| Considering the role of the colloid in the pollution distribution and elements cycling, the presence of colloids in runoff is very important to understand the colloidal phosphorus (P) transport process. Therefore, the transport of colloidal P in soils has received considerable attention in non-point pollutions in recent years, because colloid-facilitated phosphorus has been proved as a significant contributor to eutrophication. The overall objectives of this research has been to assess the effect of drying-flooding cultivation on the potential of P distribution, and to discover the characteristics of P release to water bodies from the paddy field in field scale. In the present work, a long-term experiment was set-up in 2005. From the experiment, the distribution of P in soil profile of the paddy field can be monitored, including the mobilization of colloidal phosphorus, dissolved P and Olsen P from topsoils to subsoil. Batch experiments were also conducted to investigate the equilibrium sorption isotherms for P onto particle-sized fractions. Besides, to determine the effect of water dispersible colloids derived from swine manure on the leaching of phosphorus from paddy soils, leaching experiment in saturated-flow columns packed with aggregate-sand mixture materials were investigated using manured soil and unmanured soil. The detailed results are as follows.(1) Batch experiments were conducted to investigate the equilibrium sorption isotherms for P onto particle-sized fractions (<2μm,2～20μm,20～200μm, and 200～2000μm) which were named clay, silt, fine sand and coarse sand according to international system respectively) derived from a series of long-term paddy field of Jiaxing in China. The results showed that the ratio of total P (TP) in each fraction to total P in the bulk soil followed by the order:clay (52.84%±0.93%)> fine sand (24.85%±2.47%)> coarse sand (16.72%±2.69%)> silt (9.09%±1.48%). All of he percent of TP and Olsen P adsorbed by clay of the three soils exceeded 50% and 69% of the soil, respectively. The adsorption of P by these fractions could be described by the Langmiur equation and Frendlich equation. The maximum adsorption of P (Cm) and distribution coefficient (Kd) decreased with the following order:clay>silt>coarse sand>fine sand in the three soils. By choosing particle size, organic matter, iron oxide and equilibrium pH value as parameters, multivariate statistical analysis in SPSS were employed to build the regression model of adsorption of P and to evaluate the effect of these factors on it. The results showed that particle-size fraction, organic matter have much more significantly effect on the adsorption of P than available iron.(2) Application of P with animal manure and fertilizer in amounts exceeding removal with crops leads to accumulation of P in soil, making them potential long-term diffuse sources of P loss to water. The impact of a range of manuring and fertilization practices on the TP, Olsen P, distribution of dissolve phosphorus and colloidal P, and degree of phosphorus saturation (DPS) of soil were investigated in field study. In the present work, the overall comparison of all sites with a wide range of DPS has been investigated. The results indicated that DPS was an important factor in controlling the concentration of dissolved P and colloidal P in soil. The change points at 9% and 12% DPS were noted by using a split-line model, above which Olsen P (10.8 mg P kg-1) and dissolved P (3.1 mg P kg-1) in soil profile began to rapidly increase and potentially mobile downward. Therefore, it is supposed that the leaching of dissolved P can not be neglected as a widespread environmental problem. Compare with dissolved P, colloidal P was the dominant fraction of P in water-dispersible colloid suspension of the soil profile. The significant decrease of ionic strength and the increased pH value from topsoils to subsoils can explain the high release of soil colloid. And the soil colloid was the important carriers of collidal P. Therefore, colloidal P was also high in the subsoil despite DPS was low in subsoils. Overall, the high DPS induced by manuring and fertilization was the main factor of the transportation of colloidal P. But the effect of pH value, EC on the release of colloidal P in the soil also can not be overlooked.(3) The objective of this study was to test whether new application of fertilization 24 h before an intense rainstorm or the intial P in the soil is the main source of P loss in runoff water. A rainfall simulation study compared TP, colloidal P, total dissolve phosphorus (TDP), and total particulate phosphorus (TPP) concentrations and losses in runoff water after swine manure and inorganic fertilizer were broadcast. The results showed that P loss increased with applications fertilizer or manure and initial soil P of the soil, with most occurring as TDP accounted for more than 60% of total P in the fraction<0.45μm. Colloidal P also account for 42-62% of total P in the fraction 0.1-1μm. In comparison with IP1-C and IP2-C of group 1 (without application of fertilizer), the concentration of Colloidal P, TDP were much higher after using inorganic fertilizer in IP1 and IP2 treatments of group 2, respectively. And the ratio of colloidal P and TDP to TP also increased relatively in these treatments, respectively. Howerer, in comparison with OP1-C and OP2-C of group 1 (without application of manure), the concentration of Colloidal P, TDP were also increased after the application of maure in OP1 and OP2 treatments of group 2, respectively. But the ratio of colloidal P and TDP to TP decreased relatively in these treatments, respectively. The linear regression equation analysis demonstrated that new applied fertilizer and manure were the main source of runoff P rather than intial P in soils.(4) To investigate the effect of water-dispersible colloids derived from swine manure on the potential risks of P, migration behavior of P in saturated-flow columns were compared in the presence and absence of water-dispersible colloids of manure in the inflow. It was found that total dissolved P (TDP) accounted for a majority of total P (65%-98%) in the effluent with deionized water treatments, while only accounted for 21%-45% to total P in the leachate in the manure colloid treatments. In manured soils, with the inflow of manure colloidal suspension, colloidal P in the effluent were 26.7 times higher than that of deionized water treatment (PM+W) and 1.9 times more than that of unamended soil treated with manure colloid (PO+M) in the end of the leaching experiment, respectively. Despite the initial reduction of TDP concentrations in the effluent with the presence of manure colloid, the TDP concentrations still increased smoothly and continued to transport with the effluent throughout the breakthrough experiment. This suggested that P sorption sites of the soil and the added manure colloid in the column were fastly saturated during initial stage of the experiment. The good linear correlation between colloidal P and colloidal Fe indicated that Fe hydroxides could be served as a main medium for the transportation of colloidal P. Moreover, colloidal P exhibited greater mobility under higher pH and lower ionic strength.(5) In a series of laboratory soil incubation experiments, the effect of applied phosphorus on soil test phosphorus, degree of phosphorus saturation (DPS), dissolve phosphorus in water were discussed. During the experiment, two types of degree of phosphorus saturation had an obvious increase as a result of applied phosphorus. In a series of laboratory soil, DPSox increased from 11.2% to 34%, while DPSM3 increased from 2.72% to 17.98%. In addition, in both of the surface runoff of P and mobilization of P in soil profile, the statistics analysis found significant relationships between the differient DPS and soil test phosphorus. Therefore, the DPS was a good indicator of environmental impacts of P loss potential from agricultural soils to waters. |
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