The Spatial Distribution And Storage Of Soil Organic Carbon And Nitrogen Of Hilly Region Of Loess Plateau

The loess plateau is the main ecological fragile regions in China and it is also the keyarea of ecological restoration and reconstruction. As an important carbon sink area, it gainsmore and more attention with the implementation of the Cropland Conversion to Forest andGrassland （CCFG）. At present, the soil organic carbon transformation, decomposition andaccumulation mechanism at different conditions, and the interaction with further climaticchange need to be studied under the background of vegetation restoration and construction inlarge area on the loess plateau. In this paper, the typical small and medium watersheds in theloess hilly-gully region were chosen with the aim of studying soil organic carbon reserves andthe change of soil organic C and total N in the small and medium watersheds under thedifferent land uses of typical CCFG small watersheds. The objective of this study is toprovide theoretical basis for evaluation and forecast soil carbon sequestration effect atregional vegetation restoration area, and the establishment of the regional soil organic carbonchange law. The main results are as follows:（1） The results showed that the soil organic carbon content in different soil layers wasmedium variation in this region, it increased as the soil depth decreased. The total soil organiccarbon reserves at0-60was about4.73×10⁴t, the reserves at natural grassland and forestregions was higher, which took up80%of the total amount. Under different land use, the soilorganic carbon content in the Zhifanggou watershed was significantly different, with the orderof natural grassland>shrub forest> arbor land> dam farmland>terrace farmland> slopefarmland>abandoned land>orchard.（2） The soil organic C and total N content, in the Yanhe watershed, showed thefollowing rules: the shady slope was higher than sunny slope, the half-shady slope was higherthan the half-sunny slope, the hilltop and bottom slopes were higher than the middle slope.Soil organic C and total N decreased with the altitude increasing, and had a sharp increasewith the1300-1400m then went down smoothly. In the different land use patterns, soilorganic C and total N content was as follows: forest land （arboreal lands, shrub land）> grass（natural grassland, artificial grass, the abandoned grassland）> orchard> farmland. When the land uses shift from grassland land to forest land, soil organic C and total N content not onlyshowed an increase in amount, but the distribution level also changed from the “low” to “flat”.In Yanhe watershed, Soil organic C and total N was the highest in the forest zone, followedby the forest steppe zones, the steppe zones was lowest. Soil organic C and total N contentshowed an obvious decrease during the transition zone of the forest to forest steppe, and tosteppe.（3） In the three vegetation belts in research regions, soil microbial biomass carbon （MBC）content was decreased with soil increasing. The MBC content showed an increase trend fromthe steppe zone to forest zone. Soil microbial biomass nitrogen （MBN） was higher in thesurface layer than in the lower layer. The MBN content also showed an increase trend fromthe steppe zone to forest zone. In the different land uses under the various vegetation belts, theMBC and MBN were significantly higher in the arboreal land than in the farmland. The MBCwas highest in the arboreal land, followed by farmland, natural grassland and abandoned land,the shrub land was lowest. The MBN was high in the grassland. In the forest steppe zone, theMBC was the lowest in the cropland, it was high in the abandoned land, arboreal lands andshrub land; the MBN was high in the farmland and shrub land, it was low in the abandonedland and arboreal land.（4） In different land use patterns, the change range of soil organic C density was2-3.87kg·m^-2and total N density was0.18-0.36kg·m^-2. The content and density of soil organic Chad the same change trend. Moreover, in the same land uses, the average soil organic Cdensity in the certain depth showed the feature of “surface-aggregation”. As a whole, the soilorganic C and N reserves in Yanhe watershed were2.24×10⁷t and6.03×10⁶t respectively.