Using Potential Molecular Transformation To Understand the Molecular Trade-Offs in Soil Dissolved Organic Matter

Understanding the chemical composition and molecular transformation in soil dissolved organic matter (DOM) is important to the global carbon cycle. To address this issue, ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) was applied to investigate DOM molecules in 36 paddy soils collected from subtropical China. All the detected 7576 unique molecules were divided into seven compound groups, and nine trade-off relationships between different compound groups were revealed based on principal component analysis and Pearson’s correlation. An optimized method was developed to evaluate all potential molecular transformations in DOM samples. The concept of thermodynamics was introduced to evaluate the identified molecular transformations and classify them as thermodynamically favorable (TFP) and thermodynamically limited (TLP) processes. Here, we first tried to understand the molecular trade-offs by using the potential molecular transformations. All the nine trade-offs could be explained by molecular transformations. Six trade-offs had bases of biochemical reactions, and the trade-off-related direct transformations could explain the content variations of carbohydrate-like, condensed aromatic-like, tannin-like, and lignin-like compounds in TLP. More reasonable explanations existed in the TLP rather than TFP, which demonstrated the critical role of external energy in the molecular transformation of soil DOM.解析土壤溶解性有机质（dissolved organic matter, DOM）的化学组成与分子转化过程，对全球碳循环研究至关重要。针对该科学问题，本研究采用超高分辨傅里叶变换离子回旋共振质谱（Fourier transform ion cyclotron resonance mass spectrometry, FT-ICR-MS），对采集自中国亚热带区域的36份稻田土壤中的DOM分子开展了分析。本次研究共检出7576种独特分子，并将其划分为7类化合物；基于主成分分析与皮尔逊相关性分析，揭示了不同化合物类群间的9种权衡关系。本研究开发了一种优化方法，用以评估DOM样品中所有潜在的分子转化过程。引入热力学概念对已识别的分子转化过程进行评价，并将其分为热力学有利过程（thermodynamically favorable, TFP）与热力学受限过程（thermodynamically limited, TLP）两类。本研究首先尝试通过潜在分子转化过程解析分子权衡关系，结果显示全部9种权衡关系均可通过分子转化过程得到合理解释。其中6种权衡关系以生化反应为依据，与权衡相关的直接转化过程可阐释热力学受限过程中类碳水化合物、缩合芳香类、单宁类及木质素类化合物的含量变化。相较于热力学有利过程，热力学受限过程能够提供更为合理的解释，这表明外源能量在土壤DOM分子转化过程中发挥着关键作用。