No abstract

‘Benggang’ is a local term for a widespread type of severgully erosion with steep collapsing walls in granitic, low, hilly areas of southern China, and its development and expansion are closely related to the shear strength of the collapsing wall. Plant roots play an important role in improving soil shear strength. However, the shear strength of root‐soil complexes in different layers of collapsing walls remains obscure. We selected Dicranopteris linearis fern roots and adopted the direct shear method to evaluate the effect of root weight density (RWD) (0–1.25 g 100 cm−3) on the shear properties of the lateritic, sandy and detritus layers. The results showed that roots could enhance soil shear strength, and the maximum increase in the lateritic layer was 11.53%, higher than that in the sandy (5.84%) and detritus layers (3.17%). As the root content increased, the cohesion of the sandy and detritus layers increased and then decreased, and their maximum increase in cohesion and the fitting optimal RWD were lower than those of the lateritic layer. The internal friction angle was not affected by roots. When the root content was constant, the shear strength and cohesion of the lateritic layer were significantly higher than those of the sandy and detritus layers, while their internal friction angle was significantly lower than that of the latter two layers. The average increment of soil cohesion calculated by the Wu‐Waldron model (WWM) was 10.52 kPa, which was 0.30, 3.75 and 19.38 times the measured average values of the lateritic, sandy and detritus layers, respectively. The correction coefficient k′ was 0.02–1.18, and the truek'¯$$ \overline{k\hbox{'}} $$ in the lateritic layer was the highest (0.82), followed by that in the sandy and detritus layers. By combining the modified WWM with Coulomb's formula, new shear strength equations for root‐soil complexes of D. linearis were established. The predicted shear strength compared well with the measured shear strength (R2 > 0.90, NSE >0.90). Overall, the roots only had a significant reinforcement effect on the lateritic layer, and they could still not change the mechanical properties of the collapsing wall, which were more stable in the upper layers and weaker in the bottom. Therefore, other measures should be taken in the bottom layers to improve the stability of Benggangs. Highlights Effect of D. linearis roots on the shear strength of collapsing walls in Benggang was studied. Roots could improve collapsing‐wall soil shear strength, mainly reflected in the cohesion. The roots enhancement effect in lateritic layer was better than that of sandy and detritus layers. New shear strength equations of root‐soil complexes were established based on the Wu‐Waldron model.

The vertical distribution of soil microorganisms in soil indicates the restoration degree of degraded soil ecosystems. We took the untreated bare land and vegetation restoration sample plot in the red soil erosion area of southern China as the object of study; comparatively analysed the soil bacterial community changes in the 0 to 10, 10 to 20, 20 to 30 and 30 to 40 cm soil layers; and explored the environmental factors driving the change in the soil bacterial community. The poor nutrient conditions created by soil erosion increased the competitiveness of autotrophs and made Chloroflexi the dominant phylum of bacteria. Soil erosion led to the gradual similarity of soil bacterial communities in the 0 to 10, 10 to 20 and 20 to 30 cm soil layers. However, only the relative abundance of Actinobacteria changed in different soil layers in the erosion area, mainly due to the inconsistent distribution of soil organic carbon caused by erosion affecting the change in the Actinobacteria relative abundance in the soil layer. After vegetation restoration, the soil properties of the eroded land were obviously improved, and the dominant bacterial phylum changed from autotrophic bacteria ( Chloroflexi) to heterotrophic bacteria ( Actinobacteria). The change in community structure existed only in the 0 to 30 cm soil layer in the restoration area, while the community structure changed to mainly Proteobacteria in the 30 to 40 cm soil layer. The change in the respective proportions of Chloroflexi, Proteobacteria and Actinobacteria was the main reason for the difference in soil bacterial community structure among soil layers. The change in soil aggregates caused by vegetation restoration was the main environmental factor driving the variation in soil bacterial community structure, and the formation of aggregates was closely related to soil organic carbon. The vertical distribution of Actinobacteria in different soil layers can indicate the degree of soil ecosystem restoration in the red soil erosion area of southern China, and the relationship between Actinobacteria and soil organic carbon was significant.