Soil Organic Carbon Fractions And Their Structures Under Different Types Of Forest In Subtropics

In the subtropics, broad-leaved and Masson pine （Pinusmassoniana Lamb） forests arerepresentative natural ones, whereas chestnut （Castanea sativa） and Lei bamboo（Phyllostachys Praecox） forests are two kinds of economical ones with intensivemanagements. Studying different soil organic carbon fractions and their structuralcharacteristics of the four kinds of forest stands and understanding the changes of thoseafter land use change from different stand types, especially the natural forests toeconomical forests, may help maintain balance and stability of soil carbon and guideforestry production. The soil samples of the representative forest in subtropics（broad-leaved, Masson pine, chestnut, and Lei bamboo forests） were collected to study soilorganic carbon contents and its distribution in different soil particle sizes, humus carboncomposition, light/heavy fractions and nuclear magnetic resonance（NMR） structure of totalsoil organic carbon, organic carbon in different particle sizes and humic acids. The resultsobtained from this study are as follows:（1） The four kinds of subtropical forests studied, the highest soil organic carbon content in0～40cm soil layer was obtained from Lei bamboo forest (31.56～34.78g kg^-1),followed by evergreen broad-leaved forest (10.20～19.89g kg^-1) and Masson pineforest (8.51～13.62g kg^-1), the lowest one was obtained from chestnut forest (6.25～12.17g kg^-1). Soil organic carbon content in Lei bamboo forest was significantlyhigher than that in other three forests. NMR spectroscopy of soil organic carbon in thefour kinds of forests all contained four distinct resonance region: alkyl carbon （0～50ppm）, and alkoxyl carbon region （50～110ppm）, aromatic carbon region （110～160ppm）, and the carbonyl carbon （160～220ppm） in which soil organic carbon NMRsignal intensity was marked different. Soil organic carbon in broad-leaved and Massonpine forests, alkyl carbon ratio was higher （52.33%and42.62%）, whereas soil organiccarbon in Lei bamboo and chestnut forests, alkoxyl carbon ratio was higher （68.26%and48.06%, respectively）.（2） It can be seen from the carbon distribution of different particle sizes that soil organiccarbon in evergreen broad-leaved forest, Masson pine and Lei bamboo forests wasmaily combined with coarse sand; soil organic carbon in chestnut forest was manilycombined with silt and clay. Soil organic carbon in coarse and fine sand, silt and clayall contained four distinct resonance regions and its distribution was roughly similar to one of total organic carbon in the soils. The distribution of carbon functional groups insoil organic carbon in four kinds of forest had a certain disciplinarian, namely, fromcoarse sand to silt and clay, the proportion of alkyl carbon in the organic carbonincreased, proportion of the alkoxyl carbon decreased, the ratios A/O-A as well as thehydrophobic carbon/hydrophilic carbon showed an increasing trend, but there was noconsistent trends for aromaticity.（3） The contents of light fraction material in the four kinds of soils ranged from0.5%to6.5%, in which the highest content was observed in Lei bamboo forest and thelowest one was observed in chestnut forest. The organic carbon contents in lightfraction ranged from1.40g kg^-1to13.06g kg^-1and accounted for21.16-36.51%oftotal organic carbon in the soils.（4） The proportion of humus carbon accounting for soil total organic carbon in four kindsof forests ranged from32.42%to49.49%. Fulvic acid carbon was primary conponentin humus carbon.The carbon contents in humus compositions in the soil under Leibamboo forest were significantly higher than that under other three kinds of forests.The rates of humic acid/fulvic acid （H/F） in the humus decreased in the order: Leibamboo forest> broad-leaved forest> Masson pine forest> chestnut forest. Itdecreased with soil depths. NMR structure of humic acid also contained four distinctresonance regions. However, obvious differences were found for the proportions offour carbon functional groups in humic acid among the soils under four kinds of forests.For broad-leaved forest and Masson pine forest, the humic acid structure in soil humuswas very similar but not for Lei bamboo forest and chestnut forest. Compared withbroad-leaved forest and Masson pine forest, the proportions of alkyl carbon accountingfor humic acid was significantly decresed, the proportions of alkoxyl carbon andaromatic carbon were obvious incresed, and the alkyl carbon/alkoxyl carbon （A/O-A）and aromaticity increased significantly under Lei bamboo and chestnut forests soils.This suggested that management practices not only affected the contents soil organiccarbon and the distribution of various compositions, but also had a significant impacton the structure of the humus.