Attribution Of Typical Red Earth Of Hunan Province In China Soil Taxonomy And Their Genetic Characteristics

Red earth is a group in the Chinese soil genetic classification,and it is the main zonal soil in Hunan Province,accounting for 51%of the total soil area.The complex soil-forming environment and pedogenic process of red earth result in the intricate subgroup types,unclear boundary,and overlapping or leakage of soil types.This phenomenon is not conducive to the systematic identification of soil types also not to the scientific utilization and protection of red earth.Therefore,it is imperative to comprehensively research the red earth taxonomy using the quantitative soil classification,clarify the attribution of the main categories of red earth in Chinese soil taxonomy（CST）,and explore the evolution rule of genetic characteristics under different soil types.Based on this,through the field survey and laboratory analysis of red earth derived from different parent materials（granite,GR;slate and shale,SS;limestone,LS;sandstone,SDS,and Quaternary red clay,QRC）in Hunan Province,we retrieved the classification indexes and established the taxonomy of red earth using the CST third edition.Furthermore,we intensely studied the rule of profile morphological characteristics,clay mineral composition,cation exchange properties,the fraction of iron and aluminum.We clarified their implication for pedogenesis and soil taxonomy.The main results are presented as follows:（1）The red earth in Hunan Province mainly be attributable to Ferrosols（accounting for 40%）,followed by Cambosols（36%）and Argosols（20%）,and the proportion of Primosols（4%）was the least.The red earth from GR and LS was mainly classified as Ferrosols,the red earth from QRC mostly be belonged to Argosols,and the Cambosols and Primosols mostly came from the red earth from SS and SDS.According to the pedogenic process and morphological characteristics of red earth,we suggest adding the subgroup of Plinthic Ali-Udic Argosols and Mottlic Ferri-Udic Cambosols in CST.Compared with the ST and WRB,the CST can better reflect the pedogenic characteristics in subtropical areas.It has more soil types in groups and subgroups and stronger discrimination.（2）There were significant differences in texture types,soil color,and the occurrence frequency of neogenesis of red earth derived from different parent materials.The texture of red earth derived from GR and SS are mainly sandy clay loam and silty clay loam,respectively.In contrast,the texture of red earth derived from LS and QRC was mainly clay.The hue of red earth ranges from dark reddish-brown to yellow（10R to 2.5Y）.The soil colors from QRC and LS were mainly bright reddish-brown（5YR 5/6）and bright orange（7.5YR 5/6）,respectively.The soil colors from GR and SDS were mostly bright brown（7.5YR 5/6）and orange（7.5YR 6/6）.The occurrence frequency of neogenesis in red earth from QRC and LS was higher.The horizon and profile development index（HDI,PDI）based on the profile morphological differences between the eluvial and illuvial horizons can better reflect the degree of soil development.The HDI values of particle size,color,and average in Umbric epipedon were higher than that of Ochric epipedon.The average value of HDI in the Argic and LAC-ferric horizon was higher than that of the Cambic horizon.The PDI values among different soil types were as follows:Ferrosols>Argosols>Cambosols>Primosols.（3）The dominant clay minerals of red earth in Hunan were kaolinite,followed by 2:1-type minerals,including illite and vermiculite,and a small fraction of mixed-layer minerals.Kaolinite was predominant in soils from GR and LS,while mixed-layer minerals prevailed in QRC,whereas illite and vermiculite were prevalent in SDS.Clay minerals were significantly correlated with altitude and terrain.The clay mineral in the LAC-ferric horizon was mainly kaolinite,and the content of mixed-layer minerals in the Plinthic horizon was the highest.Kaolinite was primarily distributed in Ferrosols,while the clay minerals of Primosols were mainly 2:1 type clay minerals.（4）The cation exchange properties of the red earth subsurface layer in Hunan were mainly affected by parent material.The CEC and ECEC of the subsurface layer from LS and QRC were higher than that of other soils.The sum of base cations（SBC）and base saturation（BS）of the subsurface in LS was significantly higher than that of other soils,while its Al saturation and exchangeable acidity were the lowest.The vegetation type and terrain were the main environmental factors affecting the cation exchange properties of the surface layer.There were significant differences in cation exchange properties between the different diagnostic horizons and soil types.The averaging CEC of Umbric epipedon was significantly higher than that of Ochric epipedon.The SBC and CEC of the Argic horizon were significantly higher than that of the LAC-ferric and Cambic horizon.The CEC,SBC,and BS of Argosols were significantly higher than that of other soil types.In the subsurface layer with low content of SOC,the effect of clay on CEC was higher than that of SOC.（5）The primary form of red earth iron in Hunan was Fe_d.The main form of Fe_dwas Fe_c,accounting for 93%of Fe_d,which reflects the strong development degree of red earth in Hunan.There were differences in the profile distribution of iron forms.The content of Fe_t,Fe_d,and Fe_cfirst increase and then decrease with depth.The content of Fe_odecreases with depth,while the Fe _Sishowed the opposite trend.This result indicates the Fe_dreleased from weathering Fe _Simoves downward from the upper part of the profile.The content of Fe_t,Fed,and Fe_cof red earth from LS was the highest,while the content of Fe _Siand Fe_oin red earth,respectively from GR and QRC,were higher than others.The averaging Fe_oand Fe_o/Fe_dof the diagnostic surface horizons were higher than that of the subsurface horizons.The contents of Fe_d,Fe_c,and Fe_d/Fe_tin LAC-ferric and Cambic horizon were higher than that of others,while Fe _Siwas mainly distributed in the Argic horizon.The contents of Fe_dand Fe_cin Ferrosols were highest.Fe _Siwas distributed mainly on Argosols,while Fe_owas primarily distributed in Primosols.（6）The Al forms of red earth in Hunan mainly were interlayered Al(Al _In)and noncrystalline Al(Al _Nc).The exchangeable Al(Al _Ex),Fe bound Al(Al _DCB),and Al _Ncwere mainly distributed in the subsurface layer,while the adsorbed inorganic hydroxyl-Al(Al _Hy),organic complexed Al(Al _Or),and Al _Inwere primary spread in the topsoil.The contents of Al _Hy,Al _Or,Al _DCB,and Al _Ncin the subsurface layer from LS（weak acid to neutral）were highest,while the content of Al _Exwas the lowest.The contents of Al _Exand Al _Inin the subsurface layer from acid QRC were the highest.The contents of mineral Al and total Al in the subsurface layer from sandy SS and GR were higher than others.The contents of Al _Ex,Al _DCB,and Al _Inwere the highest in the Plinthic horizon,while the content of Al _Orreached the maximum in the Umbric epipedon.The contents of Al _Hyand Al _Ncin the LAC-ferric horizon were higher than that of others.The contents of Al _Ex,Al _Hy,Al _DCB,and Al _Ncamong different soil types were as follows:Ferrosols>Argosols>Cambosols>Primosols,while Al _Inand Al _Orshowed the opposite trend.This study illustrates that the parent material and environmental conditions significantly affect the genetic characteristics of red earth（morphological characteristics,clay mineral composition,cation exchange properties,and the fraction of Fe and Al）,control the development direction and pedogenic process of soil,and affect the classification of the diagnostic horizon and soil types.