Research On The Soil Anti-erodibility And Migration Rate Of Gully Heads In The Typical Mollisol Region Of Northeast China

The Mollisol Region of Northeast China is one of the"four major Mollisol regions"in the world.The soil in this region is rich in nutrients and has a high fertility level,making it the primary supply source for commercial crops such as corn and japonica rice in China.However,since the 1950s,the continuous large-scale development and cultivation in the Mollisol Region of Northeast has caused serious soil erosion problems,mainly manifested in the extensive loss of black soil layers on sloping farmland and the formation of numerous gullies during the process of soil and water loss.Gullies damage the land,causing it to become fragmented and hindering mechanical movement,reducing the efficiency of machinery operations,and causing irreversible damage to the soil in the Mollisol region.However,research on gully erosion in the Mollisol Region of Northeast has been relatively late,with most studies focusing on the distribution,development characteristics,and influencing factors of gullies.As one of the important processes of gully erosion,gully head retreat determines the development process of gullies,and the soil erosion resistance of gully heads is an inherent factor that determines gully head retreat.However,the characteristics of gully head soil erosion resistance,gully headcut retreat rate,and its influencing factors in the Mollisol region are unclear,which is not conducive to the prevention and control of gullies in the Mollisol region.Therefore,this study selected six types of gully heads as research objects:active gully head developed in farmland（Gf1）,semi-stable gully head developed in farmland（Gf2）,gully head developed in artificial forest land（Gw）,gully head developed in naturally restored grassland（Gg）,gully head developed in abandoned land（Ga）,and gully head developed on a stable gully channel（Gz）.The study used a combination of in-situ soil and root sampling,laboratory hydraulic scouring tests,and post-rainy season gully morphology monitoring to investigate the distribution characteristics of soil properties and root systems of different types of gully heads in typical Mollisol regions,to clarify the profile changes in soil anti-erodibility of gully head（soil anti-scourability and erodibility）and their relationships with root and soil properties,and to elucidate the retreat rate of six types of gully heads during the rainy season and its relationship with influencing factors.The main conclusions are as follows:（1）The root length density of the Gz was the highest,ranging from 1.62 to 4.43 times that of other gullies,while the root biomass and root surface area density of the Gg were significantly higher than those of other gullies,with increases of 2.05%to 31.75%and 1.37%to 11.61%,respectively.The root systems of the six gully vegetation types decreased with increasing soil depth,and were mainly distributed in the 0-30 cm soil layer,with densities ranging from 1.54 to7.09 times that of the 30-200 cm soil layer.The plant roots in the entire gully profile were mainly composed of fine roots（diameter≤2 mm）,accounting for 85%to 90%.（2）The soil bulk density and saturated hydraulic conductivity of the six gully heads varied from 1.29 to 1.68 g·cm^-3 and 0.02 to 0.35 mm·min^-1,respectively,with the highest values observed in the Gf1 and the lowest in the Gw.The trend of bulk density with soil depth showed an initial increase followed by a decrease and then an increase,while the variation in saturated hydraulic conductivity differed significantly among the gully heads.With increasing soil depth to below 50cm,the soil erodibility of the six gully heads increased significantly,with the average erodibility of the Gz and Ga being 10%to 91%higher than that of other gully heads.The organic matter content and soil aggregate stability of the gully heads decreased gradually with increasing soil depth,with the highest organic matter content observed in the Gw(33.11 g·kg^-1),which was 1.36to 3.06 times that of the other gully heads.The soil shear strength of gully heads varied with soil depth in a similar manner as the saturated hydraulic conductivity,with the highest value observed in Gf1 and Gf2,which was significantly higher than those of other gully heads by nearly 2 times.（3）The soil detachment rate of the six types of gully heads ranged from 0.0037 to 0.035kg·m^-2·s^-1.The gully head developed in a stable gully channel（Gz）had the highest soil detachment rate,indicating the poorest erosion resistance,and was significantly lower than the other gully heads（P<0.05）.Similarly,the soil erodibility of Gz and Gg were not significantly different,but were significantly higher than the other four types of gully heads by 0.54 to 6.84 times.The soil detachment rate and erodibility of gully heads increased first and then decreased with increasing soil depth,but the depth at which the maximum value appeared varied depending on the type of gully head.The maximum value of Gf1 and Ga appeared at 30-50 cm,Gw at 50-80 cm,Gz and Gf2 at 80-120 cm,and Gg at 120-160 cm,and the location of the maximum value was basically consistent with the development position of the gully head’s vertical wall depression.（4）The changes in soil detachment rate and erodibility of gully heads were significantly correlated with soil bulk density,shear strength,aggregate stability,soil disintegration rate,and root length density.Random forest importance analysis showed that soil disintegration rate had the greatest impact on the changes in soil detachment rate and erodibility,followed by saturated hydraulic conductivity,shear strength,aggregate stability,and root density,indicating that soil properties had a greater impact on gully head erosion resistance and erodibility in the Mollisol region than plant roots.（5）The linear migration rate,areal migration rate,and volumetric migration rate of the six typical gully heads developed on different land uses were 3.67 m·yr^-1-81.97 m·yr^-1,5.06m²·yr^-1-233.45 m²·yr^-1,and 10.15 m³·yr^-1-380.27 m³·yr^-1,respectively.The erosion rate of the gully head developed on a stable channel（Gz）was significantly higher than those of the other gully heads.The gully erosion in this region is in an active development stage,and its headcut migration rate is higher than those in other parts of the world.The soil disintegration capacity and the upstream catchment area of gully heads were the main factors affecting gully headcut retreat,and the impact of root density on gully headcut retreat was the weakest.