Dissolved organic matter (DOM) drives many fundamental biogeochemical processes (e.g., carbon storage, nutrient cycling, and soil development) in forest soil. However, the molecular-level characteristics of DOM derived from different types of tropical forest soils are poorly understood. Here, water samples at different soil depths (0, 20, and 40 cm) from tropical legume (Acacia auriculiformis, AA) and non-legume (Eucalyptus urophylla, EU) tree plantations were analyzed using absorption and fluorescence spectroscopy, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS), and solution-state ¹H nuclear magnetic resonance (NMR) spectroscopy. The FT-ICR MS results indicated that DOM persisted in the soil, but its molecular composition notably shifted from low-mass (150–300 Da) and more-aromatic molecules to middle- (300–450 Da) and high-mass (>450 Da) and less-aromatic molecules with increasing soil depth. This was primarily mediated by consumption and mineral sorption of low-mass plant-derived DOM (e.g., low-mass carbohydrates and polyphenols) and further formation of larger microbial products (e.g., protein-like and lipid-like compounds). In addition, a higher abundance of microbial-derived molecules (e.g., protein-like and carboxyl-rich alicyclic molecules) was found at the legume plantation relative to the non-legume plantation, which suggests a faster microbial turnover of DOM. Also, the legume plantation had greater enrichment of middle- and high-mass and condensed aromatic-like DOM components in soils. These findings improve our understanding of the drivers that mediate the response of DOM to soil depth and tree species in tropical plantations.

溶解有机物（DOM）驱动着森林土壤中的许多基本生物地球化学过程（例如碳储存，养分循环和土壤发育）。但是，人们对源自不同类型热带森林土壤的DOM的分子水平特征了解甚少。在这里，使用吸收和荧光光谱，傅立叶变换离子回旋加速器分析了热带豆科植物（Acacia auriculiformis，AA）和非豆科植物（Eucalyptus urophylla，EU）人工林在不同土壤深度（0、20和40 cm）的水样共振质谱（FT-ICR MS）和溶液状态¹H核磁共振（NMR）光谱。FT-ICR MS结果表明DOM在土壤中持续存在，但其分子组成从低质量（150-300 Da）和更多芳族分子转移到中质量（300-450 Da）和高质量（> 450 Da）和芳香度较低的分子，随土壤深度的增加而增加。这主要是由低质量植物来源的DOM（例如，低质量碳水化合物和多酚）的消耗和矿物质吸附以及进一步形成较大的微生物产品（例如，类蛋白质和类脂质化合物）介导的。此外，在豆类种植园中发现的微生物衍生分子（例如，类似蛋白质和富含羧基的脂环族分子）的丰度要高于非豆类种植园，这表明DOM的微生物更新更快。也，豆科植物在土壤中具有较高的中，高品质和浓缩芳香样DOM成分。这些发现提高了我们对介导DOM对热带人工林土壤深度和树木物种响应的驱动器的理解。