Abstract
Fluxes of organic matter across Earth habitats shape ecosystem function and carbon storage. Yet, it is challenging to predict how the molecules are preferentially accumulated in specific habitats, that is, the habitat affinities, due to their diverse characteristics. Here, we develop an indicator of compositional‐level habitat affinity for dissolved organic matter (DOM) by quantifying the aggregated affinities of individual molecules that are over‐ or under‐represented relative to a reference habitat. Applying this indicator to 93 paired riverine water‐sediment sites, we find that 65.1% of molecules show non‐significant habitat affinities for water or sediment and are thermodynamically favorable and susceptible to microbial degradation. Consistently, the habitat affinities of DOM assemblages decrease with lower molecular weights and recalcitrance for waters, and with higher thermodynamic favorability for sediments. These affinities are both reduced by elevated temperatures and anthropogenic pressures, such as nitrogen loading and land‐use change. Our projection of habitat affinities of global riverine DOM shows that regions with intense anthropogenic disturbances or warm climates, such as East Asia, Western Europe, and the Amazon, tend to exhibit lower affinities. These findings suggest enhanced molecular homogenization between water and sediment habitats is associated with environmental changes and underscore the importance of habitat affinities for predicting the potential trajectories of organic carbon across ecosystems.