草酸对砖红壤的还原-溶解及其与磷素释放的耦合关系

砖红壤中铁、铝氧化物对P素的强烈吸附和包蔽是这类土壤P素生物有效性低的主要原因，P素释放与土壤的还原-溶解过程关系极其密切. 通过研究草酸对砖红壤的还原-溶解反应，以揭示溶液中c(Fe)，c(Al)和c(P)以及pH的动态变化规律，阐明溶液pH条件以及2种陪伴金属阳离子(铝离子和铜离子)对Fe，Al和P释放数量的影响. 结果表明，在所研究的24 h反应时间内，溶液c(总Fe)在第12小时达到最大值后逐渐下降，pH在第12小时达到最大值后则保持不变；溶液c(Al)逐渐升高，而c(P)则表现出下降趋势. 这说明溶液中c(Fe)，c(Al)和c(P)以及pH主要受土壤固相还原-溶解反应与离子表面吸附反应的相对强弱控制. 低pH条件有利于土壤还原-溶解反应的进行，促进Fe，Al和P释放到土壤溶液. 此外，溶液中c(Al)和c(P)差异明显，而c〔Fe(Ⅱ)〕和c(P)之间差异较小，表明草酸还原-溶解砖红壤过程中，Fe(Ⅱ)和P释放基本同步进行. 铝离子或铜离子对草酸还原-溶解砖红壤过程有抑制作用，可降低平衡溶液中c(P)和c〔Fe(Ⅱ)〕. 

Coupling of Oxalic Acid-Promoted Reductive Dissolution of Latosol and Phosphorous Mobilization

Bioavailability of phosphorus in latosol is commonly limited by its strong adsorption onto Fe and Al oxides as well as formation of insoluble Fe and Al phosphate. The mobilization of P in such acid and highly weathered soil is significantly dependent on the reductive dissolution of the soil. In this paper, the dynamic change rules of soluble Fe, Al and P concentrations as well as solution pH were investigated during a 24 h oxalic acid-promoted reductive dissolution of latosol. The final release of Fe, Al and P from the above process as affected by pH and the two accompanying cations (Al and Cu) was also examined. As the results show, the concentration of soluble Fe gradually reached a maximum after 12 h shaking time, and then decreased as the reaction time prolonged. The solution pH initially increased for the first 12 h and then remained constant. A regular decrease in the concentration of P in the solution was observed, while the reverse trend was found with respect to soluble Al concentration. The observed element profiles and progression of pH likely resulted from the interplay of soil solid phase reductive dissolution and the surface adsorption reaction. Release of Fe, Al and P was favorable under low pH conditions. Moreover, the distinct difference between the concentrations of dissolved Al and P and the close correlation between the concentrations of dissolved Fe(Ⅱ) and P indicated that the release of P was intimately coupled with reductive dissolution of Fe-oxides in the soil. The above heterogeneous reaction would be inhibited in the presence of Al(Ⅲ) and Cu(Ⅱ), resulting in the decrease in concentration of Fe(Ⅱ) and P in solution.