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Aggregate stability (MWD) is an important indicator of soil structure stability, and closely correlated to the processes of slope hydrology and soil erosion. Up to date, few studies are conducted to investigate the variation of MWD and influencing factors on the regional scale. In order to study the variation characteristics of MWD and influencing factors in the Loess Plateau, a 508 km long transect including seven sampling sites along the precipitation gradient was laid out in this study. The MWD of cropland, grassland and woodland on each sampling site were measured, in the meantime soil mechanical composition, bulk density, soil organic matter, root mass density and vegetation coverage of each sampling site were also measured. The results indicated that there existed significant (P<0.05) difference between the MWD of cropland and the MWD of grassland and woodland. Cropland had the largest mean MWD, followed by woodland; the cropland had the lowest mean MWD. The ratios of the mean MWD of grassland and woodland to the mean MWD of cropland were 2.94 and 2.39, respectively. The reason why the mean MWD of cropland was lowest could be explained by the fact that the lower soil organic matter and root mass density of cropland, and the disturbance of MWD induced by the frequent tillage operations. The MWD of cropland showed a decreasing trend from south to north along the precipitation gradient. The changing trends of the MWD of grassland and woodland were similar, which could be attributed to the similarity of the near surface characteristics (such as non-disturbance, high plant residue return) of grassland and woodland. The MWD of the Yijun was slightly lower than the MWD of Fuxian, and then the MWD of sampling sites showed a generally decreasing trend from Fuxian to Erdos. The MWD of cropland, and grassland and woodland were all influenced by soil organic matter, annual average precipitation and annual average temperature. In addition, there existed a significant (P<0.05) relationship between the MWD of cropland and soil mechanical composition, however the MWD of grassland and woodland were closely related to root mass density. The poor relationship between the MWD of cropland and root mass density were attributed to its low root mass density and frequent tillage operation. The fact that grassland and woodland had high soil organic matter and root mass density which masked the influence of soil mechanical composition well explained that no significant (P>0.05) relationship was found between the MWD of grassland and woodland and soil mechanical composition. The annual average precipitation and annual average temperature influenced the MWD of cropland, grassland and woodland by affecting root mass density and soil organic matter. No-linear regression demonstrated the MWD of cropland could be satisfactorily simulated by median soil grain size and annual average precipitation with the coefficients of determination (R2) of 0.95 and the Nash-Sutcliffe efficiency (NSE) of 0.99. The MWD of grassland and woodland could be simulated by soil organic matter, root mass density and annual average temperature. The simulated result of grassland and woodland was not as good as that of the cropland with the coefficients of determination (R2) of 0.64 and the Nash-Sutcliffe efficiency (NSE) of 0.64, which under-predicted low measured MWD and over-predicted high measured MWD. The results of this study will be of great significance in understanding the spatial variation of aggregate stability (MWD) along the precipitation gradient and its influencing factors, and analyzing the regional change law of hydrological and erosional processes. [ABSTRACT FROM AUTHOR]

Plant litter has been accumulated greatly along with vegetation restoration on the Loess Plateau. Besides covering soil surface, plant litter can be incorporated into topsoil under natural circumstances by soil splash, sediment deposition, and soil-dwelling animal activities. The distribution (covering soil surface or incorporated into surface soil) and the decomposition of plant litter can modify soil physical properties (including soil detachment capacity and soil erosion resistance) which are closely related to soil detachment process by overland flow. This study was conducted to investigate the effect of vegetation restoration on soil detachment process for black locust (Robinia pseudoacacia L.) stand with different ages of returning farmland on the Loess Plateau. The experiments were performed in the Zhifanggou small watershed (36°46'28"-36°46'42"N, 109°13'46"-109°16'03"E) in August, 2015. After a complete watershed survey, altogether 6 sampling sites were selected, including 1 corn (Zea mays L.) field and 5 black locust stands of 10, 15, 20, 30, and 40 years. Soil samples were collected from the top 5 cm soil layer using a circular steel ring with 10.0 cm diameter and 5.0 cm depth. Soil detachment was measured in a 4.0 m long, 0.35 m wide flume. Prior to the test of soil detachment capacity, soil samples were saturated in a container for 8 h and drained for 12 h. Then the soil samples were inserted into a hole (0.5 m away from the outlet of flume) on the flume bed and were scoured by flowing water under designed flow shear stress. A total of 180 soil samples were collected from different aged black locust stands and scoured under 6 flow shear stresses. In this study, 6 combinations of flow rates and slopes were applied to obtain 6 different flow shear stresses (5.4-17.4Pa) with similar intervals. Rill erodibility is defined as the increase in soil detachment capacity per unit increase in flow shear stress. Critical shear stress is a threshold parameter defined as the value above which a rapid increase in soil detachment capacity per unit increase in shear stress occurs. When the measured detachment capacity was plotted against the flow shear stress, rill erodibility and critical shear stress could be determined from the slope of the fitted straight line and its intercept on the X axis. The results showed that the density of plant litter in 0-5 cm surface soil was 0.065, 0.175, 0.316, 0.289 and 0.171 kg/m2 for 10, 15, 20, 30, and 40-year-old black locust stands, respectively, with the mean value of 0.20 kg/m2. The 40-year-old black locust stands had more capillary porosities, soil organic matter, and water stable aggregates, and smaller soil bulk density. Soil detachment capacity decreased by 49.8%, 73.9%, 85.0%, 86.0%, and 87.0% for 10, 15, 20, 30, and 40-year-old black locust stands, respectively, compared to the control. The soil detachment capacity decreased significantly over time as an exponential function (R2=0.97, P=0.006). The control (corn field) had the biggest rill erodibility (0.29 s/m), and the 40-year-old black locust stand had the smallest rill erodibility (0.04 s/m). Compared to the control, the rill erodibility for 40-year-old black locust stand was reduced by 86.3%. The black locust stand with 20 years had the biggest critical shear stress (4.78 Pa), and the control had the smallest critical shear stress (4.15 Pa). The critical shear stress increased by 10.1% for 40-year-old black locust stand compared to the control. The critical shear stress increased significantly with litter density as a linear function (R2=0.67, P=0.046). Rill erodibility was better than critical shear stress in the respect of reflecting the variation of soil detachment. The soil-plant litter system has a larger erosion resistance to flowing water than the control. Importantly, the soil-root system may have a larger structural stability and erosion resistance to flowing water than soil-plant litter system. [ABSTRACT FROM AUTHOR]

Gully erosion is one of the major contributors to severe land degradation in the Yuanmou Dry-hot Valley Region of Southwest China, due to the frequent erosive precipitation events in the rainy season and special lithological conditions. Most active gullies in the study area are characterized by steep gully headcut, and headward erosion has been recognized as the main process of gully expansion. In particular, scour holes are often developed in the open side of gully headcut, which cause the frequent collapse of overhanging layers of gully heads in the process of headward erosion, and thus accelerate gully headward development. It is, therefore, of great significance to carry out the study on the collapse of overhanging layers of gully heads during the gully headward processes. In this study, based on the field investigations, a generalized physical model of active gully head was established. Then, the stability of overhanging layer of active gully head was studied by the momental analysis method, considering the main factors i.e. headcut height, the shape of scour hole and the development of tension crack at gully head. The results showed that the stability of overhanging layer of gully head was affected by various factors, such as the soil water content, the height of scour hole, the location and depth of tension crack. The results also indicated the difficulty in studying the mechanism of the gully head collapse process. When other parameters were assigned with default values, the stability of overhanging layer of gully head was decreased with soil water content and depth of tension crack (p<0.001) as an exponential function, and decreased with height of scour hole (p<0.001) as a linear function. As the distance between the tension crack and the headcut wall increased, the stability of overhanging layer increased significantly (p<0.001). Furthermore, to test the reliability of the evaluation of stability of overhanging layers, 4 gully heads molded by man with scour holes and the flow scouring experiments were employed in the field. When the collapse of overhanging layers of gully heads occurred, soil water contents of the whole collapse soil and the collapse surface soil were determined by oven-dry method with 6 repetitions. The results showed that the measured soil water contents of the whole collapse soil and the collapse surface soil were 69.8%-87.6% and 82.0%-95.5% of the model estimated values, separately. The small difference in soil water contents between the measured values and estimated values also indicated that this evaluation model was very valuable for the prediction of the collapse of overhanging layers of gully heads in the Yuanmou Dry-hot Valley Region. Moreover, the soil water content of the collapse surface soil was much closer to the model estimated value, which demonstrated that the development of tension crack on the overhanging layers of gully heads did affect their stability. These results are helpful to supplement meaningful theories to the erosion mechanism study and conservation practices implementation of active gully heads in the Yuanmou Dry-hot Valley Region. However, due to the complexity and uncertainty of the collapse of overhanging layers, further studies are needed to focus on the process of collapse of active gully heads, which can further improve the evaluation reliability of the stability model. [ABSTRACT FROM AUTHOR]