Effect Of Iron Reduction On Phosphorus Release In The Typical Soil During The Flooding In Water-Level Fluctuating Zone Of Three Gorges Reservoir Region

The water level fluctuating zone as reservoir runoff pool area, has the potential to be both the phosphorus sink and phosphorus source. The physical and chemical properties, such as pH, Eh, conductivity, microorganism and organic matter of soil affected by periodically submerged, scouring and silting of the reservoir. Iron oxide is one of the most easily changed elements of the chemical behavior in soil during flooded, and the effectiveness of soil phosphorus in large part depend on the modal of iron transformation. So study the effects of iron oxide to phosphorus chemical behavior, can deep understand of the phosphorus migration in the water level fluctuating zone, which has important theoretical and practical significance to prevention and control of eutrophication.The water level fluctuating zone of three gorges reservoir region with a complex anti-flood environment alternating wet and dry seasons, with the change of season, temperature, light also change, while fluctuating zone soil is an important reservoir water ecosystems and terrestrial ecosystems shore pastoral areas, human activities and the impact of fluctuating zone most frequent and intense interaction, the accumulation of organic matter decomposition and transformation of light, temperature change is bound to have a significant impact on the reduction of iron, thus restricting migration and release of phosphorus. Accordingly, this study was collected from Kai County, Chongqing and Fuling Gorges Reservoir typical soil damp soil and purple gray brown purple clay as the research object, through laboratory simulation experiments to explore the temperature, light, humic acid treatment, the Riparian soil iron reduction behavior and its impact on phosphorus release. The results showed that:(1) Different temperatures (15℃,25℃and35℃), phosphorus released from anaerobic waterlogged incubation fluctuating zone soil (sediment) to the overlying water, with flooding time continuation, total phosphorus (TP) concentration in the overlying water showed from the first increase to gradually reduce and eventually become dynamic equilibrium process. Two soils are expressed as a function of temperature rise, increased phosphorus release, TP concentration in the overlying water increases. The purple tide soil suspensions phosphorus concentrations at15℃,25℃and35℃handle were reached peak35.0,48.7and50.3mg/L after60d waterflooding, the gray-brown purple soil suspension phosphorus concentrations at15℃,25℃and35℃handle were reached peak49.9,86.4and88.9mg/L after60d waterflooding, which have shown increasing temperature of the soil suspension phosphorus concentration increased. Also found in flooded soil suspension available phosphorus showed a significant negative correlation (p<0.05, n=14) between iron phosphate in various forms of inorganic phosphorus, correlation with several other forms of inorganic phosphorus is not significant. Phosphorus increased in waterflooding soil affected by iron oxides.Increased temperature promotes iron reduction, the concentration of Fe2+and amorphous iron in soil suspension is increasing with increasing temperature. Using MINTEQ model calculations mainly iron saturation coefficient solid interface (SI) of purple tide soil and brown purple clay at different temperatures, found that at different temperatures Fe (OH)2colloid, magnetite, and vivianite are already saturated from the beginning, and the rest several major iron minerals exhibit saturation with increasing temperature coefficient is reduced. Comparison of two soils at different temperatures Fe2+, amorphous iron and phosphorus correlation was found purple gray brown and purple tide soil are expressed as a function of temperature rise, Fe2+and phosphorus increasingly significant positive correlation, amorphous iron and phosphorus along with the correlation of temperature change, at15℃correlation is not significant at25℃and35℃significant correlation (P<0.05, n=14). Temperature increases in favor of iron oxide reduction and activation, promoting the electron transfer process.Iron-reducing bacteria are heterotrophic microorganisms, which use soil organic matter for carbon source, soil organic matter is the main source of electrons and effective complexing agents, and changes in CO2and CH4can characterize soil organic matter decomposition process. In the early flooding, CH4yields lower CO2to become dominant, and found that the temperature rise make the maximum amount of CO2and CH4ahead of time in gray brown purple clay, and35℃、25℃CO2and CH4production significantly higher than15℃treatment. As the temperature increases soil organic carbon gaseous conversion efficiency, correlation analysis showed that the reduction of iron and CO2and CH4production showed a significant positive correlation (P<0.05, n=14), further evidence of the temperature increases promote organic decomposition, accelerate electron transfer and promote iron reduction, thereby causing the release of phosphorus.(2) Light inhibite phosphorus release at a certain degree, TP in the overlying water of flooding purple gray brown and purple tide soil reached the maximum peak concentration were0.033and0.045mg/L under light conditions, otherwise the dark handle the peak reached0.057and0.084mg/L. The light and dark conditions, the two soil suspension phosphorus release is different, but phosphorus release all showed the dark higher than light. The correlation analysis between available phosphorus and various forms of inorganic phosphorus found that light caused flooding soil the correlation changes between available phosphorus and iron-bound phosphorus may be an important reason for the light to suppress phosphorus release.Iron oxide changes under the action of light is:illumination lead to reduced iron oxides in soil saturation, Fe2+and amorphous iron generated blocked. CO2and CH4reflect waterlogged soil organic matter decomposition, light reduce the conversion efficiency of gaseous organic carbon and accelerating inorganic electron acceptor consumption in soil, reduces the iron oxide reduction capability. Light on waterlogged soil phosphorus release and inhibition of iron reduction with waterlogged soil organic matter decomposition are closely related.(3) After add exogenous humus in fluctuating zone waterlogged soil, TP in the overlying water of control soil (without addition of exogenous humus) significantly higher than added AQDS (anthraquinone-2,6-disulfonate), AD-HA (Adamas soil humic acid) and AL-FA (Aladdin soil fulvic acid)treatment, and phosphorus release between AQDS, AD-HA and AL-FA treatment have no significant difference(p>0.05, n=14). Purple tide soil with added AQDS, AD-HA and AL-FA, after flooding60d suspension available phosphorus concentrations were higher than the control treatment5.5,8.17and8.43mg/kg. Purple gray brown soil with AQDS, AD-HA and AL-FA treatment, after flooding60d suspension available phosphorus concentrations were higher than the control treatment5.52,6.99and8.1mg/kg. Correlation analysis of soil inorganic phosphorus and available phosphorus, found that purple tide soil and purple gray brown soil adding AQDS, AD-HA and AL-FA, available phosphorus and each inorganic phosphorus forms basicly negative correlation, indicating that adding humic acid promote various forms of inorganic phosphorus conversion to available phosphorus. Found that adding exogenous humic acid change mechanism of phosphorus release caused by the change of iron phosphate.Fe2+and amorphous iron increased first and then decreased in two control soils during flooding, but amorphous iron and Fe2+have a little change among AQDS, AD-HA and AL-FA treatment during the processing of flooding, indicating that humus added inhibition iron reduction and activation. Using MINTEQ model calculations mainly iron saturation coefficient solid interface (SI) of purple tide soil and brown purple clay, Fe (OH)2colloid, magnetite and vivianite already saturated from the beginning under adding exogenous humus treatments, and the rest several major iron minerals saturation coefficient exhibit AQDS, AD-HA and AL-FA treatment is higher than the control treatment. Comparison the correlation of amorphous iron、Fe2+and available phosphorus in two soils under different treatments, found that the correlation between the amorphous iron and available phosphorus from positive correlation becomes to negative correlation in purple gray brown and purple tide soil with AQDS, AD-HA and AL-FA treatment, further confirmed humus suppression the amorphous iron generation, reducing the amorphous iron adsorption phosphate. Added humic acid to make the iron reduction is suppressed, while the soil available phosphorus is still increased, indicating that the process of phosphorus release from mechanisms of other than iron reduction.Transformation of organic matter is closely related to iron oxide conversion, CO2and CH4reflect changes of the whole system in the anaerobic environment, CO2concentration in AQDS, AD-HA and AL-FA processing is much higher than the control treatment from the first3d flooding in both two soils, CH4were inhibited before first30d in all treatment, but then rapidly grow in the control process, the growth rate reached306.8%. After flooding40d at peaking781.7nmol/L and then rapid decline, while CH4in slow growth under AQDS, AD-HA and AL-FA treatment. CO2/CH4was greater than1during60d waterlogging, indicating that in this process humic acid is mainly as an electron acceptor. Amorphous iron was negatively correlated with CH4, CO2was positively correlated with amorphous iron in both purple gray brown and purple tide soil, amorphous iron inhibits the formation of CH4. Also found compared with the control treatment, the correlation between Fe2+and CO2changes from positive to negative in purple gray brown and purple tide soil under AQDS, AD-HA and AL-FA treatment. Indicating AQDS, AD-HA and AL-FA treatment CO2and CH4generated changes thus causing Fe2+and amorphous iron changes. The specific mechanism of added humic acid inhibited iron reduction, promotion phosphorus release need to be further studied.