Universal microbial reworking of dissolved organic matter along environmental gradients

Soils are losing increasing amounts of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can increasingly offset their carbon sink capacity. DOM is more susceptible to degradation closer to its source and becomes increasingly dominated by the same (i.e., universal), difficult-to-degrade compounds as degradation proceeds.  However, the processes underlying DOM degradation across environments are poorly understood.  Here we found DOM changed similarly along soil-aquatic gradients irrespective of differences in environmental conditions.  Using ultrahigh-resolution mass spectrometry, we tracked DOM along soil depths and hillslope positions in forest headwater catchments and related its composition to soil microbiomes and physico-chemical conditions.  Along depths and hillslopes, carbohydrate-like and unsaturated hydrocarbon-like compounds increased in abundance-weighted mass, suggestive of microbial reworking of plant material.  More than half of the variation in the abundance of these compounds was related to the expression of genes essential for degrading plant-derived carbohydrates.  Our results implicate continuous microbial reworking in shifting DOM towards universal compounds in soils.  By synthesising data from the land-to-ocean continuum, we suggest these processes can be generalised across ecosystems and spatiotemporal scales.  Such general degradation patterns can be leveraged to predict DOM composition and its downstream reactivity along environmental gradients to inform management of soil-to-stream carbon losses.