No abstract

Water erosion on hillslopes is a worldwide environmental problem, which is a rainfall‐induced process, especially extreme rainfall. The great intensity of extreme rainfall strongly enhances the power of overland flow to detach soil and transport sediment. Plant litter is one of the most important constituents of ecosystems that often covers the soil surface and can be incorporated into topsoil. However, little attention has been paid to its effect on flow hydraulics owing to the veiled nature. This study aimed to examine the effects of incorporated litter on the hydraulic properties under extreme rainfall condition. To reach this goal, six litter rates of 0, 0.05, 0.10, 0.20, 0.35, and 0.50 kg m−2 and four litter types collected from deciduous trees, coniferous trees, shrubs, and herbs were incorporated into topsoil. Then, simulated rainfall experiments were performed on five slope gradients (5°, 10°, 15°, 20°, and 25°) with an extreme rainfall intensity of 80 mm h−1. The results showed that Froude number and flow velocity of the overland flow decreased, whereas flow resistance increased exponentially with litter incorporation rate. Litter type had an influence on flow hydraulics, which can mainly be attributed to the variations in surface coverage of the exposed litter and the litter morphology. Flow velocity and Darcy–Weisbach coefficient increased markedly with slope gradient. However, the variation of slope gradient did not modify the relationships between flow hydraulics and incorporated litter rate. The random roughness, resulting from heterogeneous erosion due to the uneven protection of surface exposed litter, increased linearly with litter incorporated rate. As rainfall proceeded, flow hydraulics varied with incorporated litter rate and slope gradient complicatedly due to the increases in flow rate and coverage of the exposed litter and the modification of soil surface roughness.

Biological soil crusts (BSCs) are ubiquitous living covers that have been allowed to grow on abandoned farmlands over the Loess Plateau because the “Grain for Green” project was implemented in 1999 to control serious soil erosion. However, few studies have been conducted to quantify the effects of BSC coverage on soil hydraulic properties. This study was performed to assess the effects of BSC coverage on soil hydraulic properties, which are reflected by the soil sorptivity under an applied pressure of 0 (S0) and −3 (S3) cm, saturated hydraulic conductivity (Ks), wetting front depth (WFD), and mean pore radius (λm), for the Loess Plateau of China. Five classes of BSC coverage (i.e., 1–20%, 20–40%, 40–60%, 60–80%, and 80–100%) and a bare control were selected at both cyanobacteria‐ and moss‐covered sites to measure soil hydraulic properties using a disc infiltrometer under 2 consecutive pressure heads of 0 and −3 cm, allowing the direct calculation of S0, S3, Ks, and λm. The WFD was measured onsite using a ruler immediately after the experiments of infiltration. The results indicated that both cyanobacteria and moss crusts were effective in changing the soil properties and impeding soil infiltration. The effects of moss were greater than those of cyanobacteria. Compared to those of the control, the S0, S3, Ks, WFD, and λm values of cyanobacteria‐covered soils were reduced by 13.7%, 11.0%, 13.3%, 10.6%, and 12.6% on average, and those of moss‐covered soils were reduced by 27.6%, 22.1%, 29.5%, 22.2%, and 25.9%, respectively. The relative soil sorptivity under pressures of 0 (RS0) and −3 (RS3) cm, the relative saturated hydraulic conductivity (RKs), the relative wetting front depth (RWFD), and the relative mean pore radius (Rλm) decreased exponentially with coverage for both cyanobacteria‐ and moss‐covered soils. The rates of decrease in RS0, RS3, RKs, RWFD, and Rλm of cyanobacteria were significantly slower than those of moss, especially for the coverage of 0–40%, with smaller ranges. The variations of soil hydraulic properties with BSC coverage were closely related to the change in soil clay content driven by the BSC coverage on the Loess Plateau. The results are useful for simulating the hydraulic parameters of BSC‐covered soils in arid and semiarid areas.

Plant litter can either cover on soil surface or be incorporated into top‐soil layer in natural ecosystems. Their effects on infiltration and soil erosion are likely quite different. This study was performed to compare the effects of litter covering on soil surface and being incorporated into top‐soil layer on infiltration and soil erosion under simulated rainfall. Four litter types (needle‐leaf, broad‐leaf, brush, and herb) were collected from fields and applied to cover on soil surface or to be incorporated into top‐soil layer (5 cm) at the same rate (0.2 kg/m2). The simulated rainfalls (40 and 80 mm/hr) were run at two slope angles (10° and 20°). The results showed that the mean infiltration rate of litter covering treatment was 1.4 times as great as that of litter incorporated. Litter covering enhanced infiltration via protecting surface from soil sealing. Whereas, litter incorporation affected infiltration by its water repellency. Soil erosion of litter incorporated treatment was 5.4 times as large as that of litter covered treatment, which was attributed to the changes in surface litter coverage and soil erosion resistance. Litter type affected soil erosion through the variations in litter coverage and litter morphology. For litter covering treatment, litter coverage can explain the major variance of soil loss on the slopes. Whereas, for litter incorporated treatment, both the influences of litter coverage and litter length on soil erosion resistance were considered necessary to well explain the variance of soil loss. The results also showed that the benefits of litter to control soil erosion declined with rainfall intensity and slope gradient for both covering and incorporated treatments. The results of this study are helpful to understand the mechanisms of litter influencing hydrological and erosion processes on hillslopes.

Plant litter can control land degradation by increasing soil organic matter and decreasing soil erosion by covering the soil surface or being incorporated into topsoil. However, the effect of litter incorporation on soil erosion is not clear. This study aimed to investigate the effect of litter incorporation into topsoil on interrill erosion. The poorly decomposed litter was collected from the field and incorporated into topsoil (5 cm) at six rates (ranging from 0 to 0.5 kg m −2 ). Laboratory rainfall (80 mm hr −1 ) experiments were conducted under five slope gradients (ranging from 9% to 47%).The results showed that the soil loss rates declined exponentially with increase in litter incorporation rate on slope gradients between 9% and 36%. However, on slope gradient of 47%, litter incorporation caused an increase in the soil loss rate. The efficiency of litter incorporation in controlling soil loss varied from −21.7% to 40.5% with a mean of 10.8%. It decreased linearly with the slope gradient (R 2 = 0.34). The decrease can be explained by two simultaneous but opposite effects: the effect of preventing erosion and that of promoting erosion. The former was induced by decreases in the effective erosive force of raindrop and overland flow exerted on soil, and soil erodibility, along with increases in sediment interception. The latter was caused by rapid saturation of the surface soil and concentrated overland flow. The results demonstrate the complexity of the effect of litter incorporation on soil erosion and can improve our understanding of litter in soil loss control.