The Distribution And Control Factors Of Soil Organic Carbon In The Karst Critical Zone Of Southwest China

Soil organic carbon pool is a critical part of the global carbon cycle.Soil organic carbon?SOC?sequestration is a key way to reduce soil carbon emission and mitigate global warming.Soil carbon and nitrogen nutrient cycling affect the physicochemical properties and productivity of soil,as an important factor limiting the ecological restoration in the karst region of Southwest China.The Chenqi catchment in Puding county of Guizhou province was selected as the study area of the karst critical zone.The effects of land use and slope position on soil structure,soil organic carbon and nitrogen sequestration were studied.Quantitative relationship between soil structure and SOC sequestration was studied.Indicators of the ecological restoration stage were sought.The roles of stable carbon isotope of soil aggregate-associated organic carbon in indicating C₃ and C₄vegetation composition and the decomposition rate of soil organic matter were studied.The main results are showed as follows:?1?The mass proportion of macro-aggregate,SOC content,soil organic nitrogen content,and C/N ratio of soil organic matter decreased as follows:secondary forest,bush,abandoned orchard,grassland and farmland.The distribution of soil aggregates,SOC content and soil organic nitrogen content in abandoned farmland were the same physical and geochemical characteristics as the natural land,indicating that the degraded karst soil had recovered to some extent.?2?It was assumed that the farmland was restored according to the gradient as follows:farmland,abandoned farmland,grassland,bush and secondary forest.And the mass proportion of macro-aggregate,SOC content,soil organic nitrogen content and C/N ratio of soil organic matter increased during the process of farmland restoration.The mass proportion of macro-aggregate and SOC content might be used as indicators of the karst ecological restoration stage.?3?The quantitative relationship between soil structure and SOC sequestration indicates that the carbon sequestration ability of macro-aggregate was likely to play a dominant role in the sequestration of SOC.?4?The?¹³C_SOC value in micro-aggregate was more sensitive to alteration of vegetation composition.The log-transformed SOC content was negatively correlated with?¹³C_SOC value under over C3 plant covered land,and the slope angle of the fitted line in different size aggregates decreased as follows:macro-aggregate,micro-aggregate,silt and clay size aggregate,indicating that the decomposition rate of organic matter sequentially decreases.