| Loess Plateau was with fragmented topographies , complex and diverse land uses , in which the typical erosion characteristics of gully region and hilly region were shaped at local scale. Soil organic carbon storage in Loess region was based on accurate factors on spatial distribution of SOC in local scale. In this study, the spatial distribution of SOC at different depths was studied at a typical watershed of Yanggou in hilly region of loess Plateau, based on three topographies (tableland ,slopeland, gullyland) and eight landuses (farmland, orchard, manmade and secondary grassland, manmade and secondary shrubland and manmade and secondary woodland) , 314 soil surface (0-20cm) samples and 53 profile (0-100cm) samples were collected to investigate effects of topographies and landuses on spatial distribution of soil organic carbon in the watershed. The spatial distribution map of soil organic carbon was generated by using the raster calculation function of ArcGIS9.2 to overlap the longitude map , latitude map, slope map and coded topographic map, land use map, aspect map according to multiple linear regression equation calculation, soil organic carbon content in different topographies and land uses was statistical. The main results are as follows:The surface (0-20cm) SOC density was 1.63 kg3s-2 and 1m SOC density was 5.04 kg·m-2 at the watershed scale. Soil organic carbon reserves at 1 m soil layer was 217.6×103 t , and 67.5% of SOC reserve was at subsoil (20-100cm).Topographies and landuses significantly (P <0.0001) affects spatial distribution of soil surface organic carbon in Yangou watershed, and their interaction significantly (P =0.0511) impact on the spatial distribution of soil organic carbon .Under the influence of topographies, the spatial distribution of soil surface organic carbon expressed as gullyland > slopeland> tableland Trends, and the soil surface organic carbon contents of gullyland (8.0 g·kg-1) and slopeland(7.1 g·kg-1)were as 1.9 ,1.7 times as tableland (4.2 g·kg-1) respectively.Under the influence of landuses, the spatial distribution of soil surface organic carbon performanced for secondary woodland> secondary shrubland> manmade woodland> secondary grassland> manmade shrubland> manmade grassland> farmland> orchard. Under the influence of their interaction , soil surface organic carbon of farmland, orchards and grassland were gradually concentrated on gullyland under the condition of soil slope erosion. Forests (shrubland and woodland) were different from farmland ,orchard and grassland , and reflected that soil surface organic carbon contents of the slopeland were much higher than the gullyland . The soil organic carbon content of the slopeland of secondary woodland (24.6 g·kg-1) which was 1.5 time of gullyland (16.4 g·kg-1)reached a significant level compared with gullyland.Topographies , land uses ,depths and interaction of them significantly (p <0.01) affects spatial distribution of soil organic carbon in subsurface in Yangou watershed. SOC had different spatial distribution in topographies between subsurface (10-100) and surface (0-10).In 0-10cm soil layer , the content of soil organic carbon of slopeland (10.7 g·kg-1)was the highest ,followed by gullyland(8.9 g·kg-1), the content of SOC of tableland (4.4 g·kg-1)was the lowest. But the contents of SOC every layer in10-100cm expressed as gullyland > slopeland> tableland Trends, the average contents of SOC were 5.6 g·kg-1 ,4.5 g·kg-1 and 3.2 g·kg-1 . land uses significantly (p <0.05) affects spatial distribution of SOC in subsurface in Yangou watershed. Compared with farmland , the content of SOC of orchard in 0-40 cm decreased by 21%, yet increased by 13% in 80-100cm. The content of SOC of manmade shrubland (2.6 g·kg-1) was 19% lower than farmland , while that of manmade woodland (3.4 g·kg-1) was 6% higher than farmland. The content of SOC of secondary shrubland was higher than any other land uses in 20-100cm ,but it is significantly (p <0.05)different form other land uses in 40-100cm , the average contents was 5.3 g·kg-1 , which was 66% higher than farmland. The content of SOC of secondary woodland was higher than any other land uses in 0-20cm, but it was less differences form other land uses in 40-100cm. The storage of SOC of gullyland(5.04 kg·m-2)in subsurface (20-100) was the highest , accounted for 71.4% in 1m profile , the relative storage of SOC of slopeland and tableland accounted for 63.6% and 72.6 % respectively. The storages of SOC of secondary shrubland in subsurface (20-100cm) were the highest , it was 6.0 kg·m-2 , accounted for 64.7% in 1m profile, while the relative storage of secondary woodland was the lowest , only accounted for 49.7% . The storages of SOC of farmland and orchard both accounted for more than 70% of 1m profile.Land use has greater effect on organic carbon content of the upper soil layers (0-40cm) than topography, which covered the influence of topography under soil erosion laws, but topography has greater influence on the subsoil (40-100cm), so we can see the clear significant difference between gullyland and tableland or slopleland.The topography of tableland was dominated by middle value plaque (50%) and low value plaque (48%); slopeland by middle value plaque (62%) ,followed by low value plaque (22%); slopeland by middle value plaque (70%) ,followed by low value plaque (23%). Tableland ,slopeland and gullyland were dominated by low value plaque in10-40cm, accounted for 90%, 80% and 75% respectively. Also dominated by low value plaque in 40-100 cm and low value green plaque of gullyland accounted for 34%, much higher than slopeland (8%) and tableland (13%). For land use, It was significant effect of land use on spatial distribution in 0-40 soil layers ,but no significant in 40-100 cm.The high value plaque of woodland, shrubland and grassland accounted for 18%, 47% and 10% respectively, while plain land, farmland and orchard were no high value plaque and the middle value plague of them accounted for 80%, 53%, 85%, 73%, 39% and 23% respectively. The middle value plaques of woodland, shrubland , grassland, plain land, farmland and orchard in 10-40 cm accounted for 21%, 46%, 22%, 19%, 5% and 4 % respectively. But in 40-100cm, the SOC of all land uses were at low value plaque. For aspect, the SOC contents of the area of semi-shady (East + northeast + North) were the highest, while the area of semi-sunny (West + South West + South) the lowest .
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