Soil erosion resistance factors in different types of gully heads developed in four main land-uses in the Mollisols region of Northeast China

Soil erosion resistance of gully head determines gully headcut process and further affects gully erosion. However, few studies were conducted to clarify the vertical change in soil resistance of gully heads and its influencing factors, which is not conducive to the revelation of gully erosion mechanism and gully erosion prevention. Therefore, this study aimed to investigate the vertical change in soil resistance of six kinds of typical gully heads developed in farmland (Gf1, Gf2), woodland (Gw), grassland (Gg), abandoned land (Ga) and stable gully channel (Gz) and its influencing factors in the Mollisols region of Northeast China. Soil samples were collected at seven soil depths (0-10 cm, 10-30 cm, 30-50 cm, 50-80 cm, 80-120 cm, 120-160 cm, and 160-200 cm) of each type of gully heads (including three repetitions) to measure soil and root properties. Soil detachment rate (Dr) of gully head at seven soil depths was determined by a hydraulic flume experiment under five runoff shear stresses (4.02-10.89 Pa), and then soil erodibility (K) and critical shear stress (tau(0)) of gully heads at seven soil depths were calculated by the equation [Dr=K center dot(tau-tau(0))]. Results showed that Dr and K of six types of gully heads increased firstly and then declined with soil depth increased and the most erodible position occurred at a changed depth of 50-100 cm of gully heads. On average, Dr of Gz was significantly 1.6-9.3 times greater than that of other types of gully heads. Dr of all gully heads was better simulated by stream power than by runoff velocity and runoff shear stress, and it was positively related to soil disintegration rate (DR) and negatively related to soil bulk density (SBD), saturated hydraulic conductivity (SHC), soil shear strength (SSS), mean weight diameter of aggregate (MWD) and root length density (RLD) significantly. The mean Ks of Gz and Gg were 1.5-7.8 times greater than that of other types of gully heads, and the K was positively related to DR but negatively related to SHC, SSS, MWD and RLD. tau(0) showed a great fluctuation with soil layer and no significant difference occurred among six types of gully heads. tau(0) was only related with SHC. Soil properties have the larger effect on Dr and K of gully heads than root characteristics, implying that it is not reliable to use only vegetation to control the gully headcut. It is suggested that the first consideration should be given to preventing soil from runoff scouring and improving soil erosion resistance of gully head.