桂林典型岩溶区和非岩溶区土壤剖面钙镁形态迁移对比

文章以桂林典型岩溶区和非岩溶区土壤剖面为研究对象，采用改进的Tessier元素形态连续提取法，测定岩溶区和非岩溶区土壤钙（Ca）和镁（Mg）元素离子交换态（包括水溶态）、碳酸盐结合态、腐殖酸结合态、铁锰氧化物结合态、强有机质结合态（包括部分硫化物）和残渣态等形态，探讨岩溶区和非岩溶区土壤Ca、Mg形态在土壤剖面中的迁移变化特征。结果表明:（1）岩溶区石灰土剖面中，随剖面深度加深，pH值增大，而土壤有机质、全氮（N）、全磷（P）含量减少，Ca元素主要以交换态存在且在各土壤剖面中含量相近，Mg元素主要是以残渣态赋存在土壤剖面中，随着土壤剖面深度腐殖酸结合态百分比减少，存在累积现象；（2）在非岩溶地区酸性土壤中，土壤总钙较低。在土壤剖面0~60 cm以强有机结合态为主，深层剖面（大于60 cm）以残渣态为主；Mg元素有效态含量较低，非岩溶区土壤Mg元素以腐殖酸结合态和残渣态为主，与岩溶区土壤类似，非岩溶区Mg元素在土壤中也存在一定累积。 

Comparison of soil calcium and magnesium fractions transport in classic karst and non-karst region,Guilin

The exposed karst area in the world is about 17.9 million km2, mainly distributed in China, Vietnam, France, Slovenia, the United States and other countries. Covering a total area of 780,000 km2, the karst are in south China is most widely distributed in the world, mainly in the most area of 8 provinces/cities such as Yunnan, Guizhou, Guangxi, Hunan, Hubei, Chongqing, Sichuan and Guangdong. The southern karst area represented by Guilin is a typical karst area in China, where soil is formed by the physical, chemical and biological effects of carbonate rocks in the geological background. Due to the richness of calcium (Ca) and magnesium (Mg) in carbonate rocks, alkaline soil rich in Ca and Mg is distributed in karst areas. But non-karst areas are primarily clastic rock, where soil mainly exists acidic. Besides, Ca in soil is an essential mineral nutrient for plant growth and metabolism. Soil Ca and its morphological changes affect the physicochemical properties of soil, and also affect the uptake of calcium and other nutrients by plants. Magnesium is also an essential mineral element for plant growth, ranking the fourth after nitrogen, phosphorus and potassium. Therefore, plants need to take in Mg, depending on the content and supply mechanism of Mg in soil.With a modified Tessier sequential extraction procedure, soil profiles from typical karst and non-karst areas in Guilin are analyzed to determine different fractions of Ca and Mg,the ion-exchange fraction (including water-soluble state), carbonate-bound fraction, humic acid-bound fraction, Fe-Mn oxides bound fraction, strong organic matter-bound fraction (including some sulfides) and residual fraction. The migration characteristics of Ca and Mg elements in soil from these two types of areas are explored. The vegetation type of sampling sites is mainly forest. During the sampling, a soil profile with 1 m wide, 2 m long and 1.5 m deep was dug. The profile is stratified from the surface to 140 cm, and divided into 7 horizons at intervals of 20 cm. Then soil samples are collected from the bottom layer by layer. A total of 7 soil samples in the limestone soil profile are collected from the depth of 105 cm at intervals of 15 cm. The results showed that in the limestone soil profile of the karst area, the pH value increases with the depth of the profile, while the contents of soil organic matter, total nitrogen (N) and total phosphorus (P) decrease. Except the parent material, the P content of soil is related to other factors in the process of soil formation, among which bioaccumulation and soil formation are important. The weak migration of P, together with the limited availability of limestone soil makes the P content difficulty to leach down from the profile, hence the P content of the surface layer is higher than that of the lower layer. Calcium mainly exists in the exchange fraction and its content is similar in each soil profile, while Mg mainly exists in the residual fraction. There is an accumulation when the percentage of humic acid-bound fraction decreases with the depth of the soil profile. The maximum value of the ion-exchange fraction of Ca occurs in the middle profile at the depth of 45-60 cm. In the karst area, Mg is mainly present in the residual fraction (97.45%) in the limestone soil profile. But Mg content is low in the ion-exchange fraction including the water-soluble fraction (0.61%), the carbonate-bound fraction (0.11%) and the humic acid-bound fraction , accounting for 1.80% of the total amount in the latter three fractions. Compared with the typical karst area, the total Ca content is lower in acidic soil of the non-karst area. The soil profile from 0 cm to 60 cm is dominated by strong organic bound fraction, and the deeper profile (>60 cm) by residual fraction. Calcium in the topsoil in non-karst areas is mainly present in the ion-exchange fraction (including the water-soluble fraction), the Fe-Mn oxide-bound fraction and the strong organic-bound fraction (including some sulfide). The dominance of the ion-exchange fraction (including the water-soluble fraction) indicates the relatively high mobility of Ca. In non-karst areas. The effective content Mg is low, mainly in humic acid-bound fraction and residual fraction. Similar to karst areas. Mg in non-karst areas also accumulates in the soil to some extent. In conclusion, the study explores the morphological migration of Ca and Mg in soil profiles in karst and non-karst areas, and provides scientific support for soil improvement and risk assessment of Ca and Mg leaching in soil.