Purpose Soil structures are the main course of the formation and development of collapsing gullies, which are the most severe type of erosion in south China. However, few studies have focused on the relationship between soil macropores, soil erosion, and local topography. This study aimed to quantify and compare soil properties and macropore characteristics in the collapsing gully region, and explore their influences on the formation and development of the associated erosion.
Materials and methods Soil core columns at different positions of a typical collapsing gully were excavated, and then scanned to analyze soil macropores. Moreover, soil properties and saturated hydraulic conductivity were investigated in the laboratory and in the field, respectively.
Results and discussion The results indicated that the sand content increased from the ridge to the slope and the valley, while silt and clay contents decreased for the same catena. The mean weight diameter of aggregates was largest at the ridge and lowest at the valley. The infiltrate rates were highest at the valley and lowest at the slope. The valley had the greatest macroporosity (1.09±0.33%), and the highest number (5919±703), volumes (24.7±7.5 cm3) and surface (10.4±2.6 m2) of macropores, as well as the highest conectivity (42.3), while the slope had the smallest macroporosity (0.15±0.14%), and the smallest number (1189±747), volumes (3.4±3.2 cm3) and surface (1.7±1.4 m2) of macropores. The mean pore volume of macropores larger than 1 mm3 was largest at the ridge (16.8±7.4), and smallest at the slope (10.6±2.9). The number of macropores and their macroporosity mainly decreased with increasing depth, but were influenced by the soil macrofauna as well as the erosion and sediment processes. Macropores were mainly vertical, which is affected by the roots of plants and is conducive to the vertical infiltration of water. But, there wer many horizontal macropores at the valley because of the sediment process. The equivalent pore diameter of macropores was mainly smaller than 2 mm (accouting for more than 76.3%), and the macropores larger than 5 mm were less than 1%.
Conclusions The macropore characteristics at different sites of the collapsing gullies affected the soil water infiltration and hydraulic conductivity, and further affected the processes of water erosion and mass ersion. The highest macroposities at the valley would result strong subsurface flow erosion and the loss of the base of collapsing wall. Macropores at the ridge would increase rain infiltration and promote soil collapsing. Few macropores and low infiltration abilities at the slope would strengthen the overland flow erosion. Thus, macropore characteristics had significant effects on both the formation and development of collapsing gullies.