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 thermodynami-cally 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 trans-formations. 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.

Automated summary

Understanding the chemical composition and molecular transformation of soil dissolved organic matter (DOM) is crucial for the global carbon cycle. This study used ultrahigh-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) to analyze DOM molecules in 36 paddy soils from subtropical China. By investigating the trade-offs and molecular transformations, the study revealed thermodynamically favorable and limited processes, and highlighted the role of external energy in the molecular transformation of soil DOM.