Structural characteristics and infiltration performance analysis for zonal soils under different erosion degrees

Abstract: It is important to study soil structure characteristics and its zonality difference in the process of soil degradation for understanding soil structure stability mechanism, guiding regional agricultural management, and predicting soil erosion. In this study, we selected different soil erosion degrees (feeble, light, heavy, and severe) of 3 typical types of zonal soils (yellow cinnamon, yellow brown, and red soils) of Hubei province (32°18′N, 112°09′E) and determined their soil saturated hydraulic conductivity, aggregates water stability, porosity distribution and other physical and chemical properties so that we can better understand their changes under different erosion degrees. The results showed that with the increase of the erosion degree, the soil organic matter content decreased gradually. For different zonal soils, the sequence of free iron and aluminum oxides (Fed and Ald) concentration was red soil > yellow cinnamon soil > yellow brown soil, and the free iron and aluminum oxide concentration had significant difference (P<0.05) for different soil erosion degrees. The results also showed that the sequence of aggregate mechanical stability and water stability from the 3 zonal soils was red soil > yellow brown soil > yellow cinnamon soil. Except for yellow brown soil, soil aggregate physical stability gradually increased with the increase of soil erosion degree. The >5 mm aggregate fraction and water stability mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates decreased gradually with the increase of the erosion degree in the 3 soils. Water stable aggregate (WSA) for red soils was evenly distributed among different size fractions, while WSA were distributed mainly <0.25 mm fraction for yellow brown soils and yellow cinnamon soils, and compared to dry sieving aggregates, >5 mm WSA decreased significantly (P<0.05) and it was disrupted into finer aggregate. In addition, the results showed that the soil porosity was between 38.6%-53.0%, of which 36.8%-52.7% was capillary porosities. The saturated hydraulic conductivity showed a decreasing trend with the increasing of erosion degree except for red soils, and saturated hydraulic conductivity of feeble soil erosion was higher than other eroded soils (P<0.05). In addition, saturated hydraulic conductivity of yellow cinnamon soils, yellow brown soils and red soils gradually increased except in feeble and light degree erosions. Correlation analysis showed that the aggregates mechanical stability and water stability correlated extremely significantly with Ald (correlation coefficient = 0.77 and 0.81, respectively, P<0.01), and positively significantly with Fed (correlation coefficient = 0.73 and 0.76, respectively, P<0.05). All of those illustrated free iron and aluminum oxides were the dominate factors affecting the stability of zonal soils structure in the research area. Non-capillary porosity, aggregates water stability and bulk density of soil were closely related with the saturated hydraulic conductivity of the yellow cinnamon, yellow brown, and red soils (P<0.05), and there was significant correlation between saturated hydraulic conductivity and WSA fractal dimension (correlation coefficient = 0.76, P<0.01). The study showed soil structure changes and the mechanism of soil structure stability are affected by iron and aluminum oxides in the process of soil erosion, and the results provide technical reference for evaluation of soil erosion so that subsequent prevention and management methods can be developed.