Universal microbial reworking of dissolved organic matter along environmental gradients

Abstract

Soils are losing increasing amounts of carbon annually to freshwaters as dissolved organic matter (DOM), which, if degraded, can accelerate climate change. DOM is more susceptible to degradation closer to its source and becomes increasingly dominated by the same, 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 ultra-high-resolution mass spectrometry, we tracked DOM along soil depths and hillslope positions in forest headwater catchments and related its composition to soil microbiomes and physical chemistry. Along depths and hillslopes, carbohydrate-like and unsaturated hydrocarbon-like compounds increased in 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 generalise 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.

Logging disrupts the ecology of molecules in headwater streams

“…We then correlated commonly used descriptive traits of individual molecular formulae to the strength of the correlation between the relative intensity of each formulae and the HIX value of the corresponding DOM pool across all sampling times. Molecular formulae that were more positively associated with HIX also had larger values of the formula-based estimate of aromaticity (AI mod , ρ = 0.62, p <0.001), which indicates the presence of condensed aromatic structures 63 and confirms that stream DOM was becoming more like surface soil post-logging, confirmed by leaching studies performed at the site 43 . Studies in lake sediment have found that AI mod strongly correlates with the relative abundance of lignin-like compounds 59 , further supporting a terrestrial origin to the HIX signal rather than changes produced in-stream 60, 61 .…”

“…Finally, major ions, nutrients, and metal concentrations were measured from a subset of the sampled water at the Great Lakes Forestry Centre, Sault Ste. Marie, Ontario, according to methods outlined in ref 43 .…”

“…Increased precipitation entering the soil following tree removal also enhances soil wetness and the expansion of surface saturated areas 42 . Further alteration in DOM sources can be expected due to soil disturbance, changes in nutrient dynamics, and/or inputs of woody debris leftover by harvest activities 43 . Knowledge of these mechanisms are essential to design forest management practices that protect aquatic ecosystems and minimize the potential of downstream carbon emissions.…”

Phototrophic Biofilms Transform Soil-Dissolved Organic Matter Similarly Despite Compositional and Environmental Differences

“…This finding is consistent with that of Schroeter et al who found that litter metabolomes converged over time in a lab-based decomposition experiment regardless of being derived from different sources. Others working in temperate forests and grasslands have similarly found that DOM from different sources converged as it passed through soils, and this has been attributed to microbial production and consumption of DOM over time . Our approaches could be used to assess the potential impacts of phototrophic biofilms on DOM molecular richness and persistence changes in paddy soils.…”

“…Others working in temperate forests 54 and grasslands 55 have similarly found that DOM from different sources converged as it passed through soils, and this has been attributed to microbial production and consumption of DOM over time. 56 Our approaches could be used to assess the potential impacts of phototrophic biofilms on DOM molecular richness and persistence changes in paddy soils.…”