Soil Detachment Process And Its Temporal Variation Under The Influence Of Plant Litter

Plant litter accumulated greatly along vegetation restoration on the Loess Plateau. Despite covering on soil surface, plant litter can be incorporated into topsoil under natural circumstances by soil splash, sediment deposition, and soil-dwelling animal activities. The distribution(cover on soil surface or incorporation into surface soil) and the decomposition of plant litter can modify soil physical properties which closely related to soil detachment process by overland flow(including soil detachment capacity and soil erosion resistance). However, few studies have been conducted to quantify the the effects of plant litter distribution(cover on soil surface, incorporated into surface soil) and related decomposition on soil detachment process by overland flow.This study was performed 4 experiments to investigate, respectively, the effects of litter cover on soil detachment process by overland flow, the effects of litter incorporation on soil detachment process, temporal variation in soil erosion resistance for soil incorporated with plant litters, and the effect of vegetation restoration on soil detachment process for different aged black locust(Robinia pseudoacacia L.) on afforested farmland in the Loess Plateau. The former three experiments was under controlling conditions and 390 soil samples, 480 soil samples, and 1080 soil samples were collected from corresponding controlling plots, respectively. For the last one experiment, 180 soil samples were collected from different aged black locust stands. A total of 2130 soil samples were collected for all experiments and scoured under different flow shear stresses. The results showed that:(1) A rapid accumulation of plant litter after the cropland was newly converted to black locust stands. The relationship between litter accumulation and forest age can be simulated well by logistic curve(R2 = 0.99, P < 0.001). Soil bulk density and shear strength decreased significantly with litter cover percentage and implied that litter cover could prevent soil from crusting and consolidation effectively. Soil detachment capacity was affected significantly by litter cover percentage and increased with litter cover percentage. But when litter cover percentage exceeded 50%, the differences between different percentages were not significant. No difference was found between three different litter species. Rill erodibility increased with litter cover percentage as a power function(R2=0.97). Critical shear stress decreased exponentially with litter cover percentage(R2=0.99). Rill erodibility or critical shear stress was closely related to the changes in soil bulk density and shear strength caused by sheltering effect of litter cover.(2) A widely distribution was found in surface soil in forest land of black locust. Soil-litter mixture was mainly resulted from soil splash by rainfall. The litter mass density was ranged from 0.07-1.08 kg m-2 with a mean value of 0.32 kg m-2. Litter mass density was increasing until 20 year aged black locust stands and then decreasing. The relationship between litter mass density and forest age can be simulated by peak curve(R2 = 0.94, P = 0.013). Soil detachment capacity decreased exponentially with incorporated plant litter rate. A threshold of 0.35 kg m-2 of litter was needed to be incorporated to provide protection of soil from overland flow erosion. The effects of litter incorporation rate on soil detachment capacity was not significant when the incorporation rate was greater than 0.35 kg m-2. Rill erodibility also decreased exponentially with the incorporated plant litter rate. The shape of plant litter fragments was hypothesized to account for the variations in the effects of different incorporated litter species on soil detachment capacity and rill erodibility. A distinguishable increasing linear trend was observed between critical shear stress and litter incorporated rate, but with a weak correlation. Critical shear stress also increased with incorporated rate and was related to soil cohesion.(3) Soil detachment capacity and rill erodibility were decreasing with time for all treatments of bare soil and soil with plant litters. The critical shear stress was of small data range. Plant litter in surface soil was effective to enhance soil resistance to flowing water erosion. Compared to bare soil, rill erodibilities of soils incorporated with litter of black locust, sea buckthorn, and green bristle grass decreased by 24.3%, 33.5%, and 34.8%. The temporal variations in rill erodibility of bare and litter incorporated soils were similar. Rill erodibility decreased significantly over time as an exponential function for both bare and litter incorporated soils. The relative rill erodibility of three litter incorporated soils increased over time as a power function. The fitted critical shear stress increased exponentially over time. The temporal variations in rill erodibility could be explained by the temporal variations in soil consolidation, water stable aggregate, and litter decomposition. Rill erodibility could be well estimated by soil bulk density, water stable aggregate, and litter mass density(NSE = 0.92) in the condition of litter incorporation within a growing season.(4) Soil detachment capacity decreased significantly over time as an exponential function(R2 = 0.82, P = 0.013). The rill erodibility was reduced 86.3%, the critical shear stress increased 10.1% for 40 year aged black locust stands compared to control(maize field, Zea mays L.). Rill erodibility was better than critical shear stress in the respect of reflecting the variation of soil detachment. Rill erodibility could be well estimated by water stable aggregate, litter mass density in surface soil(NSE = 0.99) under the interactive effects of litter cover, incorporation, and decomposition in the black locust stands with 0-40 age bracket.