| In this research, on the basis of the simulating studies indoors and field, systematic researches were carried out to study related key problems of above-underground biomass dynamics, soil conservation mechanics of canopy, stems, litter, and root, and vertical changes of sediment yield on different soil depth of the vegetation on the natural and abandoned grassland. Based on the preliminary discussion on the soil and water conservation mechanics of grassland vegetation, following researches can be reached:1. Revealing the dynamics of vegetative cover characters, soil properties and vertical root distribution of vegetation on grassland in its rehabilitating succession process.During the rehabilitating succession process, both above and underground biomass of vegetation on returned farmlands changed with the rehabilitating time. Its above biomass increased with the increase of abandoned time. Compared to pioneer herbaceous species, perennial herbaceous grasses have larger above biomass, even under the similar vegetative cover. There were regular changes in root distribution patterns. During the initial period of the land abandoned, more roots were distributed in deeper soil. When the abandoned time was over 15 years, vegetation root system tended to concentrate in surface soil layer. To the vegetation on natural grassland, its root system was concentrated in surface 0~40cm soil layer. All the other root indexes, including root length, root surface area etc, showed similar distribution patterns.During the vegetation rehabilitation process, soil properties were changed correspondingly. Both soil organic matter content and water stable aggregate content (>0.25mm) decreased with the increase of soil depth, and increased with the increase of abandoned time, among which water stable aggregate with its diameter between l-2mm has the similar distribution pattern to that of the vertical root distribution characters.2. Effect of grassland vegetation on soil and water conservation increased with the increase of rehabilitating time.Simulating rainfall results indicated that under the cover of pioneer herbaceous species, both average and stable soil infiltration was higher than that of perennial herbaceous vegetation. Under the cover of the perennial herbaceous vegetation, soil loss was decreased clearly compared to that of the pioneer species. To the vegetation at the same succession stages, with the increase of vegetation cover, its stable infiltration decreased, together with its soil loss. The existence of microbiotic crust under the cover of perennial herbaceous vegetation lowered stable soil infiltration ratio.3. Effect of different human induced activities on soil and water loss process was different. Fire destroyed the effect of vegetation on soil and water conservation most seriously, followed with mowing and grazing.Effect of human activities on soil and water loss was simulated in this study. Results indicated that fire destroyed most surface vegetation, exposed soil under the direct raindrop splash, and increased the risk of soil erosion. On the other hand, fire helped to destroy the surface microbitic crust, which increased soil infiltration and improved the utilization of rainwater. Mowing was also a local common human disturbance to vegetation, it decrease the protection to vegetative cover to soil, and increased soil erosion. But its removing vegetation canopy also helped to destroy the surface microbiotic crust, and increased soil infiltration, which was similar to that of fire. Other human activities, including grazing, are also effective in destroy the surface microbiotic crust, and increased soil infiltration, which helped to improve the utilization efficiency of rainwater.4. The effects of different herbaceous vegetation parts on soil erosion were different. The existence of canopy diminished raindrop energy, stems and litter dispersed runoff and consequently runoff erosivity, root system improved soil resistance to runoff scouring.Simulating results in field indic...
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