Characteristics Of Soil Fe-Mn Nodules And Their Pedogenetic Significance In South China

With the rapid development of economy and increase of population,anthropogenic activities strongly impact soil genesis and evolution,the study of anthropogenic activities on the influence of soil evolution is of importance.Fe-Mn nodules are important soil new formation,and regarded as the pedogenetic products.Being relative stability of their structure,soil Fe-Mn nodules recorded pedogenetic processes and can trace soil forming environments.Therefore,Fe-Mn nodules can act as a“key”for deciphering pedogenetic information,and are of great significence for pedogenesis.In this study,Fe-Mn nodules in soils derived from Quaternary red earth（QRE）,Fluvial deposits（FD）and Basalt（B）with different land use in South China were collected,macro-micromorphology,element geochemical characteristics,mineral composition and metal adsorption-desorption characteristics etc.,were studied by steromicroscope,scanning electron microscopy equipped with an energy-dispersive X-ray spectrometer（SED/EDS）,chemical analysis,X-ray diffraction analysis（XRD）.The aim of this study was to reveal Fe-Mn nodules characteristics and interpret pedogenetic processes.The results showed as follows:1)Fe-Mn nodules in soils derived from different parent materials exhibited different characteristics of macro-micromorphology,which were observed and analysed by munsell,steromicroscope and SEM/EDS.The internal microstructures of spherical Fe–Mn nodules in soil derived from QRE exhibited a differentiated structure by well-expressed bands with surface color ranging from orange to black;tubular Fe–Mn nodules in soil derived from FD exhibited a homogeneous cross-section with surface color ranging from turbid yellowish orange to grayish yellowish brown;nodules in soil derived from B,including spherical,subangular and irregularly shaped exhibited a homogeneous cross-section with some quartz grains,and surface color ranging from grayish brown to turbid reddish brown.The macro-micromorphological characteristics of Fe-Mn nodules differed with land use,particularly the surface color and cross-section structure.For instance,the color of surface were blackish brown and dark reddish brown dominating the Fe-Mn nodules of upland derived from QRE,whereas the dark brown,brown,turbid orange,orange nodules were exhibited in paddy field.This was mainly controlled by difference for cycles of wetting and drying conditions under different land use,and indicated the impact of soil pedogentic process by the morphological characteristics of the nodules.2)The chemical composition of Fe-Mn nodules in soils derived from different parent materials were different,Fe-Mn nodules in soils derived from QRE was comprised of SiO₂,Al₂O₃,Fe₂O₃,and MnO,Fe-Mn nodules in soils derived from FD and B of SiO₂,Al₂O₃,and Fe₂O3.The chemical composition of Fe-Mn nodules were different with the difference of Fe-Mn nodules size,the contents of Al₂O₃,Fe₂O₃,P₂O₅,Cr,V,As,Zr decreased,whereas SiO₂,MnO,TiO₂,CaO,Pb,Cd,Ni,Co,Mo,Li,Ba,Sr,Rb,∑REE increased with nodule size in soil derived from QRE,and the element content had no obvious pattern in nodules derived from FD and B.The pearson correlation and NMDS analysis of enrichment factor of elements in Fe-Mn nodules,indicated that SiO₂,Al₂O₃,TiO₂,K₂O,MgO,Zr,Li,Sr and Rb are closely related in together which were elemental composition of phyllosilicate minerals,Fe₂O₃ was strongly associated with P₂O₅,V and As,whereas MnO is positively correlated with Pb,Cd,Ni,Co,Mo,Ba and REE in iron manganese nodules,Cu and Zn were influenced by both Fe₂O₃ and MnO.Light rare earth elements（LREE）occurred mainly in studied Fe-Mn nodules and soil matrices,and The chondrite-normalized curves all showed"right tilt with a relative enrichment of LREE".The Fe and Mn oxides in nodules derived from different parent materials were mainly DCB-extractable Fe and Mn.The ratio oxalate-extractable Mn to DCB-extractable Mn（Mn_o/Mn_d）was significantly higher than that of the ratio oxalate-extractable Fe to DCB-extractable Fe（Fe_o/Fe_d）.3)Chemical index of alteration（CIA）,chemical index of weathering（CIW）,andδEu in nodules were undifferentiated between nodules and soil matrices,and both showed B>QRE>FD,these evaluation indexs indicated that Fe-Mn nodules can reflect the difference of pedogenesis derived from different parent materials;whereasδCe reflected the variation of soil environment.4)The XRD pattern suggested that the constituents of phyllosilicate minerals in Fe-Mn nodules were similar to soil matrices,they both contain kaolinite,illite,smectite and1.42 nm intergradient minerals derived from QRE;kaolinite,illite and smectite were contained in nodules and soil matrices derived from FD;similarly,kaolinite and gibbsite were contained in nodules,saprolite and soil matrices derived from B.The results indicated parent material sources of Fe-Mn nodules were strongly related to corresponding soil matrices.The changes of the relative contents of phyllosilicate minerals between Fe-Mn nodules and soil matrices were comparatively remarkable,the relative contents of illite and smectite in nodules derived from QRE were higher soil matrices,the relative content of illite in nodules derived from FD was higher soil matrices,the relative contents of kaolinite and gibbsite in nodules derived from B were similar to saprolite,whereas different to soil matrices.Hematite and goethite were contained both nodules and soil matrices,goethite was major constituent of iron oxide minerals in nodules,whereas,hematite was major constituent of iron oxide minerals in soil matrices,the results demonstated the formation of Fe-Mn nodules were controlled by the longer periods of saturation in the soil environment and higher oxidation potential.5)The adsorption isotherms of Pb（Ⅱ）,Cd（Ⅱ）,the As（Ⅴ）by Fe-Mn nodules accorded with Langmuir equation.Compared with the soil matrices,nodules had stronger adsorbed ability.The variation of adsorbed ability of nodules derived from different parent materials indicated that there were some differences,and then reflected the difference of pedogenesis in different parent materials.