Abstract:【Objective】As an important component of agricultural soil a major element for plant growth, soil organic carbon (SOC) plays an important role in the earth ecosystem. Huge amounts of organic carbon are stored in the soil, and even a minor change in soil carbon could significantly change the concentration of CO2 in the atmosphere, which drives global climate change. How SOC is distributed and stored is a very important factor affecting soil quality and SOC fixation.【Method】In this study, soil samples were collected from 5 soil layers in each of the 0～100cm soil profiles distributed in a 150m rectangular grid pattern in fields different in slope position (slope top, upper slope, middle slope, and down slope) and in land use (cultivated land, forest land, grassland, shrubland, terrace) in the Wangmaogou watershed of the Loess Plateau. A total of 1080 soil samples were gathered for analysis of how topographies and land use patterns infuence on content and distribution of SOC via Kriging interpolation.【Result】 Among the slope positions, the upper slopes were the highest, (4.49g•kg-1) and followed by the middle slopes (4.30g•kg-1), the down slopes (3.97 g•kg-1）and slope tops (3.34 g•kg-1) in average SOC content of the 0~100cm soil profiles, while among the land use patterns. the forest land (4.31 g•kg-1) was the highest, and followed by the terraced field (4.25 g•kg-1), grassland (4.12 g•kg-1), shrub land (3.82 g•kg-1) and cultivated land (3.47g•kg-1). Obviously the forest land, terraces, grassland and shrub land was 24.2%, 22.4%, 18.7% and 10.1%, respectively, higher than the cultivated land. SOC in the topsoil was more susceptible to environmental factors, like terracing and some other soil erosion control practices, which can obviously sequestrate SOC in the deep soil layers (>20 cm). Variance component estimation shows that interactions between land use, topography, depth, land use and topography had extremely significant influences on spatial distribution of SOC content (P <0.01). Topography contributed the most to total SOC, reaching 32.50%. And interactions between land use & topography explained 7.4% of the variability of SOC. Spatially, SOC was distributed in patch and increased with depth in the watershed, and turned to be homegeneeous in distribution. 【Conclusion】All the findings in this study may serve as theoretical foundations for water and soil conservation and evaluation of carbon sequestration effects.

Abstract:Engineering measures, and reasonable and stereoscopic collocation of shrub-grass vegetation were performed over on large tracts of barren hills and slopes. The results showed that in the forth growth year, the root system of Prunus davidiana reached 320~360 cm deep, soil water content in the soil layer 0~500 cm deep decreased by 2.1%~3.3% compared to that before reforestation and the thickness of dry soil layer was 150 cm. And in the eighth growing year, the roots distributed as deep as 480 cm, the dry-layer reached 300 cm in thickness, the lowest and highest soil water content was 4.2% and 8.4% respectively. And in the twelfth year, the dry soil layer became very distinct, reaching 350 cm in thickness, and, especially in the 50~400 cm soil layer, the lowest and highest soil water content was 5.0% and 8.6%, respectively. While in the sixteenth year, the dry soil layer distributed in the depth of 50~350 cm, and its thickness was 300 cm, with the lowest and highest soil water content reaching 4.3% and 6.6%, respectively. But by building leveled terrace fields, contour ditches and fishscale pits, soil water content increased by 0.7%~6.3% in the depth of 0~100 cm, 0.6%~4.6% in 100~300 cm and 1.4%~4.6% in 300~500 cm compared to that in the barren hill, respectively. It showed that applying reasonable combined measures plus afforestation could regulate soil moisture region with significant effect.