Iron (Fe)-bearing mineral phases contribute disproportionately to adsorption of soil organic matter (SOM) due to their elevated chemical reactivity and specific surface area (SSA). However, the ...spectrum of Fe solid-phase speciation present in oxidation–reduction-active soils challenges analysis of SOM–mineral interactions and may induce differential molecular fractionation of dissolved organic matter (DOM). This work used paired selective dissolution experiments and batch sorption of postextraction residues to (1) quantify the contributions of Fe-bearing minerals of varying crystallinity to DOM sorption, and (2) characterize molecular fractionation using Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). A substantial proportion of soil SSA was derived from extracted Fe-bearing phases, and FT-ICR-MS analysis of extracted DOM revealed distinct chemical signatures across Fe-OM associations. Sorbed carbon (C) was highly correlated with Fe concentrations, suggesting that Fe-bearing phases are strong drivers of sorption in these soils. Molecular fractionation was observed across treatments, particularly those dominated by short-range-order (SRO) mineral phases, which preferentially adsorbed aromatic and lignin-like formulas, and higher-crystallinity phases, associated with aliphatic DOM. These findings suggest Fe speciation-mediated complexation acts as a physicochemical filter of DOM moving through the critical zone, an important observation as predicted changes in precipitation may dynamically alter Fe crystallinity and C stability.
While the importance of organic matter adsorption onto reactive iron-bearing mineral surfaces to carbon stabilization in soils and sediments has been well-established, fundamental understanding of ...how compounds assemble at the mineral interface remains elusive. Organic matter is thought to layer sequentially onto the mineral surface, forming molecular architecture stratified by bond strength and compound polarity. However, prominent complexation models lack experimental backing, despite the role of such architecture in fractionated, compound-dependent persistence of organic matter and modulating future perturbations in mineral stabilization capacity. Here, we use kinetic assays and ultrahigh resolution Fourier transform ion cyclotron resonance mass spectrometry under high temporal frequency to directly detect the molecular partitioning of organic matter onto an iron oxyhydroxide during adsorption. We observed three sequential intervals of discrete molecular composition throughout the adsorption reaction, in which rapid primary adsorption of aromatic compounds was followed by secondary lignin-like and tertiary aliphatic compounds. These findings, paired with observed differential fractionation along formulas nitrogen and oxygen content and decreasing selective sorption with reaction time, support “zonal” assembly models. This work presents direct detection of sequential molecular assembly of organic matter at the mineral interface, an important yet abstruse regulator of carbon stabilization and composition across temporal and spatial scales.
Iron (oxyhydr)oxides are highly reactive, environmentally ubiquitous organic matter (OM) sorbents that act as mediators of terrestrial and aqueous OM cycling. However, current understanding of ...environmental iron (oxyhydr)oxide affinity for OM is limited primarily to abiogenic oxides. Bacteriogenic iron (oxyhydr)oxides (BIOs), common to quiescent waterways and soil redox transitions, possess a high affinity for oxyanions (i.e., arsenate and chromate) and suggests that BIOs may be similarly reactive for OM. Using adsorption and desorption batch reactions, paired with Fourier transform infrared spectroscopy and Fourier transform ion cyclotron resonance mass spectrometry, this work demonstrates that BIOs are capable of sorbing leaf litter-extracted DOM and Suwannee River Humic/Fulvic Acid (SRHA/SRFA) and have sorptive preference for distinct organic carbon compound classes at the biomineral interface. BIOs were found to sorb DOM and SRFA to half the extent of 2-line ferrihydrite per mass of sorbent and was resilient to desorption at high ionic strength and in the presence of a competitive ligand. We observed the preferential sorption of aromatic and carboxylic-containing species and concurrent solution enrichment of aliphatic groups unassociated with carboxylic acids. These findings suggest that DOM cycling may be significantly affected by BIOs, which may impact nutrient and contaminant transport in circumneutral environments.
Permafrost contains a large (1700 Pg C) terrestrial pool of organic matter (OM) that is susceptible to degradation as global temperatures increase. Of particular importance is syngenetic Yedoma ...permafrost containing high OM content. Reactive iron phases promote stabilizing interactions between OM and soil minerals and this stabilization may be of increasing importance in permafrost as the thawed surface region (“active layer”) deepens. However, there is limited understanding of Fe and other soil mineral phase associations with OM carbon (C) moieties in permafrost soils. To elucidate the elemental associations involved in organomineral complexation within permafrost systems, soil cores spanning a Pleistocene permafrost chronosequence (19,000, 27,000, and 36,000 years old) were collected from an underground tunnel near Fairbanks, Alaska. Subsamples were analyzed via scanning transmission X-ray microscopy–near edge X-ray absorption fine structure spectroscopy at the nano- to microscale. Amino acid-rich moieties decreased in abundance across the chronosequence. Strong correlations between C and Fe with discrete Fe(III) or Fe(II) regions selectively associated with specific OM moieties were observed. Additionally, Ca coassociated with C through potential cation bridging mechanisms. Results indicate Fe(III), Fe(II), and mixed valence phases associated with OM throughout diverse permafrost environments, suggesting that organomineral complexation is crucial to predict C stability as permafrost systems warm.
Tropical forest soils contribute disproportionately to the poorly-characterized and persistent deep soil carbon (C) pool. These soils, highly-weathered and often extending one to two meters in depth, ...may contain an abundance of iron-(Fe) bearing mineral phases. Short-range-order (SRO) minerals are of particular interest due to their high reactive surface areas and capacity for soil C stabilization through sorption or co-precipitation. We hypothesized that SRO minerals might serve as primary contributors to soil C accumulation and storage in surface (0–20cm) and subsurface (50–80cm) soils of the Luquillo Critical Zone Observatory (LCZO) in northeast Puerto Rico. Oxisol and Inceptisol soils obtained from 20 quantitative soil pits, stratified across quartz-dominated granodiorite and clay-rich volcaniclastic parent materials, were subjected to selective dissolution procedures to extract Fe-C associations: sodium pyrophosphate (PP) to isolate colloidal or dispersable Fe, HCl-hydroxylamine (HH) and ammonium oxalate (AO) to isolate SRO Fe, and inorganic dithionite-HCl (DH) to isolate more crystalline pedogenic Fe. Pyrophosphate extraction of colloidal or dispersable Fe also extracted the greatest concentrations of soluble C across all samples. Dissolved molar C:Fe ratios >1 observed solely in the PP extracts indicated the presence of organic-rich non-sorptive associations, the stability of which may have stronger control on accumulation of total soil C in these soils than those of extractable SRO and pedogenic Fe. Pedogenic and SRO Fe phases were the dominant extractable minerals in both soil types, at the surface and at depth, and notably, correlated well with extracted C. This suggests that these phases are strongly associated with a smaller, but substantial, fraction of total soil C. Direct observations of the limited extractability of soil C (<50% in surface soils) during selective dissolution of Fe and the lack of correlations between extractable Fe minerals and total soil C concentrations did not support the common hypothesis that SRO mineral phases provide a dominant mechanism for soil C accumulation. Instead, SRO phases may control only a fraction of total soil C, and non-extractable Fe-bearing minerals and non-sorptive mechanisms may play more important roles than previously thought.
•Tropical soil C down-profile hypothesized to be controlled by SRO mineral phases.•Inorganic selective dissolution allows for direct quantification of Fe-bound C.•Low-molecular weight complexations dominate Fe-C associations in Luquillo soils.•Mass balance analyses suggest Fe-associated pool of C is limited (<50% total C).•Alternative mechanisms likely responsible for large C stocks observed.
While the contribution of iron (Fe)-bearing minerals to organic carbon (C) stabilization in terrestrial systems is well-described, the influence of Fe solid-phase speciation on organomineral ...associations is unclear in highly dynamic, oxidation-reduction (redox)-active soils. In humid tropic forest soils, fluctuations in redox state accelerate weathering of Fe-bearing mineral phases, producing a spectrum of mineral sizes and bonding environments available for C stabilization, and confounding our understanding of C stability. Characterizing these Fe-bearing phases can improve predictions of the response of redox-active soil systems to climatic changes that may alter Fe mineral crystallinity and solubility, such as precipitation intensity, storm event frequency and temperature. Leveraging inorganic selective dissolution techniques, 57Fe Mössbauer spectroscopy (MBS), specific surface area (SSA) analyses and X-ray diffraction (XRD), we investigated mineral speciation in surface soils of contrasting lithologies from the Luquillo Critical Zone Observatory (LCZO), Puerto Rico. The LCZO provides a model investigatory framework in which high C inputs to surface horizons by similar vegetation, topography and climatic forcings are intercepted by highly-weathered, volcaniclastic Oxisols or quartz diorite-derived Inceptisols, producing a gradient of Fe content and speciation. Strong correlations observed between Fe concentrations and extraction-induced changes in SSA indicated target Fe phases contribute substantially to SSA of the bulk mineral matrix. MBS analysis of untreated soils reveal both Oxisol and Inceptisol soils are largely composed of FeIII-oxyhydroxides, accompanied by substantial FeII and silicate FeIII contributions in Inceptisol soils. FeIII-oxyhydroxides in the Oxisol soils were largely short-range-ordered (SRO), and notably, a fraction of particularly low-crystallinity FeIII-oxyhydroxide mineral phases in these soils appear protected against harsh reductive dissolution, whereas the overall higher crystallinity Fe phases in the Inceptisol soils do not. These findings suggest that some high-SSA, SRO FeIII phases, which likely also have high C sorption capacities, may be immobilized against reduction in these Oxisol soils. Consequently, C associated with these FeIII phases may be preferentially stabilized in Oxisol soils, potentially driving disparate C mineralization and CO2 production rates across contrasting lithologies.
We have carried out a small-scale deep-sea field test of the hypothesis that CH4 gas can be spontaneously produced from CH4 hydrate by injection of a CO2/N2 gas mixture, thereby inducing release of ...the encaged molecules with sequestration of the injected gas. Pressure cell studies have shown that, under some pressure and temperature conditions, this gas mixture can induce formation of a solid N2/CO2 hydrate with no associated liquid water production. We transported a cylinder of pure CH4 hydrate, contained within a pressure vessel, to the sea floor at 690 m depth off shore Monterey, CA, using the remotely operated vehicle (ROV) Ventana. Upon opening the pressure vessel with the vehicle robotic arm, we emplaced the hydrate specimen on a metal stand and covered this with a glass cylinder full of a 25% CO2/75% N2 gas mixture, thereby fully displacing the surrounding seawater (T = 4.92 °C). We observed complete and rapid dissociation of the CH4 hydrate with release of liquid water and creation of a mixed gas phase. This gas composition will undergo transition over time because of the high solubility of CO2 in the displaced water phase. We show that the experimental outcome is critically controlled by the injected gas/hydrate/water ratio.
The record-setting wildfires that ravaged the western United States throughout 2020 released high concentrations of organic carbon (C) into the environment, including the adjacent Pacific Ocean. Yet ...little is known about the fate of marine wildfire-derived C, solubilized as dissolved organic matter (DOM), despite growing observations of ash deposition in such systems. We sought to quantify and characterize DOM inputs to Pacific surface waters spanning the California coastline from August 1 to October 31, 2020. Across over 290 field samples, dissolved organic C concentrations peaked 2- to 4-fold higher after the eruption of fire systems than immediate pre-wildfire levels. C concentrations were well correlated with atmospheric pyrogenic proxies PM2.5 and ozone, supporting pyrogenic sourcing. Molecular characterization of DOM by ultrahigh-resolution FTICR-MS revealed both a diversity of formulas, supporting a growing consensus of pyrogenic heterogeneity, and temporal shifts conserved across sites. An initial increase in highly aromatic, oxygen-containing compounds aligned with PM2.5 concentrations, burn extent, and C deposition. Over time, transformation to increasingly aliphatic DOM occurred. The latter is hypothesized to be a result of complex interplay between biotic and abiotic processes, warranting further study. Our observations suggest that wildfires are a substantial yet dynamic source of marine surface organic C.
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