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. However, a more holistic perspective of the molecular characteristics of DOM and underlying mechanisms across Earth systems and climates remains under study. 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 waters, land, plant, petroleum, and atmosphere systems and climatic regions from the 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 in the other systems, while their O/C ratios rank between plant and atmosphere systems. In the waters and land systems, the H/C ratios of DOM vary from the highest to the lowest in the habitats of the 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 mid-latitudes of approximately 40°-50° in river water, lake water, and forest soil. The two ratios are primarily controlled by 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 of global carbon cycle processes under future global change.