Geochemical characteristics of typical tobacco-planting soils inZhangjiajie mountainous area

Tobacco-planting mountain region have ecological effects on tobacco quality in central south China. The climate, topography, geomorphology, soil parent materials and soil nutrient status are highly heterogeneous factors in the area. Different parent materials and soil types have different degrees of weathering which result in diverse composition and content of soil geochemical elements. The interconnected factors consequently influence the growth and quality of tobacco in mountain regions. Zhangjiajie is a typical mountain tobacco-planting region in Hunan Province of central south China where high quality tobacco is produced. To lay the scientific basis for high quality tobacco cultivation and production, the effects of soil weathering, the abundance or deficiency of nutrients and the migration or enrichment of geochemical elements on Zhangjiajie mountain tobacco cultivation region were investigated. A total of 23 typical soil profiles were sampled and analyzed for 25 types of geochemical elements (or their oxides). The results showed that limestone, plate-shale and purple-sandstone efflorescence of soil parent materials in the region were strongly weathered. The chemical weathering index (CIA) of the soil parent materials were in the range of 73.19~88.03. The soils were characterized by both the depletion of Ca and Na and enrichment of Fe and Al elements. Purple soils developed from purple-sandstone efflorescence showed strongly weathering and eluviation properties. Some of soil types developed from limestone efflorescence exhibited a lower degree of allitization. Most of the geochemical elements of yellow soil, paddy soil and purple soil were highly enriched which was critical for tobacco yield. There were higher contents of P, B, Se, Mn, Zn, Cu, Ni, Y, Sc, La, Fe²O³ and SiO² in the eluvial soil horizon, higher than the median of these soil elements in China or in the world. However, the contents of N, S, Cl, Mo, Co, Sr, Br, I, CaO, MgO, K²O, Na²O and Al²O³ in the eluvial soil horizon were low. Both the macro elements of N, P, S, CaO and MgO and the trace elements of Mo, Se, Mn, Zn, Br and I significantly changed along the soil profile. The hierarchical cluster analysis method was used to classify the enrichment coefficient of geochemical elements into three groups. The groups consisted of N, P, S and Se with strong enrichment characteristics; Br, I, Mn and CaO with strong migration characteristics; and then the relatively stable elements. B, Zn and Cu trace elements and Y, Sc and La rare earth elements were both abundant and relatively stable along the soil profile. The geochemical elements of N, P, S, Se, B, Zn, Cu, Y, Sc and La were the possible characteristic elements with vital influence on tobacco quality in Zhangjiajie mountain tobacco cultivation region.