Assessment Of Soil Organic Carbon Distribution, Reserves And Vegetation Restoration Potential In Karst Rocky Desertification

In the karst region of Guizhou,special geological conditions and physical geography background rendered it a relatively weak ecological environment.Worsened by vegetation deterioration,soil erosion and quality degradation as a result of improper human activities,vast expanse of stony desertification landscape were formed,whose most protruding feature is run off of topsoil and nutrients and discontinuous distribution of complex habitats formed by exposed rocks and earth cover separated by rocks.Soil quality,as the key to restore vegetation,relies on organic carbon(material)to improve soil fertility.To research distribution of soil organic carbon and to accurately estimate organic carbon reserves during vegetation restoration process thus plays a crucial role in restoring vegetation and controlling soil nutrients in stony desertificated area.Special geological background and complex landform in karst region,however,lead to a high degree of spatial heterogeneity of soil,bringing various uncertain factors to estimate of organic carbon reserves of soil.How to accurately estimate organic carbon reserves of soil in karst region has long been an important and debatable issue.In consideration of this,small watershed soil of plateau-type karst was chosen as our research object,from whose 2,755 soil profiles we have collected 23,536 soil samples in total using grid method.On the basis of this,we studied characteristics of soil organic carbon content distribution,corrected various indexes thereof and reasonably optimized calculation formula thereof.By comparing and studying different estimate methods,we identified one method specific to estimate of soil organic carbon content in karst area and tried to work on vegetation restoration potential in karst area,so as to provide science support to development and reasonable utilization of soil resources in karst area as well as restoration of the degraded and fragile ecosystem.The main conclusions of our research are as follows:(1)Distribution pattern of Soil organic carbon content and its influencing factors in Karst rocky desertificationThe stony desertificated soil in karst area has a high but quite variable content of organic carbon.The topsoil of the watershed has an average organic carbon content of 25.07g/kg,varying within 1.61-119.11g/kg.The entire profile soil of the whole watershed has an average organic carbon content of 20.71 g/kg,varying within 1.35-119.11g/kg,with variable coefficient of 52.68%-75.28%,belonging to moderate degree variation.91.7% sample points from the topsoil exhibit an organic carbon content of higher than 11.6g/kg,indicating that the organic carbon content in the watershed soil is at a relatively high level.With the increase of soil depth,organic carbon content of soil gradually decreases,with decreasing amplitude varied among different soil genus.For different slope locations and directions,the sequence of organic carbon content is: sunny slope > shady slope,forest land > shrub and grass land > dry land > paddy field.The organic carbon content of soil shows a significantly positive relation with slope gradient,elevation,and rock outcropping ratio,and a significantly negative relation with soil layer thickness and soil bulk density.Due to high heterogeneity of soil in karst area,the correlation between gravel contents and organic carbon content of soil is not significant.The PCA(principal component analysis)results indicate that elevation,slope gradient and soil type are dominant factors affecting distribution of organic carbon in Houzhai River watershed soil,which must be paid with high attention during estimate of organic carbon reserves in soil.(2)Spatial heterogeneity of organic carbon in karst soil under different sampling sizes and its influencing factorsGrid sampling method was adopted and the organic carbon content obtained from samples collected from 150 m×150m grids was taken as the reference data.Five grid sizes(300m×300m,450m×450m,600m×600m,750m×750m and 900m×900m)were adopted for sample subset collection.Ordinary Kriging interpolation method was employed to analyze and predict spatial variation and pattern of samples.The study results indicate that the sampling size has a significant effect on the prediction accuracy of spatial distribution of soil organic carbon.In the karst watershed scale,sampling size of 450m×450m is a relatively proper size for soil samples collected for soil organic carbon research.With increase of sampling size,the terrain factors(elevation and gradient)have a relatively small effect on sampling precision,while the rock outcropping ratio and gravel content have a relatively large effect on sampling precision.The number of optimal samples obtained by Cochran method is obviously on the low side.Only using Cochran formula on the karst watershed scale may lead to inaccurate space prediction of soil organic carbon;therefore the spatial variation of organic carbon in karst soil should be fully taken into consideration.Rational spacing of soil sampling points played a more crucial role than simply increasing number of sampling points.(3)Spatial heterogeneity of organic carbon reserves in karst soil and estimation method optimizationIn Houzhai River watershed,organic carbon reserves are 1.48×108 kg for soil with depth of 10 cm,2.65×108 kg for soil with depth of 20 cm,3.43×108 kg for soil with depth of 30 cm,and 5.39×108 kg for soil with depth of 100 cm.C0 – nugget of organic carbon reserves of each soil layer,increases with depth of soil layer and reaches its maximum value for soil with depth from 0 to 100 cm.All the four types of soil in different thicknesses exhibit a trend of low in the middle part,high in the four corners and lowest in the south part.Elevation,slope gradient,rock outcropping ratio,and gravel content are dominant factors affecting spatial heterogeneity of organic carbon reserves in karst small watershed soil.With a relatively high isomerism,different indexes affect estimate of organic carbon reserves in karst soil in the following degree sequence: soil thickness > rock outcropping ratio > gravel content > soil organic carbon content > soil bulk density.Using the modified calculation formula,the 20 cm thick soil in the watershed was estimated to have a density of 3.53-5.44kg/m2,1.24 kg/m2 on average;the soil with depth within 100 cm was estimated to have a density of 4.44-14.50kg/m2,12.12 kg/m2 on average.(4)Variation of organic carbon reserves in karst watershed soil among different estimate methods and precision studyWith soil profile adding method as the reference method,applicability of soil type method,land utilization type method and slope direction method(all the methods based on corrected rock outcropping ratio and gravel content)to estimate of organic carbon reserves in karst soil with different depths were explored.The results indicate that rock outcropping ratio and gravel content have a relatively large impact on the estimate results of soil organic carbon reserves.Based on the corrected rock outcropping ratio and gravel content,the organic carbon reserves for soil with depth of 0-20 cm,0-30 cm,and 0-100 cm are 3.42×108 kg,4.49×108 kg,and 7.38×108 kg,respectively if using soil type method,in which the rock outcropping ratio and gravel content have the greatest effect on white sandy soil;the aforementioned values are 3.20×108 kg,4.16×108 kg,and 6.07×108 kg,respectively if using land utilization type method,in which the rock outcropping ratio and gravel content have the greatest effect on wasteland;the same values are 3.19×108 kg,4.12×108 kg,and 6.28×108 kg,respectively if using slope direction method,in which the rock outcropping ratio and gravel content affect the southward slope soil the most.The same values are 3.07×108 kg,3.99×108 kg,and 6.48×108 kg,respectively if using space interpolation method with corrected rock outcropping ratio and gravel content.In terms of watershed scale,the estimate mode was expanded to “land utilization type method” and “slope direction method” from “vertical stratification + horizontal classification” mode,which can estimate organic carbon reserves in a relatively accurate manner.Slope direction method delivered the best estimate for topsoil,while for soil of 100 cm depth land utilization type method delivered the best estimate.(5)Quality characteristics of soil in karst area and influence factors thereofAs a result of complex and diverse terrain in karst area,quality of soil in different sample points varied.Soil bulk density,averaged at 1.17-1.41 g/cm3,showed a trend of increase with soil layer depth firstly and then stabilized.Thickness of soil varied among different types of soil,with that of yellow mud field averaging at 86.98±22.58 cm which is the highest value and that of white sandy soil averaging at 36.21±27.98 cm which is the lowest value;in Houzhai River watershed,the area with soil layer thickness over 100 cm accounts for 51.32% of the total watershed area.Gravel content in the studied area took a percentage of 0-20.15% on average and gradually reduced to zero with increased depth of soil layer;its maximum value of 20.15% appeared in depth of 0-10 cm and minimum value of zero appeared in depth of 80-90 cm and 90-100 cm.The nine types of soil genus exhibited relatively large variation of gravel content at the same depth level;within the depth of 0-10 cm of topsoil,yellow limestone soil has a gravel content of 31.03% which is the highest while the large soil clay has a gravel content of only 2.34%,which is the lowest.The soil organic matter,averaged at 28.27g/kg,was at a relatively high level;82.18% soil sample points belonged to the first three levels with 33.21% sample points in the first level.Among the various correlating factors that affect karst soil quality,rock outcropping ratio,gravel content and slope gradient are dominant influence factors for soil quality of Houzhai River watershed.Based on the principal component function model,this paper evaluated the soil quality of the backward River Basin,and found that the two major,grade three and grade four.Total soil areas are 60.84km2,Accounting for 82.03% of the total area.The soil quality was generally inthe middle and lower levels in the Houzhai River watershed.(6)Physical and chemical properties of karst soil under different revegetation types and revegetation potential thereofThe soil bulk density showed a trend of firstly increase and then decrease under different revegetation types.Vegetation exerted the most obvious effect on bulk density of topsoil.The soil thickness averaged at 30.07-56.03 cm under different vegetation types.For soil of the same thickness but under different restoration types.Organic matter content of soil under different vegetation types all reduced with increased depth of soil layer,from which an almost straight line decline was seen for shrubwood and grassland soil,a relatively large decline was seen for high-forest soil and a relatively slow decline seen for abandoned land and scrub-grassland.The organic matter content of economic fruit forest soil manifested a decreasing speed of three stages of “slow-fast-slow” with increased depth of soil layer.The arbor land has a relatively good soil quality,followed by grassland;the abandoned land has a relatively poor soil quality.Fuzzy mathematical method was employed to evaluate vegetation restoration potential in karst area,with results as follows: the arbor land has the highest vegetation restoration potential index-level IV;the shrubwood land and scrub-grassland has a vegetation restoration potential index of level III and that of grassland and economic fruit forest land is level II;the abandoned land showed the poorest restoration potential.