Variation in soil property and its response to vegetation restoration in areas of collapse mounds in mid-subtropical China

The term of collapse mound, is called Benggang by local residents, represents the most severe eroded ecosystem in the red soil region of southern China. We studied the response of vegetation restoration on the characteristics of soil properties in different parts of the collapse mound system,which is an important reference to reveal the soil environmental effects under the intensity erosion environment and to guided the ecological restoration in the collapse mound area. Therefore, we selected three adjacent collapse mounds with different vegetation coverage（2%, 20%, and 95%） as the study object. The physicochemical properties of 63 soil sample plots were investigated in 7 sites, including upper catchment, collapsing wall（top, middle, and bottom section）, colluvial deposit（upper and lower section）, and channel outlet. The variation and correlation of soil properties were also analyzed. The results showed that the soil bulk density, slit content pH, ammonium nitrogen（NH+4-N）, nitrate nitrogen（NO-3-N）, available phosphorus（AP）, and available potassium（AK） content decreased significantly, while the organic carbon（SOC）, total nitrogen（TN）, total phosphorus（TP） and total potassium（TK） content increased considerably with process of the vegetation restoration. From the upper catchment to channel outlet（via collapsing wall and colluvial deposit）, the soil bulk density and sand content showed up-down-up trend, whereas the content of silt and clay showed decreasing trend. The content of SOC, TN, TP and TK in the upper catchment and the colluvial deposit were significantly higher than that other sites, whereas their contents were lowest in the top collapsing wall and channel outlet, respectively. The spatial distributions of NH+4-N and NO-3-N content showed downward and upward fluctuating trend, respectively, and the spatial variation of AK content showed a "V" tendency and its lowest content appeared in the bottom of the collapse wall, whereas AP and pH showed a significant increasing trend along with the runoff erosion direction. In summary, the soil structure and nutrient content were poor in the collapse mound ecosystem. and the soil physicochemical properties present obvious spatial differences in different erosion sites, they were improved by natural vegetation restoration.