Mardi 29 mai 2018
Wuhan, Huazhong Agricultural University
The behavior of clay minerals in different size fractions of soil colloids are very particular, mostly in the nanoparticle (25-100 nm) fractions, where clay minerals are very reactive and have a significant role in the transport of nutrients. In this work, the characterizations of clay minerals in the subsoil particle fractions (< 2000, 450-2000, 100-450, and 25-100 nm) of Alfisol 1 (Alf1) from north, Alfisol 2 (Alf2) and Ultisol (Ult) from Central and Oxisol (Oxl) from South China were studied by inductively coupled plasma-optical emission spectroscopy (ICP-OES), conventional and synchrotron X-ray diffraction (XRD), fourier-transform infrared spectroscopy (FTIR), thermogravimetric analysis (TG), scanning electron microscopy analysis (SEM) and high-resolution transmission electron microscopy (HRTEM) in order to : analyze the chemical elements in different subsoil size particles; identify the type of clay minerals and their evolution characteristics in the subsoil size particles; understand the transformations of clay minerals and their crystalline structures with the decreasing size particle fractions and from north to south China; reveal the change of the stacking layer of clay minerals from macro to nanoscale in the subsoil particles and from north to south; and reveal the influence of soil formation factors on the clay mineral transformation in the particle size fractions. The study revealed that:
1) From north to south, the degree of pedogenesis, the desilication as well as the allitization were observed in the four soils. Clay minerals suites in the Alf1 and Alf2 are illite, vermiculite, kaolinite and trace of illite-vermiculite and illite-kaolinite mixed layers. Kaolinite, hydroxy interlayer mineral (HIV), few vermiculite and illite constituted the clay minerals phases in the Ult. However, kaolinite, hydroxy interlayer (HIV) and trace or minor illite dominated in the Oxl.
2) As the size particles decreased to nanoparticles, the degree of pedogenesis and the desilication as well as allitization slowdown in the north while these process were intense with decreasing size particles in central and south region. In the same way, the silica-alumina ratio slightly increased with decreasing size particles in north and significantly reduced as the size particles reduced to nanoparticles in central and south China. With reduction of size particles to nanoparticles, the dissolution of 1:1 clay minerals occurred in the north while the transformation of 2:1 into 1:1 was found in the central and south region. Illite was transformed into vermiculite in the < 2000, 450-2000 and 100-450 nm size particles of Afls from north and central China.
3) With the decrease of size particles, the trioctahedral phase weathered gradually and the dioctahedral phase became dominant in nanoparticles. With decreasing of altitude, the weathering of trioctahedral phase progressively accelerated and rich the maximum in south China; the 2:1 clay minerals gradually reduced while the 1:1 clay minerals increased and gibbsite as well as some iron oxides appeared. Clay minerals in nanoparticles changed from strong dominance of illite to total dominance of kaolinite.
4) The disorder in the crystalline structure of illite and kaolinite was more obvious in nanoparticles compared to the larger particle fractions (< 2000, 450-2000 and 100-450 nm). The stacking layers of clay minerals in these soils changed from well-stacking to defect stacking layers as the size particles reduced to nanoparticles. From north to south, the disorder in the crystalline structure as well as the defect stacking layers of clay minerals progressively decreased and became insignificant in south China.
5) During their transformation, clay minerals in the larger particles size (< 2000, 450-2000 and 100-450 nm) weathered to give clay minerals in nanoparticles. From north to south, the main transformation of clay minerals was illite → vermiculite → HIV → kaolinite → gibbsite. Some soil formation factors may have contributed to the transformation of clay minerals in this study; however the influence of the climate factor remained significant in the four soils.