Grain Size Distribution, Element Migration, and Stable Carbon Isotope Characteristics of Dolomite Weathering Profiles in Xinpu, North of Guizhou Province
The relation of the vertical distribution and cycle characteristics of soil organic carbon (SOC)with the elements in the soil is an important area of research on the soil geochemical circulation and climate change. At the same time, the distribution characteristics of soil elements can also affect the cycle rate of soil organic carbon. To identify the parameters that may influence the turnover rate of soil organic carbon in karst terrain and their relations with the geochemical behavior of chemical elements, we selected two dolomite weathering profiles for analysis (the surface layer is grass and new cultivated land), which are located in Zunyi, Guizhou Province. On the basis of particle size distribution, of the delta~(13)C value of SOC, and the quality migration coefficient of the elements, we discuss the trends of soil organic carbon and delta~(13)C values with the deepening of the red weathering crust in this area and the geochemical characteristics of the elements. Pearson′s correlation coefficients were calculated to quantify the relations. Our results indicated that SOC concentrations in the grass profile decreased with the deepening of soil layers, but in the farmland profile, SOC content of the surface layer increased rapidly at the depth of 25 cm, and then decreased with deepening of the profile. The pH value of the surface layer was acidic in both profiles, and the acidity of the grass profile was stronger than that in the farmland profile. The pH value increased with the deepening of soil layers, but the rising trend of the grass profile was more obvious:Soil grain size fraction showed that the silt fraction (550 mum)made up 5080% of the soil, whereas the sand fraction was the least prevalent. The share of each particle size varied with the deepening of the profile. By means of mass balance approximation and by selecting Ti as an inactive reference element, we obtained a quality migration coefficient of each relevant element. The results showed that almost all elements in the profile showed a certain degree of loss or enrichment. In the grass profile, Fe, K, Ba, V, and Zn showed some enrichment; Na and Co were enriched in the weathering protolith; Mn was enriched in the dolomite powder. In contrast, Mg and Ca were almost absent in the profile. In the farmland profile, V and Zn showed certain enrichment. Fe and K were enriched at the interface of the soil and dolomite; Co was enriched in the weathering protolith, and Ba showed weak enrichment at the surface. Analysis of the delta~(13)C value of SOC showed that this metric was between-26.4 and-23.6. The delta~(13)C value of both the original soil samples and the silt fraction samples decreased with increasing profile depth, and there was no obvious difference among soil grain fractions. In the grass profile, delta~(13)C and pH significantly correlated with most elements; SOC correlated negatively with Fe, K, and Li (P < 0.05); the migration of elements in the farmland profile was mainly affected by pH. Among all of the elements, there was a significant relation in both soil profiles.Thus, it is possible that the basement carbonate in the vertical direction is inhomogeneous in the Xinpu region, and the local hydrogeological conditions have some influence on the distribution of soil particles. The distribution and migration of elements in the profile are related to soil pH, SOC, and clay content in a soil-forming process; there is also a good covariant relation among the elements.