Study On The Spatial Distribution Of Soil Organic Carbon And It's Influencing Factors In Small Watershed Of Karst Landform

Soil organic carbon of Karst landform plays an important role in global carbon cycle. but study on soil organic carbon is very difficult because of it's characteristic geological background, vegetation overcast and soil with signif icant spatial heterogeneity. Hou Zhai river's drainage basin belongs to small watershed of Plateau Karst landform with fractured landscape, characteristic geological background and complex spatial distribution of soil. That lead high degree of difficulty to the study of soil organic carbon. Soil profile was made at this watershed based on 150m×150m grid method, then soil sample was collected in layers, while background information was surveyed. In the next moment laboratory analysis and GIS technique has been used to ttudy the spatial distribution of soil organic carbon and it's influencing factors. This has theoretical and practical significance of reasonable development of it's soil resources as well as restoration of Karst ecosystem. The main results are as follows:1.The spatial distribution characteristics of soil organic carbon in watershed(1)Soil organic carbon in watershed has characteristic of high contents, high variability and low density. The average soil organic carbon content in topsoil is 25.07g·kg-1, ranges from 1.61g·kg-1 to 119.11g·kg-1; the average soil organic carbon content in profile soil is 20.71g·kg-1, ranges from 1.35g·kg-1 to 119.11g·kg-1. The variable range of soil organic carbon content of each soil layer is 52.68~75.28%, showing a moderate transformation with a high variability. Soil organic carbon content in watershed stay at a high level, 91.7% of it at surface soil samples are greater than 11.6g·kg-1. But the soil organic carbon density is a little bit small, with a average soil organic carbon density of 102.05 t C·hm-2, lower than the national average level( 105.30 t C·hm-2).(2) Soil organic carbon content in watershed shows a fragmentized, patch distribution pattern, decreased form scope of upstream to scope of downstream. The southeast contents more than north west, agrees well with the changes of landform in general. The deeper the soil layer,the more serious the fragmentized, patch distribution. The spatial distribution characteristics of soil organic carbon density in each layer conforms with the distribution characteristics of soil organic carbon content in each layer, but the distribution of profile soil organic carbon density has homogenize tendency, the area whose organic carbon density is higher distributes scattered, except for a few obvious area which organic carbon density is low, plaque broken severely,the area which organic carbon density is high and the area which organic carbon density is low distribute Interleaved.(3)The average soil organic carbon content and density in watershed decreased with the increasing of soil depth and have obvious surface-aggregation. The soil organic carbon content and density of 0~40cm layer decreases rapidly with soil depth increase,the reduction variation range of the layer?40cm becomes small and has stable tendency. The soil organic carbon content and of 0~5cm soil layer is highest,it's 29.28g·kg-1,the soil organic carbon density of 0~10cm soil layer is highest, it's 29.57 t C·hm-2, the contribution that soil organic carbon density of 0~50cm soil layer to all profile is 69.97%, the upper layer of soil is the main storage of organic carbon.2.Influencingfactors of soil organic carbon in watershedSome factors like landform, environment, human element and other soil genes is greatly influencing the soil organic carbon content and density.(1)Soil genesis factors:The order of soil organic carbon content in the different parent rock is the soil of limestone and dolomite development>the soil of marl development>the soil of yellow Quaternary clay and sand shale;The density of soil organic carbon range from 97.21~111.01 t C·hm-2,and the order is: the soil from limestone>the soil from sand shale>the soil from development of yellow Quaternary clay>the soil from development of dolomite>the soil from development of limestone?The high content of soil organic carbon is lime soil,the followed is paddy soil,the minimum is yellow soil;the density of soil organic carbon in the different soil genus sectional ranged from 74.60~140.62 t C·hm-2,and the order is 813>551>821>531>552>511>553>351>554?Generally,the content of soil organic carbon is increased with the decreasing of soil density and thickness.The more shallow soil,the smaller soil bulk density,the higher bare rock,the higher content of the soil organic c arbon is, the effect of gravel content related to the content of soil organic carbon is more complex, Overall the density of soil organic carbon increase with the increasing of soil bulk density and thickness, and decreases with the increasing of gravel content. the effect of bare ratio of rock in the density of soil organic carbon is more complicated.(2)Landform factors:Higher ground at high soil organic carbon content, low content of low-lying place, Bit different slope uphill soil organic carbon content>crest>middle slope>slope>toe>depression. Different slope positions sectional soil organic carbon density in the range of 82.57~112.29 t C·hm-2, Order:depression>toe>middle slope>crest>Downhill>uphill; different aspect of soil organic carbon content of the order on the southern slope>North Slope>West Slope>East Slope>no aspect, the Soil organic carbon a density in the range of 90.95~109.85 t C·hm-2, order: no aspect>North slope>east slope>western slope>south slope.Generally, Soil organic carbon content and soil organic carbon density with slope and elevation increases.The Soil organic carbon density decreases with the slope. The soil organic carbon density in different altitude level area difference is not signif icant.(3)Environment and human element factors:Soil organic carbon content is higher than the shrub grassland,mixed higher than pure forest. Different vegetation soil organic carbon content of the order:evergreen and deciduous broadleaf forest>sparse shrub>coniferous forest>coniferous evergreen forest>deciduous broad-leaved forest>evergreen broadleaf forest>shrub land. Cross-section of different vegetation soil organic carbon density was not significantly different.Different niche soil organic carbon content in the order of rock soil surface>pit>gully>stone cistern>soil surface, The Soil organic carbon density is not significantly different.The order of soil organic carbon content in different land use Type is: forest land>shrub land>unused land>dry land>paddy field. The Soil organic carbon density ranged from 86.71~123.07 t C·hm-2, order: paddy>dry land>unused land>forest land>shrub land.3.Main inf luencing factors of soil organic carbon(1) Soil organic carbon content of profiles soil has a signally positive correlation with slope gradient, altitude and the rate of outcrop rock. while has a signally negative correlation with soil thickness. But soil organic carbon density of profiles soil contrast to that, it has a signally positive correlation with soil thickness and soil bulk density.(2)The result of PCA shows that landform factors(gradient, aspect, slope positions, slope length) are the main influencing factors of soil organic carbon in Hou Zhai river watershed and need pay high attention to it. Soil genes is factors(soil bulk density, parent material and soil thickness) are secondary factors that must be treated. Vegetation factors are third premeditated factors.