太湖地区水耕人为土中漂白层的成因探讨

CAUSES OF FORMATION OF ALBIC HORIZON IN STAGNIC ANTHROSOL IN THE TAIHU LAKE REGION,JIANGSU PROVINCE,CHINA

采集太湖地区4个具有漂白层的水耕人为土剖面（E—A1、E—A2、E-A3、E—A4）及1个发育于相同母质上的一般水耕人为土剖面作为参照（Ref），描述了土壤剖面的形态特征，分析了土壤颗粒组成、黏粒矿物组成、铁氧化物以及大量和微量元素等，并探讨了漂白层成因。5个剖面土壤颗粒组成以粉粒为主，存60％～75％之间，且剖面中各层次变异系数较小，小于10％；大量元素和微量元素在剖面各层次问变异系数也小于10％，指示漂白层与上覆和下伏土层发育在相似物质来源的土壤母质上；而E—A1剖面漂白层中较高的粉粒含量（75．04％）、较大的中细粉粒和中细粉粒／粗粉粒比值的变异系数（38．05％、61．85％）表明该剖面母质具有不均一性。说明E—A2、E—A3、E—A4、Ref剖面土壤性质的变化主要是由于成土作用形成的，而E．Al剖面土壤的性质变化除受成土作用影响外，还受沉积环境的影响。E．A2、E—A3、E-A4剖面的漂白层中蒙脱石和蛭石、粉粒和粗粉粒、SiO2、CaO、Zr等含量高于上下土层，而水云母和绿泥石、游离铁、A12O3、Rb、Li、Ba、稀土元素（Rareearthelement，REE）等则相反；同时，参照剖面亦有与E—A2、E—A3、E—A4剖面类似的特点，只是漂白作用出现在耕作层，不能定义为漂白层；而E—A1剖面的部分性质则显示出空间上的不一致性，其粉粒含量并非漂白层最高，而是随剖面土层深度向下越来越大。这些变异表明，太湖地区水耕人为土漂白层的形成，可以由黄土性泻湖相沉积物经过水耕过程中水分的周期性淹水和排干，通过机械淋洗和活性铁的淋溶而形成（剖面E—A2、E．A3、E—A4），也可以是母质沉积的过程中受湖水水面升降，导致机械淋溶，然后水耕过程叠加活性铁的淋溶而形成（剖面E-A1）。

The formation of an albic horizon in stagnic anthrosol has a special significance to genesis and development of the soil. To explore causes of the formation,four profiles of stagnic anthrosols with an albic horizon( E-A1,E-A2, E-A3 and E-A4) and one( Ref),the same in parent material but without the horizon,as reference,in the Taihu Lake region were sampled for analysis of morphological characteristics,soil mechanical composition,clay mineral composition,contents of iron oxides,and geochemical characteristics of macro and traceelements. Of the five soil profiles,soil particle size compositions were dominated with silt,which accounted for 60% ~ 75% and did not vary much between soil layers in the profile,with variation coefficient being < 10%. Contents of macro and trace elements did not vary much either,with CVs being less than 10%. All these indicated albic horizons developed from parent materials similar to those the overlying and the underlying layers did from. However,in Profile E-A1,the albic horizon was quite high in silt content( 75. 04%) and inCV of medium-fine silt( 38. 05%) and medium-fine silt / coarse silt( 61. 85%) as well,which indicates that the profile is not uniform in parent material. All these demonstrate that the variations of soil properties in Profiles E-A2,E-A3,E-A4 and Ref were caused by soil formation,while that in Profile E-A1 was also affected by the environment of sedimentation,besides soil formation. The albic horizons in Profiles E-A2,E-A3 and E-A4 are higher than their overlying and the underlying horizons in contents of smectite,illite,silt,coarse silt,SiO2,CaO and Zr,but lower in contents of hydromica,chlorite,free iron, Al2O3,Rb,Li,Ba and REE. Profile Ref was quite similar to Profile E-A2,E-A3,E-A4 in all characteristics except that the albic process appeared only in the plow layer,which is not fit to be defined as albic horizon. In Profile E-A1,some soil properties were not spatially homogeneous,for instance,silt content is not the highest in the albic horizon,but increased with depth in the soil profile,which suggests that the formation of an albic horizon in stagnic anthrosols in the Taihu Lake region is a result of the subjection of loessial lagoon sediments to periodical alternation of flooding and draining mechanical washing and leaching of active iron during anthrostagnic process( Profiles E-A2,E-A3 and E-A4),or a result of the subjection of parent material to mechanical washing caused by rise and fall of the water surface in the lake during its sedimentation process and then to leaching of active iron during repeated anthrostagnic processes( Profile E-A1).