Variations in weathering characteristics of soil profiles and response of the Atterberg limits in the granite hilly area of South China

Red loam hills with granitic parent material are susceptible to gully erosion under continuous runoff. Laterite soils from granitic parent material in South China are continuously weathered. Soil profile weathering and Atterberg limits, as indirect indicators of soil erosion resistance, are often neglected. Here, three typical granite weathering profiles in Hubei (HB), Fujian (FJ) and Guangxi (GX) were selected to investigate the granite weathering zonation characteristics and their relationships with the Atterberg limits. The main soil profile oxides were SiO2, Fe2O3 and Al2O3, and their total content exceeded 80.00%. Moreover, the chemical weathering indicators consistently showed highly weathered states and increase in weathering degree from bottom to top in the soil profiles; the weathering degree had zonal characteristics from north to south, and the weathering mantle thickness gradually increased. The liquid limit (LL), plastic limit (PL), shrinkage limit (SL), plasticity index (PI) and shrinkage index (SI) decreased as the soil profile depth increased, while the liquidity index (LI) decreased, indicating more stability and less susceptibility to erosion in the surface soil layers. During heavy rainfall, the soil moisture content in the sandy layer was more likely to approach the PL or even LL, which easily cause soil loss. Further, the red soil layers tended to shrink under higher moisture contents, thereby forming fissures and leading to preferential flow, which increased the interstitial pressure of the soil and led to a reduction in shear strength, and resulted in gully erosion. Regression analyses showed that Si mobility was significantly positively correlated with LL (R2 = 0.712), PL (R2 = 0.732), SL (R2 = 0.669), SI (R2 = 0.709), and PI (R2 = 0.592) and significantly negatively correlated with LI (R2 = 0.554). The correlations between Fe mobility and Atterberg limits were the opposite of those for Si. According to path analysis, the Fe2O3 content and Atterberg limits had the largest decision coefficients, indicating that the weathering intensity was significantly correlated with the Atterberg limits.

Automated summary

Red loam hills with granitic parent material are susceptible to gully erosion under continuous runoff, and Atterberg limits are important indicators affecting soil erosion resistance. The weathering intensity is significantly correlated with Atterberg limits, with zonal characteristics from north to south.