Effects of land restoration years on physical properties of rocky desertified soil in trough valley of the southeast China karst region

Severe deforestation has caused the water loss, soil erosion (including underground leakage), and rocky desertification in the trough valleys of the southeast China Karst region, limiting local social and economic developments. Land restoration projects, such as the Grain for Green project initiated in 1999 in China, become important to improve the local ecological environment and restore vegetation. In order to evaluate the soil quality that improved by land restoration, it is necessary to examine some physical properties of the soils (such as bulk density, compactness, porosity and water content) after the periods of restoration. Here, 38 sampling sites were selected in the range of 0-10° on the bedding slopes based on the remote-sensing image analysis, field investigation, and in-home personal interviews at the trough valley. The soil with thin layers on these slopes become venerable to erosion in this case. These sampling sites were divided into seven groups according to the years after the land restoration (0-5, 5-10, 10-15, 15-20, 20-25, 25-30, 45-50 a). In each sampling site, a examining pit with the depth of 30cm was dug to establish a soil profile align to the cross-section of the soil layers during the examination test. According to the depth range of the soil layer, each soil profile was divided into two layers, 0-15 and 15-30 cm. In these two layers, the compactness of soils was measured by a compaction meter before soil sampling. Soil samples were then collected from both layers by sampling rings in order to measure some physical properties of soils, such as the bulk density, porosity and water content, after field investigations. Vegetation and landforms around the sampling sites were also noted during the samples collection. This study also proposed a feasible mechanism of land restoration to promote the local environment of sampling sites in the through valley of this Karst region. Three main findings can be made from these examined results. 1) Physical properties, especially in the shallow layer of soils, were still deteriorated from the start of land restoration to the period of 15-20 a. The water contents and porosities of the soils in the shallow layer were reduced by 12.28%-14.75% and 8.79%-11.14%, respectively, whereas the bulk densities and compactness were increased by 10.06%-13.82% and 54.09%-58.43%, respectively. 2) In the period of 20-50 a, the water content and porosity of soils in the shallow layer reached 40.65% to 60.38%, respectively, indicating an increase by 24.11% to 9.06% compared with those in the period of 0-5a. In the period of 45-50 a, the bulk density and compactness of soils were 0.93 g/cm3 and 7.57 kg/cm2, indicating much higher soil quality of the ecological system than that of other short periods after land restoration. The low-growing plants, such as grasses and shrubs, were dominant in the period of 0-20 a after land restoration, as well as there was no litter biomass layer on the ground surface of the soil. But in the long run, the planting species then gradually changed to an arbor-shrub-grass type. Arboreal plants flourished and the litter biomass layer appeared in the period of 40-50 a, indicating an obvious improvement of the water-retaining capacity and fertility of the soils. 3) As the soil layers in the Karst region were quite thin, there was no transition interlayer. A new methodological concept, "roasted effect", was therefore proposed in this study based on the traits of the quite thin soil layers, and it agrees with the transformation pattern of vegetation species from the shrub-grass to the arbor-shrub-grass type that occurred in the restoration period of 15-20 a. This study also demonstrates the combined effects of the ecological restoration on the dynamics mechanism of physical properties in the soil layers, particularly emphasis on the significance of the arboreal plant breeding in the entire restoration process. These findings can provide a potential theoretical guideline for further application of the ecological restoration at Karst regions in the southeast of China.