Patterns And Determinants Of Soil Organic Carbon Fractions And Carbon Decomposition Across Temperate Grasslands Of Inner Mongolia

Soil is one of the most important carbon pools in terrestrial ecosystem,and plays a significant role in regulating the feedback between the terrestrial carbon cycle and climate warming.However,there is some uncertainty regarding predictions of soil organic carbon（SOC）dynamics in Earth System Model,particularly due to the inadequate understanding of the spatial variation and drivers of SOC fractions as well as carbon decomposition rate.Therefore,based on the soil and plant sampling along a transect conducted on Inner Mongolia Plateau,the contents of SOC and three carbon fractions by soil fractionation,and measurements soil carbon dioxide（CO₂）release rate based on laboratory incubation.this study looks at the spatial variation of SOC,SOC fractions and decomposition rate of the topsoil in temperate grasslands of Inner Mongolia.Meanwhile,the data of environmental factors,including climate,soil,plants,minerals and substrate,etc.are collected and measured.By employing Pearson correlation analysis and least squares regression,this study also investigates the relationship between SOC,SOC fractions and soil CO₂release rate and the variables of climate,soil,plants,minerals and substrate.In addition,variance partitioning analyses is used to explore the relative importance of the factors mentioned above as to their influence on SOC,SOC fractions and soil CO₂release rate.The main results are as follows:（1）SOC content,SOC fractions and soil CO₂release rate of the topsoil in temperate grasslands of Inner Mongolia exhibits large spatial variation and demonstrates obvious distribution patterns,with a trend of increasing SOC and decreasing soil CO₂release rate from southwest to northeast.The variation of SOC、coarse particulate organic matter（CPOM）、microaggregate-associated carbon（m）and silt and clay associated carbon（s+c）ranges from 3.9 to 46.9 g kg^-1,0.9 to 19.8 g kg^-1,0.8 to 10.6 g kg^-1and from 0.3 to 7.9 g kg^-1,respectively.The organic carbon content in the CPOM fraction is the highest,which accounts for about 43.2%of the SOC content,followed by the organic carbon content in the m and s+c fractions,accounting for36.8%and 20.0%of the SOC content respectively.Finally,the variation of soil CO₂release rate ranges from 8.7 to 35.1μg CO₂-C g^-1OC h^-1.（2）Environmental factors in this study also show considerable spatial variation.Specifically,the mean annual temperature（MAT）ranges from-0.6 to 7.2°C and the mean annual precipitation（MAP）ranges from 169.6 to 394.0 mm.Pondus hydrogenii（p H）ranges from 6 to 8.9.Aboveground net primary production（ANPP）ranges from15.6 to 269.5 g m^-2.The variations of poorly crystalline oxyhydroxides Fe/Al oxide（Fe_o+Al_o）,organically complexed Fe/Al oxide（Fe_p+Al_p）,exchangeable Ca²⁺(Ca_Exch)content and clay and silt content（Clay+silt）range from 1.3 to 6.9 g kg^-1,0.1 to 1.5 g kg^-1,0.6 to 2.7 g kg^-1and from 5.6%to 48.1%,respectively.And the variation of r1620/（2850+2920）ranges from 0.1 to 1.9.（3）The results of Pearson correlation analysis demonstrate that correlations exist between SOC、SOC fractions and soil CO₂release rate and environmental factors.One the one hand,the content of SOC,CPOM,m and s+c has significant negative correlations with MAT and p H,and has significant positive correlations with MAP,ANPP,Fe_o+Al_o,Fe_p+Al_p,Ca_Exch,and Clay+silt.On the other,soil CO₂release rate has significant positive correlations with MAT and p H,and has significant negative correlations with MAP,ANPP,Fe_o+Al_o,Fe_p+Al_p,Ca_Exch,Clay+silt and r1620/（2850+2920）.（4）It is showed in the results of both variance partitioning analysis and least squares regression analysis that mineral and climate are the key factors driving the spatial variation of SOC,SOC fractions and CO₂release rate.The spatial variation of SOC is mainly influenced by minerals and climate,among which the foremost variables are MAP（R²=0.59,p<0.001）and Fe_o+Al_o（R²=0.72,p<0.001）respectively.The spatial variation of CPOM fractions is collectively regulated by climatic,mineral,soil and plant related factors,among which the main contributors are climate and minerals,whose foremost variables are MAP（R²=0.51,p<0.001）and Fe_o+Al_o（Fe_o+Al_o;R²=0.64,p<0.001）respectively.The spatial variation of the m fraction is controlled by climate and minerals,with the most important variables being（MAT;R²=0.66,p<0.001）and（Fe_o+Al_o;R²=0.64,p<0.001）;and the spatial variation of the s+c fraction is jointly regulated by minerals and climate,with the most important variables being Clay+silt（R²=0.33,p<0.001）and MAP（R²=0.20,p<0.01）respectively.The spatial variation of soil CO₂release is first influenced by minerals,climate,plants and substrates,among which the mineral factor is the strongest,whose most important variable is Fe_o+Al_o（R²=0.66,p<0.001）,followed by the climatic factor,with the most important variable being MAP（R²=0.57,p<0.001）.In summary,this dissertation reveals the spatial distribution patterns and the determinants of topsoil SOC,SOC fractions and soil CO₂release rate in temperate grasslands on the Inner Mongolia Plateau.It is found that spatial variation exists in the topsoil SOC,SOC fractions and soil CO₂release rate.Meanwhile,environmental factors also show great spatial variation.Furthermore,there are significant correlations between SOC,SOC fractions and soil CO₂release rate and environmental factors.Minerals and climate are the key factors driving the spatial variation of SOC,SOC fractions and soil CO₂release rate.What’s more,the relative importance of minerals for SOC and SOC fractions varies,namely,with the decreasing in the size of soil particles leading to the growing importance of minerals.The results above provide experimental evidence for understanding the soil carbon cycle of grasslands,and also provides enlightenment for the Earth System Model to predict SOC dynamics under climate warming scenarios.