Infiltration and shear strength characteristics of gully heads soil of typical vegetation on the gullied Loess Plateau, Northwest China.

Gully head is the main active part of gully erosion, which seriously affects the occurrence of gully headcut erosion. To investigate root distribution and soil physical and mechanical characteristics of typical vegetation gully head, we analyzed the infiltration, root distribution, physical and mechanical properties of soil-root complex of soil in different layers (0-1 m) in natural restoration gully head and artificial restoration gully head. The results showed that the variability of soil bulk and total porosity among different vegetation gully heads was low, with bulk density ranging from 1.10 to 1.37 g·cm-3 and total porosity ranging from 48.3% to 58.4%. Infiltration index of different vegetation gully heads generally decreased with increasing soil depth. The infiltration rate of different soil layers in natural restoration gully head tended to stabilize in 20-30 min, while that of artificial restoration gully head tended to stabilize in 40 min. The infiltration capacity and average infiltration rate of artificial restoration gully head were generally higher than those of natural restoration gully head in all soil layers. Root length density, root surface area density, and average diameter all tended to decrease with increasing soil depth. Except for the 20-40 cm soil layer, root length density, root surface area density and average diameter of natural restoration gully head were all lower than those of artificial restoration gully head. Root system of both vegetation gully heads mainly consisted of 0-0.5 mm roots, accounting for 84.2%-93.6% of the total root length. In the vertical depth, with the increases of water content, the cohesion force decreased linearly with the deepening of soil layer, ranging from 0.42 to 22.67 kPa. The average cohesion force of artificial restoration gully head was higher than natural restoration gully head at each level of water content. The study revealed the effects of vegetation on the gully head cut erosion, which could provide scientific basis for the effective prevention and control of soil erosion in the region.

Response of soil erodibility of permanent gully heads to revegetation along a vegetation zone gradient in the loess-table and gully region of the Chinese Loess Plateau.

Revegetation has been proven to significantly affect soil erodibility of gully heads, and climate conditions are expected to affect soil erodibility by determining the vegetation characteristic. However, there are crucial scientific/knowledge gaps regarding the change in the response of soil erodibility of gully heads to revegetation along a vegetation zone gradient. Therefore, we selected the gully heads with different restoration years along a vegetation zone gradient encompassing the steppe zone (SZ), forest-steppe zone (FSZ), and forest zone (FZ) on the Chinese Loess Plateau to clarify the variation in soil erodibility of gully head and its response to soil and vegetation properties from SZ to FZ. Furtherly, we systematically and comprehensively reveal driving factors of changes in soil erodibility in three vegetation zones. Results showed that: (1) Vegetation and soil properties were affected positively by revegetation and differed significantly in three vegetation zones. (2) Soil erodibility of gully heads in SZ was significantly higher than in FSZ and FZ, by 3.3 % and 6.7 % on average, respectively, and it showed a significantly different decrease with restoration years in three vegetation zones. (3) Standardized major axis analysis proved that the sensitivity of response soil erodibility to vegetation characteristics and soil characteristics presented a significant difference as the revegetation proceeded. Vegetation roots were the primary driver in SZ, but soil organic matter content dominated the change in soil erodibility in FSZ and FZ. (4) Structural equation modeling indicated that climate conditions played an indirect role in regulating soil erodibility of gully heads by mediating vegetation characteristics. This study offers essential insights for assessing the ecological functions of revegetation in the gully heads of the Chinese Loess Plateau under different climatic scenarios.

Revegetation-induced changes in vegetation diversity improve soil properties of gully heads.

Revegetation is among the most efficient methods to improve gully headcut erosion. However, the influencing mechanism of revegetation on the soil properties of the gully head (GHSP) is still unclear. Thus, this study hypothesized that the variations in the GHSP were influenced by vegetation diversity during nature revegetation, and the influence pathways were mainly root traits, aboveground dry biomass (ADB), and vegetation coverage (VC). We studied six grassland communities of the gully head with different natural revegetation ages. The findings showed that the GHSP were improved during 22-year revegetation. The interaction effect of vegetation diversity, roots, aboveground dry biomass, and vegetation coverage on the GHSP was 43 %. In addition, vegetation diversity significantly explained >70.3 % of the changes in the root traits, ADB, and VC of the gully head (P < 0.05). Therefore, we combined vegetation diversity, roots, ADB, and VC to establish the path model to explain the GHSP changes, and the goodness of fit of the model was 82.3 %. The results showed that the model explained 96.1 % of the variation in the GHSP, and the vegetation diversity of the gully head affected the GHSP through roots, ADB, and VC. Therefore, during nature revegetation, vegetation diversity dominates the improvement of the GHSP, which has important significance for designing an optimal vegetation restoration strategy to control gully erosion.

Evaluation of the effects of long-term natural and artificial restoration on vegetation characteristics, soil properties and their coupling coordinations.

Vegetation restoration is the most important factor to restrain soil and water loss in the Chinese Loess Plateau, and its effect is long-term. Among them, the coupling and coordination relationship between vegetation and soil is the key to the smooth implementation of ecological restoration and the project of returning farmland to forest and grassland. However, people have neglected whether the choice of vegetation restoration method is suitable for the development of ecological environment in this region, and whether vegetation and soil coexist harmoniously. In this paper, the typical watersheds with similar terrain environment but different vegetation restoration methods were selected as the research objects, which were Dongzhuanggou (natural restoration, NR) and Yangjiagou (artificial restoration, AR). Through vegetation investigation and soil physical property experiment, the comprehensive evaluation function was used to quantify the impact of restoration methods on vegetation characteristics and soil properties, and the vegetation-soil coupling model was used to explore the coordinated development of vegetation and soil under different restoration methods. The results showed that there were significant differences between the two restoration methods in terms of vegetation characteristics (P < 0.05). The vegetation diversity indices of NR were 1.59-4.81 times that of AR. For root characteristic indices, NR was 1.05-2.25 times that of AR. For soil physical properties, there was no significant difference between the two restoration methods (P > 0.05). The comprehensive evaluation function of vegetation (VCE) and soil (SCE) under NR were 0.74 and 0.42, respectively, while those under AR were 0.55 and 0.63, respectively. The comprehensive function showed that the vegetation population performance under NR was slightly better than that under AR, while the soil restoration effect was opposite. Under the two restoration methods, the vegetation-soil coupling relationship was barely coordinated (NR: 0.53; AR: 0.54), and both were the intermediate coordinated development mode. The vegetation diversity, tending level and soil management level should be improved simultaneously during the process of vegetation restoration on the Chinese Loess Plateau.

[Characteristics of soil anti-scouribility in gully head wall of grass-covering on the gullied Loess Plateau, Northwest China]. / 黄土高塬沟壑区草地沟头立壁土壤抗冲性特征.

Vegetation restoration in the Chinese Loess Plateau has significantly changed soil erosion process of gully head wall. In order to investigate the characteristics and controlling factors of soil anti-scour properties of gully head covered by grasses, we carried out indoor undamaged soil trench scouring tests. By using barren gully head as the control, the physical and chemical properties and anti-scouring characteristics of soil in different soil layers (0-10, 10-20, 20-40, 40-60, 60-80, 80-100 cm) of the vertical wall of gully with grass cover were analyzed. The results showed that water-stable aggregate content and cohesion in barren and grass-covering gully head decreased with soil depth. Soil organic matter content and soil anti-scouribility coefficient in barren gully head decreased with soil depth, while the two indicators for gully head covered by grass increased firstly and then decreased with soil depth, with the maximum value (24.30 g·kg-1 and 58.86 L·g-1) in 10-20 cm soil layer. Meanwhile, the soil anti-scouring coefficient in each soil layer of grass-covering gully head was 1.7-9.3 times of that in soil layer of barren gully head. Soil organic matter content, water-stable aggregate content, cohesion and root length density all presented significantly positive correlation with soil anti-scouribility, among which soil organic matter content had the highest coefficient (r=0.98). Results of this study might provide basic data for the study of headcut erosion mechanism in Loess Plateau gully region, and scientific basis for effective control of soil and water loss in this region.