Effects of Wind-Water Erosion and Topographic Factor on Soil Properties in the Loess Hilly Region of China.

Wind and water erosion processes can lead to soil degradation. Topographic factors also affect the variation of soil properties. The effect of topographic factors on soil properties in regions where wind and water erosion simultaneously occur remains complicated. To address this effect, we conducted this study to determine the relationships between the changes in wind-water erosion and soil properties in different topographic contexts. We collected soil samples from conical landforms with different slope characteristics and positions in the wind-water erosion crisscross region of China. We examined the soil 137Cs inventory, soil organic carbon (SOC), total nitrogen (TN), soil particles, soil water content (SWC), and biomass. 137Cs was applied to estimate soil erosion. The results show that the soil erosion rate followed the order of northwest slope > southwest slope > northeast slope > southeast slope. The soil erosion rate on the northwest slope was about 12.06-58.47% higher than on the other. Along the slopes, the soil erosion rate decreased from the upper to the lower regions, and was 65.65% higher at the upper slope than at the lower one. The change in soil erosion rate was closely related to soil properties. The contents of SOC, TN, clay, silt, SWC, and biomass on the northern slopes (northwest and northeast slopes) were lower than those on the southern slopes (southeast and southwest slopes), and they were lower at the upper slope than at the lower one. Redundancy analysis showed that the variation in soil properties was primarily affected by the slope aspect, and less affected by soil erosion, accounting for 56.1% and 30.9%, respectively. The results demonstrate that wind-water erosion accelerates the impact of topographic factors on soil properties under slope conditions. Our research improves our understanding of the mechanisms of soil degradation in gully regions where wind and water erosion simultaneously occur.

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The matching of root system is a key factor driving the resistance of plant community to soil erosion. In this study, Amoeba graphical method was used to establish a root framework erosion resistance index (ERIrf, %) from three dimensions of root morphology, quantity and spatial concerns to quantify the effective of root erosion resistance by plant community. We analyzed root growth characteristics of plant community from abandoned land, Caragana korshinskii and Robinia pseudoacacia communities in loess hilly area. The results showed that the parameters of constructing the root framework erosion resistance index included the acting coefficient of root framework (α), root density (Rb, kg·m-3), root framework degree (S), soil bulk density (ρ, g·cm-3), and soil and water conservation coefficient (φ). The equation could be expressed as ERIrf=α×Rd×S×φρ×100%. This root framework erosion resistance index well represented the erosion resistance effects of plant root system. Logarithmic function could better fit the relationship between soil erosion resis-tance ability and root framework erosion resistance index. Our findings would provide scientific reference for the construction of anti-erosion vegetation community and assessment of ecological construction.

Effects of land-use patterns on soil carbon and nitrogen variations along revegetated hillslopes in the Chinese Loess Plateau.

In water-limited areas, revegetation of abandoned croplands may lead to extensive land-use changes and considerable variations on soil carbon (C) and nitrogen (N). However, the impact of land-use patterns (i.e., the spatial combinations of different land-use types) on soil C and N variations following revegetation remains unclear. In this study, we measured soil organic carbon (SOC), total carbon (TC), and total nitrogen (TN) stocks to a depth of 200 cm in grassland (GL), shrubland (SL), young forestland (YF), and mature forestland (MF) under four land-use patterns in a catchment located in the Chinese Loess Plateau. The highest SOC, TC and TN stocks occurred in MF and the lowest was found in GL. Compared to every single land-use type, soil C and N stocks significantly increased under different land-use patterns. The highest SOC stock (6.51 kg m-2) was found in the GL-YF-SL pattern, and the highest TC stock (47.25 kg m-2) and TN stock (0.70 kg m-2) were both observed in the MF-YF pattern. SOC stocks showed significantly positive correlations with TC and TN stocks under different land-use patterns (p < 0.05), except for the GL-MF. The soil C-N interactions were stronger in the MF-SL and GL-YF-SL patterns compared to the GL-MF and MF-SL. Redundancy analysis indicated that the SOC, TC, and TN variations were well explained by aboveground biomass and land-use patterns, with accumulated variance of 41.6% and 54.2% in Axis 1 and Axis 2, respectively. The differences of soil C and N accumulation among land-use patterns were mostly related to different vegetation coverage and the intensity of soil erosion. This study indicates that creating proper spatial distribution of land-use types on hillslopes could benefit soil C and N sequestrations and ecosystem restoration in semi-arid environments.

The effect of climatic and edaphic factors on soil organic carbon turnover in hummocks based on δ13C on the Qinghai-Tibet Plateau.

Hummocks (thúfur, pounus) are peculiar landforms usually formed by repeated freeze-thaw processes and differential frost heave, and are common in frost soil regions, especially in the Qinghai-Tibet Plateau. However, little is known about the response of δ13C in soil organic carbon (δ13CSOC) to soil and climate properties in hummocks. The ß value indicates the decomposition rate of soil organic carbon (SOC) in soil, and was obtained from the slope of the regression between the log10-transformed SOC concentration and δ13CSOC in soil depth profiles. In this study, we investigated δ13CSOC and SOC contents along a soil profile (0-60 cm), together with edaphic and climatic properties, both in hummocks and control plots (alpine grasslands) on the northeastern Qinghai-Tibet Plateau. Then, the variations in δ13CSOC and ß values, and the main factors affecting them, were analyzed. The results show that δ13CSOC increases with soil depth, while SOC decreases both in the hummocks and control plots. However, ß values in the hummocks were significantly (P < 0.05) higher than in the control plots while δ13CSOC showed no difference between hummock and control. Redundancy analysis showed that altitude is the main control factor for δ13CSOC and ß in the hummocks. Climate type was the main factor affecting δ13CSOC in the control plots, while mean annual precipitation and soil fractal dimension were the main factors controlling ß. Overall, climate, rather than soil, is the key factor that affects the carbon turnover rate in the hummock in the northeastern QTP. The findings of this study will expand our understanding of the soil carbon cycle and δ13CSOC changes, especially in the case of hummocks.

Relative contributions of wind and water erosion to total soil loss and its effect on soil properties in sloping croplands of the Chinese Loess Plateau.

Wind and water erosion are two dominant types of erosion that lead to soil and nutrient losses. Wind and water erosion may occur simultaneously to varying extents in semi-arid regions. The contributions of wind and water erosion to total erosion and their effects on soil quality, however, remains elusive. We used cesium-137 (137Cs) inventories to estimate the total soil erosion and used the Revised Universal Soil Loss Equation (RUSLE) to quantify water erosion in sloping croplands. Wind erosion was estimated from the subtraction of the two. We also used 137Cs inventories to calculate total soil erosion and validate the relationships of the soil quality and erosion at different slope aspects and positions. The results showed that wind erosion (1460tkm-2a-1) on northwest-facing slope was responsible for approximately 39.7% of the total soil loss, and water erosion (2216tkm-2a-1) accounted for approximately 60.3%. The erosion rates were 58.8% higher on northwest- than on southeast-facing slopes. Northwest-facing slopes had lower soil organic carbon, total nitrogen, clay, and silt contents than southeast-facing slopes, and thus, the 137Cs inventories were lower, and the total soil erosions were higher on the northwest-facing slopes. The variations in soil physicochemical properties were related to total soil erosion. The lowest 137Cs inventories and nutrient contents were recorded at the upper positions on the northwest-facing slopes due to the successive occurrence of more severe wind and water erosion at the same site. The results indicated that wind and water could accelerate the spatial variability of erosion rate and soil properties and cause serious decreases in the nutrient contents in sloping fields. Our research could help researchers develop soil strategies to reduce soil erosion according to the dominant erosion type when it occurs in a hilly agricultural area.

Effects of revegetation and precipitation gradient on soil carbon and nitrogen variations in deep profiles on the Loess Plateau of China.

Precipitation is one of the most important factors affecting the variations in soil carbon (C) and nitrogen (N) following revegetation. However, the effects of revegetation and precipitation gradients on soil organic carbon (SOC), total nitrogen (TN) and C-N interactions in deep profiles over large scales are poorly understood. This study measured the SOC and TN stocks to depth of 300 cm in three revegetation types (grassland, shrubland and forestland) and paired cropland stands at seven sites along a precipitation gradient with mean annual precipitation (MAP) from 280 to 540 mm yr-1 in the Loess Plateau of China. The results showed that the SOC and TN stocks in the 0-300 cm profile increased along the precipitation gradient. Revegetation did not always result in accumulation of SOC and TN stocks, which depended on the precipitation condition and varied among different vegetation types. Grassland restoration resulted in more SOC and TN accumulation than shrubland and forestland in areas with MAP < 510 mm, whereas there were losses in SOC and TN following grass plantation in sites with MAP > 510 mm. The changes in SOC and TN stocks following revegetation (∆SOC and ∆TN) were significantly correlated with MAP in only the 0-20 cm layer, whereas the changes in the C/N ratio of each depth were significantly and negatively correlated with MAP. The correlations between ∆SOC and ∆TN were stronger in the 0-60 cm layer than that in the 60-300 cm layer, and an accumulation of 1 g TN was associated with approximately 7.9 g increase of SOC in the 0-300 cm profile following revegetation. This study indicated that the changes in soil C and N stocks following revegetation had different patterns along precipitation gradient and among depths, and grassland restoration and N fertilizer input benefitted soil C and N sequestration in drier areas.

[Impact of wind-water alternate erosion on the characteristics of sediment particles].

Wind and water are the two dominant erosion agents that caused soil and water losses in the wind-water alternate erosion region on the Loess Plateau. It is meaningful to study the impact of wind-water alternate erosion on the characteristics of soil particles for understanding the response of soil quality and environment to erosion. Through wind tunnel combined rainfall simulation, this paper studied the characteristics of the erosive sediment particles under the effect of wind-water alternate erosion. The results showed that the particles of 0-1 cm soil were coarsened by wind erosion at the wind speeds of 11 and 14 m x s(-1) compared with no wind erosion. Soil fine particles (< 0.01 mm) decreased by 9.8%-10.8%, and coarse particles (> 0.05 mm) increased by 16.8%-20.8%. The physical property of surface soil was changed by the wind erosion, which, in turn, caused an increase in finer particles content in the sediment. Compared with no wind erosion, fine particles (< 0.01 mm) in sediment under the water-wind alternate erosion increased by 2.7%-18.9% , and coarse particles (> 0.05 mm) decreased by 3.7%-9.3%. However, the changing trend of erosive sediment particles after the wind erosion at wind speeds of 11 and 14 m x s(-1) was different along with the rainfall intensity and duration. The erosive sediment particles at the rainfall intensities of 60, 80, 100 mm x h(-1) changed to greater extents than at the 150 mm x h(-1) rainfall intensity with longer than 15 min runoff flowing.

[Sediment-yielding process and its mechanisms of slope erosion in wind-water erosion crisscross region of Loess Plateau, Northwest China].

Due to the coupling effects of wind and water erosions in the wind-water erosion crisscross region of Loess Plateau, the slope erosion in the region was quite serious, and the erosion process was quite complicated. By using wind tunnel combined with simulated rainfall, this paper studied the sediment-yielding process and its mechanisms of slope erosion under the effects of wind-water alternate erosion, and quantitatively analyzed the efffects of wind erosion on water erosion and the relationships between wind and water erosions. There was an obvious positive interaction between wind and water erosions. Wind erosion promoted the development of microtopography, and altered the quantitative relationship between the sediment-yielding under water erosion and the variation of rainfall intensity. At the rainfall intensity of 60 and 80 mm x h(-1), the sediment-yielding without wind erosion decreased with the duration of rainfall and tended to be stable, but the sediment-yielding with wind erosion decreased to a certain valley value first, and then showed an increasing trend. At the rainfall intensity of 60, 80, and 100 mm x h(-1), the sediment-yielding with the wind erosion at speeds of 11 and 14 m x s(-1) increased by 7.3%-27.9% and 23.2%-39.0%, respectively, as compared with the sediment-yielding without wind erosion. At the rainfall intensity of 120 and 150 mm x h(-1) and in the rainfall duration of 15 minutes, the sediment-yielding with and without wind erosion presented a decreasing trend, but, with the increase of rainfall duration, the sediment-yielding with wind erosion showed a trend of decreasing first and increasing then, as compared with the sediment-yielding without wind erosion. The mechanisms of wind-water alternate erosion were complicated, reflecting in the mutual relation and mutual promotion of wind erosion and water erosion in the aspects of temporal-spatial distribution, energy supply, and action mode of erosion forces.