Understanding the quantity and chemical structure of dissolved organic matter (DOM) in subsoil and their response to land-use change is essential for evaluating the regional soil organic matter dynamics and soil carbon stability.In the present study,four adjacent land-use systems,including natural forest (control treatment),Chinese fir plantation,Chinese chestnut orchard,and sloping tillage were selected from a subtropical hilly landscape unit.The soil type was red soil derived from granite.Chemical analysis combined with spectrum scanning technology,including ultraviolet spectrum,two-dimensional fluorescence spectra,and near infrared spectroscopy,were used to analyze the quantity and chemical structure of DOM in the topsoil (0-0.2 m) and subsoil (0.2-1 m) in different land use areas.The results showed that DOM was mainly stored in the subsoil,accounting for 58-87％ of all DOM in the soil profile.Among the four land-use systems,natural forest contained the most DOM and showed simpler chemical structures of DOM in the subsoil (mainly carbohydrate and proteoid) than in the topsoil.After the natural forest was changed to other land uses,the subsoil suffered more losses in DOM (26％-41％) than the topsoil (12％-49％),which was more obvious in winter than in summer.This indicates that the quantity of DOM in the subsoil was highly sensitive to human disturbance and vegetation changes.In addition,the chenical structure of DOM in the subsoil was generally more complex,and chemical recalcitrant components (aromatics,alkanes,and alkenes) accumulated at 80-100 cm depth following land use change.The spectrum curve,specific absorption peak,and characteristic value of soil DOM were sensitive to land use change and can be used as ecological indicators of vegetation and soil organic matter change after strong human disturbances.This study suggests that the change from natural forest to other land uses not only reduced the quantity of subsoil DOM,but also decreased the quality of soil organic matter,which may weaken carbon stability and the carbon sequestration capacity of subsoil in the long term.%了解底土溶解性有机质(DOM)的数量和化学结构对土地利用变化的响应,对科学评价区域土壤有机质动态和碳库稳定性具有重要意义.通过选取花岗岩红壤丘陵区同一景观单元的天然林地(常绿阔叶林)以及由此转变而来的杉木人工林、板栗园和坡耕地,采用化学分析结合光谱扫描(紫外光谱、二维荧光光谱和傅里叶变换红外光谱)技术,研究底土(0.2-1 m)和表土(0-0.2 m)DOM数量和结构对土地利用变化的响应差异,结果表明:58％-87％的DOM贮存在底土中.天然林地土壤的DOM数量最为丰富,底土DOM的宏观化学结构比表土更为简单,以碳水化合物、类蛋白为主.天然林转变为其他利用方式后,底土DOM的损失量(26％-41％)超过表土(12％-49％),冬季比夏季更为凸显;这反映底土DOM数量对人为干扰和植被变化的高度敏感性.同时,底土DOM宏观化学结构趋于复杂化,芳香类、烷烃类和烯烃类的化学抗性物质出现积累的现象.DOM光谱曲线形状、特定峰值、特征值对土地利用的响应敏感,对人为干扰后植被、土壤有机质的变化具有生态指示意义.研究显示,天然林地转变为其他利用方式后,不仅导致底土DOM的损失,也显著降低土壤有机质品质,长期上削弱底土的碳库稳定性和碳吸存能力.
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