Adsorption and Molecular Fractionation of Dissolved Organic Matter

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Adsorption and molecular fractionation of dissolved organic matter on iron-bearing mineral matrices of varying crystallinity Elizabeth K Coward, Tsutomu Ohno, and Alain F. Plante Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b04953 • Publication Date (Web): 12 Jan 2018 Downloaded from http://pubs.acs.org on January 12, 2018

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Environmental Science & Technology

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Adsorption and molecular fractionation of dissolved organic matter on

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iron-bearing mineral matrices of varying crystallinity

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Elizabeth K. Coward1,2*, Tsutomu Ohno3, Alain F. Plante2

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Delaware Environmental Institute, University of Delaware, Newark DE 19716-7310, USA.

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2

Department of Earth & Environmental Science, University of Pennsylvania, Philadelphia PA

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19104-6316, USA.

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School of Food & Agriculture, University of Maine, Orono ME 04469-5763, USA.

ACS Paragon Plus Environment

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Abstract. Iron (Fe)-bearing mineral phases contribute disproportionately to adsorption of soil

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organic matter (SOM), due to their elevated chemical reactivity and specific surface area (SSA).

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However, the spectrum of Fe solid-phase speciation present in oxidation-reduction-active soils

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challenges analysis of SOM-mineral interactions and may induce differential molecular

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fractionation of dissolved organic matter (DOM). This work used paired selective dissolution

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experiments and batch sorption of post-extraction residues to 1) quantify the contributions of Fe-

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bearing minerals of varying crystallinity to DOM sorption, and 2) characterize molecular

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fractionation using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS).

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A substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-

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MS analysis of extracted DOM revealed distinct chemical signatures across Fe-OM associations.

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Sorbed carbon (C) was highly correlated with Fe concentrations, suggesting that Fe-bearing

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phases are strong drivers of sorption in these soils. Molecular fractionation was observed across

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treatments, particularly those dominated by short-range-order (SRO) mineral phases, which

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preferentially adsorbed aromatic and lignin-like formulae, and higher-crystallinity phases,

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associated with aliphatic DOM. These findings suggest Fe speciation-mediated complexation

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acts as a physicochemical filter of DOM moving through the critical zone, an important

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observation as predicted changes in precipitation may dynamically alter Fe crystallinity and C

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stability.

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INTRODUCTION

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Humid tropical forest soils contain a substantial portion (~500 Pg) of the global terrestrial

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carbon (C) pool1, yet the persistence of soil organic matter (SOM) in these systems remains

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unclear, largely due to mechanistic uncertainty in the biogeochemical controls on soil C.

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Organomineral complexation, driven by high concentrations of iron (Fe)-bearing mineral phases

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accumulating in surface horizons, has been framed as a potential C stabilization mechanism2-4.

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However, humid tropical soils often contain a diverse array of Fe-bearing minerals produced by

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pedogenesis5,

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confounding our understanding of Fe contributions to C accumulation and stabilization.

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Particularly, the solid-phase speciation of Fe minerals in soils, such as their size, structure, and

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surface area, can regulate the dynamics of C sorption reactions8-11, which have been linked to

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long-term retention of C in soils12,

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volcanic soils underlying many humid tropical forests14, are exceptionally reactive due to their

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high specific surface area (SSA) and large numbers of hydroxyl groups present on mineral

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surfaces15. As such, even small quantities of these minerals may provide an important sorbent for

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organic compounds.

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and frequent oxidation-reduction (redox) oscillations in surface horizons7,

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. Short-range-order (SRO) minerals, abundant in the

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Tropical systems are also characterized by large dissolved organic matter (DOM) input to

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surface soils, a highly mobile and reactive pool of SOM 16. In many tropical forests, fine litterfall

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accounts for ~60% of aboveground net primary productivity17 and rainfall is both frequent and

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abundant. As such, the movement of litter-leached DOM represents an important, near-constant

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input of C that fuels substantial heterotrophic respiration and could account for a large proportion

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of soil CO2 fluxes18. This DOM is thought to be comprised of both largely plant-derived

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compounds, such as lignin-derived phenols, mobilized through leaching, root tissue and


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exudates19, 20, and char21. Recent evidence has shown DOM is also highly processed by microbial

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assimilation and turnover21-23.

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This abundance of DOM entering humid tropical soils may undergo molecular

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fractionation at the mineral interface, driven by the molecular diversity of DOM and the

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spectrum of reactive minerals present. DOM is comprised of numerous small molecular

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components bound by hydrophobic interactions and hydrogen bonds, which may be broken when

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external forces from mineral surfaces are greater than the intermolecular forces between the

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molecular components of DOM24. Molecular components with greater affinity to mineral

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surfaces, often hydrophobic-, aromatic-, and carboxylic-rich fractions, will likely be bound while

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DOM characterize by lower affinity functional groups will remain in solution, leading to the

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molecular fractionation of DOM25, 26. The study of such molecular fractionation has, however,

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been historically limited to spectroscopic methods, which provide moiety-scale assessment of

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DOM chemistry, but are not capable of resolving molecular formulae. The chemical

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characterization of DOM composition has recently been enhanced by the ultrahigh resolution

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capabilities of electrospray ionization Fourier transform ion cyclotron resonance mass

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spectrometry (FT-ICR-MS), capable of resolving thousands of individual OM components27.

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This work uses FT-ICR-MS analysis to investigate the role of Fe-bearing mineral

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crystallinity in adsorbing and fractionating DOM in humid tropical soil matrices. Such chemical

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fractionation of DOM by the physicochemical filter of Fe mineralogy may modulate the

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composition of DOM reaching subsoils and the critical zone at large. Thus, characterizing the

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contribution of mineral matrices with varying Fe-bearing solid-phase speciation may aid in

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understanding the fate of DOM at the watershed or ecosystem level, in reconciling the wide


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range of C turnover times observed in tropical systems, and in disentangling the complex

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response of a substantial, but understudied, C reservoir to projected changes in climate.

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We aim to assess the contribution of Fe content and crystallinity to these organomineral

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complexes via two methodologies: 1) molecular characterization of DOM extracted with various

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Fe-bearing mineral phases, and 2) assessment of the contribution of Fe-bearing mineral phases to

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adsorptive molecular fractionation of DOM. Several recent studies have used FT-ICR-MS

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analysis to provide strong evidence of adsorptive fractionation of organic acids and DOM by

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synthetic metal oxides 24, 28, 29 and a temperate soil matrix 30. However, the role of Fe solid-phase

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speciation in governing molecular adsorptive fractionation of complex, litter-derived DOM

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remains uncertain Inorganic selective dissolution of distinct Fe-bearing mineral phases from

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humid tropical Oxisol soils and associated OM were followed by FT-ICR-MS analysis of the

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extracts to detect in-situ molecular fractionation of OM. Fe-induced fractionation was also

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investigated in a laboratory batch sorption experiment. Soil residues post-dissolution, a

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Ferrihydrite-coated (Fh+) soil and untreated control soil, comprising five distinct matrices

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varying in iron content, crystallinity and SSA, were then reacted with DOM in a batch sorption

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experiment. Unsorbed and post-sorption DOM solutions were analyzed for C concentrations to

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assess bulk sorption effects and via FT-ICR-MS to determine preferential adsorption of OM

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components.

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MATERIALS AND METHODS

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Soil Sampling and Selective Dissolution.

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Soil samples originated from an extensive sampling campaign completed in 2010 31 spanning

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the tropical montane forests of the Luquillo Critical Zone Observatory (LCZO). For the present

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study, one surface (0 - 20 cm) horizon from a quantitative soil pit of Oxisol soil order was


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selected to assess the contribution of matrix Fe-bearing mineralogy to OM sorption and

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molecular fractionation. Detailed site information and soil characteristics may be found in the SI.

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Six soil subsamples from the source pit were subjected to three parallel inorganic chemical

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extractions (n=18) for quantification of extractable Fe-containing secondary weathering phases

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and associated C, as previously detailed in Coward et al.3.

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Preparation of Sorbent Matrices and Dissolved Organic Matter.

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The iron oxyhydroxide Ferrihydrite (Fh) was synthesized according to methods described 32

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by Herbel and Fendorf

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NaOH until stable at pH 7.5. The suspension was allowed to equilibrate for at least 2 h, during

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which time the pH was adjusted to maintain a value of 7.5. Synthesized Fe oxyhydroxides were

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repeatedly washed (4×) by centrifugation, re-suspended in Milli-Q water and immediately added

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to substrates at 25 °C: one of 30 g soil and another of 30 g 50-mesh quartz sand. Each slurry was

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shaken horizontally for 48 hours until dry; this process was repeated three times to maximize

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surficial coating. Ferrihydrite coatings were isolated from quartz sand samples via suspended

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sonication and centrifugation for mineralogical confirmation. Mineral identity was established by

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powder X-ray diffraction (pXRD) methods (Fig. S1).

. Briefly, a ferric chloride solution was rapidly titrated with 1.0 N

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DOM was extracted from O horizon material collected during the 2010 sampling

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campaign. The composition of the original litter was 31.0 % C and 1.1 % N31. The extract was

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obtained by shaking a 1:10 soil:Milli-Q water solution for 24 hours. The supernatant was

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obtained by centrifugation at 5300 × g and vacuum filtration through 0.22 µm nylon filters. The

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DOM solution was kept in the dark at 4 °C until TOC analysis via a persulfate digestion and

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spectrophotometric method (Method 10173, Hach Company, Loveland CO). Initial DOC

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concentration was 200 ± 1.48 mg C L-1, which is relatively high but within the range of

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concentrations used in previous sorption studies (e.g., Feng et al.33). The high DOC


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concentration was used in the aim of saturating the system and reaching maximum sorption

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potential. pH of the final extract was 4.4 due to the acidic character of the water-soluble

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compounds in the initial litter.

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Sorption Experiments.

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Batch sorption experiments were conducted by mixing 0.5 g aliquots of soil with 15 mL of

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DOM in a 50 mL Erlenmeyer flask. Batch sorption experiments included four replicates of each

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sorbent matrix and four control samples consisting solely of DOM solution (~200 mg C L-1) to

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determine the extent of microbial DOM degradation during the experiment. Thus, a total of 24

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Erlenmeyer flasks were shaken for 48 h on a reciprocal shaker (120 rpm). A biocide was not

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used and experiments were performed at room temperature (25 °C). After sorption, suspensions

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were centrifuged at 5,300 × g, filtered through 0.22 µm Whatman filters and stored in the dark at

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4 °C. Supernatants from the batch sorption experiments were analyzed for DOC as described

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above. The concentration of DOM adsorbed to the mineral matrices was calculated by the

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difference between concentrations in the initial and post-sorption solutions. No detectable

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differences in DOM control solutions were detected, and thus no correction was used in this

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calculation.

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FT-ICR-MS Analysis.

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FT-ICR-MS analysis was used to characterize DOM of (i) selective dissolution extracts,

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(ii) initial DOM, and (iii) post-sorption supernatants. Solid phase extraction (SPE) was

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performed on all the samples to desalinate and concentrate the DOC solution according to

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Dittmar et al.34. Briefly, extracts were prepared by diluting the extracts to 100 mg/L DOC with

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mass spectrometry grade water and processing through Agilent PPL™ solid-phase extraction

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cartridges. The adsorbed organic matter was eluted with mass spectrometry grade methanol for


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FT-ICR-MS analysis, which was conducted with a 12-T Bruker Daltonics Apex Qe FT-ICR-MS

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housed at the College of Sciences Major Instrumentation Cluster at Old Dominion University.

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The solid-phase extraction processed samples (eluted in methanol) were diluted by a factor of 5

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with LCMS grade methanol and water to give a final sample composition of 50:50 (v/v)

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methanol:water. Further details on SPE and FT-ICR-MS parameters are included in the

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Supplementary Information.

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RESULTS & DISCUSSION

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Selective Dissolution of Fe-associated OM.

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Chemical properties of soil samples and selective extractants post-dissolution (Table 1)

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were consistent with previous applications of this selective dissolution method3. Briefly,

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inorganic dithionite-HCl (DH) extracted crystalline and SRO Fe oxides, hydroxides, and

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oxyhydroxides derived largely from pedogenesis. HCl-hydroxylamine (HH) extracted SRO Fe-

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bearing mineral phases, and sodium pyrophosphate (PP) extracted chelated and dispersable Fe

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phases. Hydroxylamine- and dithionite-extracted Fe were found to be the dominant extractable

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Fe phases, suggesting crystalline and SRO Fe phases are prevalent in these soils. The highest

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concentrations of C were solubilized by pyrophosphate extraction, however this extractant

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solubilized the lowest observed concentrations of Fe. A maximum molar ratio of adsorption C to

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Fe oxides is 1.0 for the co-extracted species35-37. Co-precipitation, chelation or aggregation of

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organic ligands generates low- density, organic-rich structures with higher (> 1) C:Fe values Fe-

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bearing mineral phases, largely via available surface area.36,

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indicate that hydroxylamine (C:Fe = 0.88 ± 0.054) and dithionite (C:Fe = 0.54 ± 0.038) extracts

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likely consist of Fe-C bound via inner-sphere adsorptive mechanisms, while pyrophosphate-

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Resultant molar C:Fe ratios


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extractable C (C:Fe = 7.44 ± 0.32) may be associated via OM-rich mechanisms, such as co-

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precipitation or aggregation.

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Ultrahigh resolution mass spectrometry of the extracts was used to probe differences in

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OM chemical composition liberated by each extract (Fig. 1). After post-spectral processing to

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identify peaks present in at least two out of three analytical replicates, > 3,800 mass spectral

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peaks were present in all three extractants, and elemental compositions were assigned in the

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range 300 < m/z < 800. van Krevelen diagrams and relative distributions (Fig. 1A-C) of chemical

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classifications reveal substantial differences in classification class abundances among extracts,

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while highlighting the predominance of lignin-like compounds across all extracts. The strong

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adsorption of lignin-like compounds to mineral surfaces has been well documented, and is likely

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a function of both ligand exchange reactions through prevalent carboxyl groups20,

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efficiency of microbial degradation of lignin and lignin-like compounds40,

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persistence of such compounds in DOM.

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and low

, leading to the

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Beyond the abundance of lignin-like molecular formulae in all three extractant types, we

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observed strong chemical fractionation among the three extractants, indicating OM associated

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with Fe phases of varying crystallinity differs at the molecular level. Most notably,

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pyrophosphate-solubilized DOM (Fig. 1A) differed substantially from DOM solubilized in other

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extracts (Figs. 1B, 1C, S2). The relative abundances of compounds in each group reveal

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pyrophosphate-extracted OM contained greater unsaturated aliphatic and saturated carbohydrate-

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like contributions and fewer condensed lignin and aromatic contributions. DOM molecular

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formulae in the aliphatic region (H/C > 1.5) of van Krevelen space are likely to be microbially-

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derived, but may also may be sourced from root necromass or other root inputs to soils, such as

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root exudates42. Carbohydrates in DOM are derived from decaying microorganisms, and are


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mainly composed of large aggregates of carbohydrates such as polysaccharides as well as

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proteins and peptides43. The predominance of carbohydrate-based compounds associated to

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chelated or colloidal Fe is in accordance to previous findings43, 44 that these inherently polar and

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highly soluble compounds may not be adsorbed directly to mineral surfaces. More broadly,

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chelated or colloidal OM appears to be largely microbial in origin, and, due to the weaker

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electrostatic interactions stabilizing these structures, may represent a more rapidly cycling pool

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of C. In accordance with the “cytoplasmic solute” hypothesis proposed by Fierer and Schimel45,

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such hydrophilic compounds may be easily dissolved, and subsequently mineralized, during the

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frequent precipitation (i.e., Fe-reducing) events in humid forest soils.

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Hydroxylamine-extracted (Fig. 1B) and dithionite-extracted (Fig. 1C) OM occupied less

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aliphatic space (H/C > 1.5), and were instead comprised of formulae with higher O/C and higher

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double-bond equivalence (DBE), a measure of the number of double bonds and rings in a

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molecule (Fig. S2, modified from analysis presented by Riedel et al., 201246), comprised of

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polycyclic aromatic, aromatic and lignin components. Preferential adsorption of aromatic

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structures to hydrous Fe phases is well-established25,

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solubilized by these two extractants were expected, due to the likely extraction of SRO Fe phases

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by both, differences between the Hydroxylamine-extracted (Fig. 1B) and dithionite-extracted

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(Fig. 1C) OM were observed. Hydroxylamine-solubilized OM contained relatively lower

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contributions of lignin-like components than those observed in dithionite-solubilized OM,

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accompanied by greater polycyclic aromatic contributions. The increase in dithionite-extracted

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lignin-like components is likely due to overall increased mineral dissolution and subsequent

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breakage of ligand-exchange bonds. We suggest that polycyclic aromatic compounds may be

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preferentially bound to those SRO mineral phases solubilized by hydroxylamine extraction.

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. While similarities in the formulae


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DOM sorption by mineral matrices.

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Post-extraction solid-phases residues, Fh+ soil, and untreated soils, were exposed to O

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horizon-derived DOM in a batch sorption experiment to assess the contribution of Fe mineral

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crystallinity and reactivity to matrix adsorption capacity. All sorbent matrices were characterized

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for total C and Fe content and SSA prior to sorption (Table 1, Figure 2). The mineral matrix

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treatments represent a wide range of Fe concentrations, which are strongly correlated (R2 = 0.90)

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with measured SSA (m2 g-1) (Fig. 2A). The strength of the observed correlation confirms the

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hypothesis that Fe-bearing mineral phases are a critical driver of available surface area in these

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highly-weathered tropical soils.

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Measurements of total C and SSA were obtained post-sorption to determine the

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concentration of C sorbed and C loadings pre- and post-sorption. Notably, sorbed C

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concentrations, derived from initial and post-sorption DOC measurements, were well-correlated

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(R2 = 0.76) with Fe content (Fig. 2B). Such robust correlations suggest that Fe-bearing mineral

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phases, largely via available surface area, are strong controls on C accumulation in these soils. It

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is important to note untreated soils adsorbed anomalously low C concentrations, which we

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suggest may be due to desorption or exchange of native SOM or to the existing occlusion of Fe-

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bearing minerals by OM, as the other treatments either removed Fe but also C (selective

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dissolution), or added Fe phases without increasing OM content (Fh coating). Comparison of C

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loadings (mg C m-2) pre- and post-sorption (Fig. 2C) revealed an expected increase in loadings

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due to sorption across all treatments, consistently correlated with Fe content (R2 = 0.79, R2 =

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0.80).

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Molecular fractionation of matrix-sorbed OM.


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FT-ICR-MS characterization of pre- and post-sorption DOM were used to identify

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compounds that were selectively removed from the solution due to adsorptive molecular

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fractionation of the bulk DOM (Fig. 3). After post-spectral processing to identify peaks present

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in at least two out of three analytical replicates, each treatment contained > 2,000 mass spectral

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peaks that were assigned elemental compositions in the range 220

Adsorption and Molecular Fractionation of Dissolved Organic Matter

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