Climate change and human pressures erode habitat preferences of riverine dissolved organic matter

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, i.e. the habitat preferences, due to their diverse characteristics. Here we develop an indicator of compositional-level habitat preference for dissolved organic matter (DOM) by quantifying the aggregated preferences 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 preferences for water or sediment and are thermodynamically favorable and susceptible to microbial degradation. Consistently, the habitat preferences of DOM assemblages decrease with lower molecular weights and recalcitrance for waters, and with higher thermodynamic favorability for sediments. These preferences are both reduced by elevated temperatures and anthropogenic pressures, such as nitrogen loading and land-use change. Further projecting habitat preferences of global riverine DOM show lower preferences in regions with intense anthropogenic disturbances or warm climates, such as East Asia, West Europe, and the Amazon. These findings suggest enhanced molecular homogenization between water and sediment habitats driven by environmental changes and underscore the importance of habitat preferences for predicting organic carbon’s fate.