Global patterns and drivers of dissolved organic matter across Earth systems: Insights from H/C and O/C ratios

Dissolved organic matter (DOM) is ubiquitous and contains a complex pool of thousands of distinct molecules, and their chemical characteristics help us inform the fate of global carbon. Yet, a more holistic perspective of molecular characteristics of DOM and underlying mechanisms across Earth systems and climates remain understudied. Here, we present a comprehensive analysis of the molecular characteristics of DOM using two abundance-weighted average indices, i.e., H/C and O/C ratios by compiling 2,995 samples from 317 studies covering the waters, land, plant, petroleum, and atmosphere systems, and the climatic regions from tropics to tundra. H/C ratios are lower on average in waters (H/C = 1.15 ± 0.005) and land (H/C = 1.20 ± 0.010) than the other systems, while their O/C ratios rank between plant and atmosphere. In the waters and land systems, the H/C ratios of DOM vary from the highest to the lowest in the habitats of land-to-ocean continuum generally as snow > glacier > marine ≥ freshwater/soil > groundwater. The H/C ratios show predictably U-shaped patterns along latitudinal gradients indicating the lowest abundance of more hydrogen saturated molecules at around mid-latitudes of 40°-50° in river water, lake water, and forest soil. The two ratios are primarily controlled by the environmental factors such as pH, dissolved oxygen, and carbon and nitrogen contents. We further unveil additional and considerable links between the ratios and the extremes of climatic factors such as precipitation of warmest quarter and maximum temperature of warmest month. Our synthesis provides molecular-level perspectives to characterize the global distribution and underlying drivers of DOM, which is complementary for our understanding global carbon cycle's processes under future global change.溶解态有机质（Dissolved Organic Matter，DOM）广泛分布于自然环境中，由数千种不同分子构成复杂组分，其化学特征可为全球碳归宿研究提供关键依据。然而，当前针对地球各系统与不同气候背景下溶解态有机质的分子特征及其潜在调控机制的系统性研究仍较为匮乏。本研究整合了317项已发表研究中的2995组样本，涵盖水体、陆地、植物、石油与大气五大系统，研究区域覆盖从热带至苔原的各类气候区，采用两种丰度加权平均指数——即氢碳比（H/C）与氧碳比（O/C）——对溶解态有机质的分子特征开展了综合性分析。结果显示，水体（平均H/C = 1.15 ± 0.005）与陆地系统（平均H/C = 1.20 ± 0.010）中溶解态有机质的H/C均值显著低于其他系统，而其O/C比值则介于植物与大气系统之间。在水体与陆地系统中，溶解态有机质的H/C比值沿陆洋连续体生境呈现从高到低的排序：雪>冰川>海洋≥淡水/土壤>地下水。在河水、湖水及森林土壤中，H/C比值沿纬度梯度呈现可预测的U型分布，表明在南北纬40°-50°的中纬度区域，高氢饱和分子的丰度最低。上述两种碳比值主要受pH、溶解氧、碳氮含量等环境因子调控。本研究进一步揭示，该比值还与极端气候因子存在显著关联，如暖季降水量与暖月最高气温。本综合研究从分子层面揭示了全球溶解态有机质的分布特征及其潜在驱动机制，可为未来全球变化背景下理解全球碳循环过程提供重要补充。