| Phosphorus (P) and Arsenic (As) belong to one family. They have similar chemical properties and similar chemical behaviors in soil-plant-systems. The presence of P would affect the uptake of As by plants and furthermore affect the migration and conversion of As in plants. In previous literatures, both synergism and antagonism between P and As in soil-plant-systems were obtained. In this study, exogenetic arsenate (60 mg As·kg-1 soil) was added into the soils, then phosphate was added into these soils after the stabilization of soil labile As. Diffusive gradients in thin films (DGT) technique was employed to determine the soil labile As concentrations and labile P/As molar ratios during the incubation and under the effect of P in different experimental conditions (various additive amount, different water conditions and disturbance of plants), further the kinetic transport of As in soils were estimated by using DGT results and DIFS (DGT Induced Fluxes in Soils) model. This study was to reveal the effect law and mechanism of exogenetic phosphate on labile As in the soils. The main results are as follows:1. Exogenetic phosphate (total of 400 mg·kg-1) was added into the soils after stabilization of soil labile As. Then the soils was cultured under dry conditions (30% MWHC). For most soils used in this study, the lability of soil As increased with increasing addition of P, indicating that competition effect between P and As in these soils. As competitive adsorption effect of P in the soil, stably bounded As was desorbed from the adsorption site, which increased the lability of As in these soils. However, different effects were obtained in soils with the highest pH value and lowest soil available P concentration. The lability of As in all experimental soils all increased when a high amount of phosphate (400 mg·kg-1) was added.2. Under the conditions of non-P application,200 mg·kg-1 P application, and 400 mg·kg-1 P application, generally the varying pattern of the resupply abilities (R values) of soils is in contrary to the varying pattern of soil labile As. The resupply abilities of the soils, which has a downtrend of labile As after P addition, increased with the addition of exogenetic phosphate; the resupply abilities of the soils, which has a uptrend of labile As after P addition, decreased with the addition of exogenetic phosphate. For most soils in this study, the distribution coefficients (Ka) decreased with the addition of exogenetic phosphate, indicating that the released As, due to the addition of P, would mainly exist in the soil solution, or the adsorbed As, due to the addition of P, was mainly from the weakly combination state of As.3. Under the conditions of different water management modes (maximum field capacity, alternating wet and dry, moderately dry), labile As concentration in the experimental soils preformed different trends before and after the application of exogenetic phosphate. In most soils, the lability of As increased after P application while the lability of As in the BLACK SOIL and YELLOW SOIL decreased, which contained the highest (127.5 mg·kg-1) and lowest (0.1 mg·kg-1) original available P content respectively. The competed mechanism between P and As in soils are very complicated, which may relate to the soil moisture conditions or different adsorption types in soils. The resupply abilities of soil solid phase (R values) would decrease after P application, but the Kd values in different soils had shown inconsistent trends.4. The concentration of As in soils and plant tissues presented different trends after P addition in the soil-plant-system. The lability of As in all experimental soils all increased after P addition (200 mg P·kg-1 soil) into the soils. However, the concentration of As in plant tissues presented different trends, the As concentration all increased in the plants grown on the soils collected from Hunan province, to the contrary, the As concentration all decreased in the plants grown on the soils collected from the other areas. The results of mutiple-analysis suggested that, the difference may attributed to the demands of phosphate for plants. In all experimental soils, the uptake of As by plant (phyto-availability of As) decreased with the application of exogenetic phosphate. However, when labile P/As is no smaller than 1.7, extra addition of exogentic phosphate wouldn’t reduce the phyto-availability of As significantly.The effect of exogenous phosphate on arsenic in soil-plant-system was studied, and the results indicated that competition was the main interaction between P and As on the surfaces of soil particles and plant roots, and this competition was more powerful under higher soil water contents. However, this competition would be affected by soil pH and the original phosphorus status in soil. DGT measured soil labile P/As molar ratio had a negative influence on the plant-uptaked-As. The critical value of the influence is 1.7. Through the study of interactions and mechanisms between P and As in soil-plant-system would provide some proposals for the control and reduction of phyto-toxicity of As in soil by accurate P fertilization. |
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