fulvic acid and goethite can increase the activity of soil phosphorus, the order of facilitation was: goethite > fulvic acid > humic acid. In purple alluvial, the proportions of Olsen-P content were increased to 21.12%, 28.21%, 92.03% by humic acid, fulvic acid and goethite, respectively. In the gray-brown purple soil were increased to 49.39%, 55.55%, 82.62%.2. Soil phosphorus release capacity was influenced by soil phosphorus saturation, culture time and soil type after the exogenous phosphorus into the soil. The release ability of soil was exponential decay, and the equation was fitted by Ct=ae-kt+b equation. The rate of phosphorus release in purple alluvial was faster, and the phosphorus release ability of the gray-brown purple soil was higher. Soil phosphorus activity and release capacity were restricted by phosphorus absorption saturation, when the phosphorus sorption saturation was greater than or equal to 50%, the Olsen-P content balance and phosphorus equilibrium desorption of purple alluvial and gray-brown purple soil increased significantly,50%Qm is the change-point of P activity and leaching risk in purple alluvial and gray-brown purple soil. Soil Olsen-P content and single extraction was a significant linear relationship(y = a+bx, P <0.01), and Olsen-P can be used to predict the leaching and release risk of soil phosphorus. In the experiment of continuous extraction and release of phosphorus from soil, after the first extraction, the amount of single extraction of soil with 25%Qm exogenous phosphorus accounted for respectively 30.7%(purple alluvial) and 41.3%(gray-brown purple soil). The single extraction of P from purple alluvial and gray-brown purple soil with more than 50%Qm exogenous phosphorus accounted for 46.6%-57.5% and 50.4%-55.9% of the exogenous phosphorus amount. After continuous extraction by five times, most of the phosphorus in the soil is released, the accumulated extraction of p from purple alluvial and graybrown purple soil accounted for 73%-81.3% and 85.9%-93.8% of the exogenous phosphorus amount. After continuous extraction of 11 times, after addition of 25%-100% Qm exogenous phosphorus, the maximum extractable amount(maximum release amount) accounted for 88.4%-95.3%(purple alluvial) and 95.3%-116.6%(gray-brown purple soil) of exogenous phosphorus. The exogenous phosphorus was completely released from the gray-brown purple soil with less than 50%Qm exogenous phosphorus, and the original part of phosphorus in the soil and some who had been also released. The effect of active agents on the phosphorus release ability of the soil varies with the different types of soil. Under the conditions of 50% Qm initial phosphorus(purple alluvial: 500 mg/kg, gray-brown purple soil: 454.5 mg/kg), in purple alluvial and graybrown purple soil without activity control agent, the proportions of single extraction amount of phosphorus in purple alluvial and gray-brown purple soil accounted for 7.3% and 25.49% of exogenous phosphorus content. Montmorillonite inhibited the release of phosphorus, the proportion of phosphorus release from purple alluvial treated by montmorillonite decreased to 4.66%, which was reduced to 24.52% in the gray-brown purple soil. Kaolin promoted the proportion of P single extraction amount rose to 23.68% in purple alluvial. On the contrary, the proportion of single extraction amount decrease from 25.49% to 24.80%. Goethite, humic acid and fulvic acid could promote the release of phosphorus in soil and the order was goethite > fulvic acid > humic acid. The above three activity control agents made the ratio of the phosphorus amount in purple alluvial increase to 12.59%, 14.11% and 69.71% respectively, also that of the amount of phosphorus in gray-brown purple soil increased to 28.54%、28.26% and 45.72%.3. Adding 0-100%Qm exogenous phosphorus and after 30 days of conversion balance, most of the exogenous phosphorus was transformed into Ca2-P and Ca8-P forms, followed by Fe-P, Al-P, the change of O-P(Occluded phosphorus) and Ca10-P was not obvious and not be formed in a short time. The total content of Ca2-P and Ca8-P accounted for approximately 50%(purple alluvial) and 60%(gray-brown purple soil) of exogenous phosphorus respectively, 22.6%~33.8%(purple alluvial) and 25.3%~35.4%(gray-brown purple soil)were converted to Al-P and Fe-P. Regression analysis and path analysis showed that, the content of Olsen-P in purple alluvial and gray-brown purple soil was directly from Ca2-P and Al-P respectively, and Ca10-P and Fe-P played a direct role in the negative. The change of phosphorus release ability in purple alluvial was mainly affected by Ca2-P, Ca8-P plays a direct positive effect, Fe-P plays a direct role to negative, and that in gray-brown purple soil was by Ca2-P, which played a direct role, Ca8-P played a direct role in the negative. Under the conditions of 50% Qm initial phosphorus(purple alluvial: 500 mg/kg, gray-brown purple soil: 454.5 mg/kg) and after 15 days of conversion balance, in compared with the soil with exogenous phosphorus, the five activity control agents could promote the conversion of OP(organic P) to IP(inorganic phosphate) in different degrees. Montmorillonite in both of soils can promote Fe/Al-P form transformation into Ca-P form, the other four kinds of activity control agents can promote the conversion of Ca-P transformation into Fe/Al-P in varying degrees. The content of Ca-P in the soil treated by kaolin was lower than the Ca-P content in the soil treated by montmorillonite, while that in soils treated by humic acid and goethite was more lower than the Ca-P content in the soil treated by clay mineral. In addition, the order of Fe/Al-P content in the soils treated by the five activity control agents above-mentioned was: goethite > fulvic acid > humic acid > kaolin > contrast > montmorillonite.4. The adsorption-desorption characteristics of soil phosphorus were influenced by the physical and chemical properties of soil and the properties of the active agents. The activity control agents not only affected the maximum adsorption capacity of the soil, but also the soil phosphorus adsorption strength. Under the condition of soil without exogenous phosphorus and after 15 days of conversion, the Langmuir equation and Freundlich equation were used to fit the phosphorus adsorption desorption curve of soil with different activity control agents, and the fitting degree was above 90%, the Tempkin equation was relatively poor, and the fitting degree was about 80%. Montmorillonite and kaolin, goethite could increase soil phosphorus Xm, while humic acid and fulvic acid were in the opposite. The k value of purple alluvial treated by montmorillonite and kaolin is decreased, while the k value can be increased by HA and FA. The k value of purple alluvial treated by goethite was decreased more obviously. In gray-brown purple soil, the k value of soil treated by montmorillonite, kaolin and goethite was increased, and the k value of soil treat by HA and FA was also increased. In addition, montmorillonite and kaolinite could inhibit the desorption of soil phosphorus, HA, FA and goethite can promote the desorption of soil phosphorus; under the conditions of the phosphorus concentration of 200mg/L, the soil phosphorus desorption rate in purple alluvial soil and grey brown purple with montmorillonite and kaolin compared with the soil phosphorus desorption rate of contrast decreased, the ratio of desoption decreased from 20.73%, 20.20%(contrast) to 17.74%, 18.25%(montmorillonite) and 19.57%, 19.02%(kaolin). The soil phosphorus desorption rate in purple alluvial soil and grey brown purple with HA, FA, goethite compared with the soil phosphorus desorption rate of contrast increased, the desorption rates were from 20.73, 20.20%(control) increased to 24.19%, 26.42%(HA); 27.37%,32.75%(FA) and 22.85%, 20.56%(goethite). There was a nonlinear relationship between desorption and adsorption capacity of soil, which could be fitted by exponential equation, the fitting degree above 90%.5. Adding 0-100% Qm exogenous phosphorus and after 30 days of conversion balance. With the increase of phosphorus content, phosphorus release amount and the proportion of phosphorus increased gradually, the proportion of P release content was higher in gray-brown purple soil(8.03-21.02%) than in purple alluvial(2.97-3.71%). Different phosphorus content had little effect on the equilibrium time of P content in the overlying water, the equilibrium times of P concentration of water from the soil with different phosphorus saturation were about 40 d. At this time, the concentration of TP came from purple alluvial and gray-brown purple soil with 0-100 Qm exogenous phosphorus were 0.15-19.24 mg/L and 0.13-4.60 mg/L, which was much higher than that of phosphorus in eutrophic water 0.02 mg/L. Under the conditions of 50% Qm initial phosphorus(purple alluvial: 500 mg/kg, gray-brown purple soil: 454.5 mg/kg) and after 15 days of conversion balance, soil phosphorus release content was mainly affected by the active control agent, flooding time and soil type. Among them, montmorillonite can inhibit the release of soil phosphorus, kaolin, humic acid, fulvic acid and goethite could promote the release of soil phosphorus in different degrees. However, the promotion effect of kaolin in purple alluvial was more obvious, and it was not obvious in the gray-brown purple soil. The order of promotion effect of other three activity control agents was: goethite > fulvic acid > humic acid. Goethite and kaolin could shorten the equilibrium time of the phosphorus concentration from soil in the overlying water, and the equilibrium time of soluble phosphate were about 15 d and 22 d respectively, the P concentration of overlying water from the gray-brown purple soil treated by kaolin was not obvious. In addition, montmorillonite, humic acid and fulvic acid had little effect on the equilibrium time of phosphorus concentration in the overlying water. The water soluble phosphate content and TP content of the water from the soil with different phosphorus load and different active control agents can be fitted with Elovich equation, and the fitting degree were above 90%(except the purple alluvial treated by goethite). |
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