古红土的发育与演变

DEVELOPMENT AND EVOLUTION OF RED PALEOSOLS

分布于江南丘陵地的第四纪红色粘土 ,通常由均质红土层、网纹层和砾石层所构成 ,它是第四纪初期的沉积物 ,但在一定程度上又具有成土作用的特征 ,故称为古红土。深厚的红色风化壳、酸性和高度富铝化是古红土的特点 ,而古红土上发育的红壤既继承、保留了古红土的原有特征 ,又在生物作用和人为活动共同作用下形成生物富集的、表层复硅的、高度风化、发育的特征。因此 ,具有昔日和现在环境双重特征的地表红壤是由古红土发育、演变而成的典型残余土壤。

Quaternary red clay widespreads in the south of China,especially in the hills of southern Yangtze River,usually consisting of uniform red soil,plinthitic and gravel layers.It is a sediment formed during the early Quaternary,and also has some characteristics of soil formation to a certain extent.In authors’opinion,therefore,it is called red paleosols.The red paleosols of Xuanzhou,Anhui Province were covered with Xiashu Loess,yellowish brown in color,while the red paleosols of Qingjiang,Jiangxi Province were exposed in surface.The red paleosols are commonly characterized by dark red weathering crust with several meters in thickness,acid reaction and allitization. Evolution of the red paleosols is actually the forming process of red earths under the effect of natural factors and human activities.The red earths with horizons A,AB and B were first differentiated from uniform red horizon of the red paleosols formed at the end of the middle Pleistocene,and developed on plinthitic horizon formed in the early of the middle Pleistocene as a parent material. The red earths were inherited from some characteristics of the red paleosols,such as red in color,acid to strong acid in reaction and Fe Mn illuvial horizon in a profile,as a result of high content of free iron,higher exchange acidity and amorphous iron oxide and so on.However,the red earths also have some new properties differing from the red paleosols.In statistics,organic matter content of diagnostic surface horizon in the red earths apparently increases to 15 8—18 7 g/kg from 3 8 g/kg of uniform red horizon in the red paleosols.Of course,change of OM content with depth in a profile also shows its bio enrichment feature. In the course of development of the red earths,surface horizon,even subsurface horizon showed resilicification and migration amount of SiO 2 being as low as -24 67%.And some weathering forming indexes,such as SiO 2/Al 2O 3,SiO 2/R 2O 3 and BA value,show that development degree of the red earths is much greater than those of the red paleosols.Thus,the red earths,which have a double feature of both past and present environment,are a typical relict soil.