Variations In Soil Organic Carbon Fractions Among Forests Of Contrasting Climate And Stand Types

Exploring the patterns of variations in soil organic carbon fractions with climate and forest types helps to understand the mechanisms underlying the environmental regulations of soil organic carbon,thereby providing theoretical basis for accurately predicting soil organic carbon dynamics under global climate change.In this dissertation,the effects of climate and forest types on the chemical and physical fractions of soil organic carbon were studied by collecting soil samples plots representing three typical forest types(broadleaf forest,coniferous forest,and mixed-wood forest)in three sub-climate zones(southern sub-tropical,warm temperate,and temperate)under humid conditions.Soil organic carbon fractions were assessed by means of solid-state 13C cross polarization 13C nuclear magnetic resonance(CP-MAS 13C NMR),density fractionations,particle size distributions,and aggregation fractionations.Resulting data were evaluated by principle component analysis(PCA)and redundancy analysis(RDA)along with soil climatic and physiochemical variables.At the global scale,a meta-analysis was conducted on over 80 sets of data of soil organic carbon fractionations and associated environmental variables from 30 publications,in order to elucidate the generic pattern of the effects of climate and forest type on variations in soil organic carbon fractions and the underlying mechanisms.The main results are summarized below:(1)Climate exerted significant effects on soil organic carbon fractions.Regardless of forest types,with mean annual temperature(MAT)increasing from 2.1? to 20.8?,the total organic carbon concentration declined from 89.2 g-kg-1 to 57.7 g·kg-1;the proportion of organic carbon in large aggregations declined from 44.0%to 35.9%;the proportion of chemically more stable alkyl and aromatic carbon increased from 24.1%and 12.2%to 27.4%and 14.0%,respectively;and the organic carbon bound to fine silt and clays increased from 1.5%and 6.9%to 3.1%and 8.5%,respectively.(2)The influence of stand types on soil organic carbon fractions showed that the coniferous forests had lower proportion of labile soil organic carbon fractions(P<0.001),lower proportions of organic carbon bound to clays(P=0.004)and in large aggregations(P=0.032),and lower proportions of free organic carbon(P<0.001),than broadleaf forests.(3)Meta-analysis of data from literature shows that the MAT was negatively correlated with both total organic carbon(P=0.003)and free organic carbon(P=0.008)of the surface soil layer,and that vegetation type significantly affected the distribution of soil organic carbon fractions(P=0.024),especially the organo-mineral carbon(P=0.030)and free organic carbon(P=0.026).Overall,results in this study shows that with increasing MAT and mean annual precipitation(MAP),total soil organic carbon decreases,but becomes more stable,implying that the forest soil organic carbon in higher latitudes would be more susceptible to climate change.Forest type results in variations in soil organic carbon fractions by influencing litter quality.