Using treated municipal wastewater (TMW) for agricultural irrigation can help reduce stress on freshwater resources. However, using TMW could expose agricultural environments to antibiotic resistance determinants (ARDs, including antibiotics and antibiotic resistance genes [ARGs]), ultimately impacting human health. In this study, we investigated two filtration systems, conventional sand, and sand mixed with 2 % (w/w) dairy manure biochar, to remove the antibiotic trimethoprim (TMP) commonly present in TMW. While the performance of the sand filtration system ultimately declined to 20-40 % TMP removal efficiency, the sand/biochar filtration system maintained >94 % TMP removal efficiency throughout the study. This high performance was achieved using biochar derived from a common, low-cost agricultural waste (dairy manure) under realistic treated wastewater conditions, emphasizing economic and reuse feasibility. Despite the highly efficient TMP removal, both systems released bacteria and ARGs into the effluent after several weeks, raising concerns about the microbiological quality of the water. Subsequent microcosm studies indicated that sand/biochar filtration reduced the presence of TMP in soil following irrigation. Moreover, filtration effectively reduced the development of several ARGs that need a short period to develop (e.g., ttgA, ttgB), but did not significantly impact ARGs that need more time to develop. These findings offer practical and economical strategies for mitigating antibiotic dissemination and antibiotic resistance development following TMW irrigation while emphasizing the importance of appropriate design and operational management to limit ARGs dissemination.

Treated municipal wastewater (TMW) is an underutilized water source for crop irrigation that can reduce the pressure on fresh water. However, the source and quality of the TMW will determine its suitability for the safety of the produced crop. We determined the yield, mineral nutrients, antioxidants, and heavy metals in spinach and radish irrigated with TMW in soil amended or not with cow manure. Both tap water (TAPW) and TMW provided equivalent concentrations of macro and micronutrients. The treatments were TAPW control, TMW, TAPW + 2% manure, and TMW + 2% manure. Although TAPW and TMW alone did not affect leaf nutrient accumulation or yield for either species, manure significantly increased leaf accumulation of K and P, mainly in spinach. When combined with TAPW or TMW, manure significantly decreased leaf Na in spinach due to the increased accumulation of K leading to a significant increase in spinach yield. Results were similar for radish, although not significant. Neither treatment affected the antioxidant capacity of spinach or radish but radish leaves had over 3-fold the antioxidant capacity (ORAC) of spinach. The concentrations of heavy metals in TMW, soil, or manure were not of concern to consumers being below the levels of concern established by the EPA. Our results support the use of TMW as an alternative to fresh water, and cow manure as an organic soil amendment to boost the yield of irrigated vegetable crops cultivated in sandy loam soils. This information will be valuable for addressing concerns about the safety of TMW as an alternative to freshwater, and of manure as a safe source of organic matter used to mitigate salinity and to boost fresh vegetable yield in arid and semiarid soils poor in organic matter and in climates where freshwater is scarce.

The use of aqueous film-forming foams at military bases potentially leads to the contamination of soils and groundwater by per- and polyfluoroalkyl substances (PFAS). Herein, an analysis of data from US Air Force bases was conducted to demonstrate that high levels of soil and groundwater PFAS contamination at such sites are common, particularly in testing and training areas, suggesting that prevention of PFAS leaching from such soils is a critical priority. Nonsaturated soil column experiments were conducted to optimize the use of soil-applied biochars as a low-cost strategy for adsorbing C8 perfluorooctanesulfonic acid (PFOS) and C4 perfluorobutanesulfonic acid (PFBS) and preventing their downward leaching. A wood-based biochar produced at >900 °C and added to the soil at 1% (m/m) was found to exhibit physio-chemical properties that facilitated excellent (>99%) retention of PFOS in a sandy loam soil. Moreover, postproduction thermal treatment (400 °C in air) of the biochar led to excellent retention of short-chain PFBS, essentially yielding nondetectable PFBS levels in the column leachate. The application of carefully selected biochars to areas with high levels of PFAS contamination at military facilities is likely a useful and low-cost approach for protecting groundwater from PFOS and PFBS (and perhaps other PFAS) contamination.

Treated wastewater application in the environment to alleviate stress on traditional water sources introduces antibiotic resistance genes (ARGs) in extracellular DNA (eDNA) into the environment, enhancing antibiotic resistance risk. This study aimed to mitigate this risk by using biochar to immobilize eDNA from treated wastewater. The interactive role of biochar production conditions were probed across a range of pyrolysis temperatures (300-800 degrees C) and feedstock classes (e.g., grass clippings (GC), manure (MN), walnut shells (WS) and pine pellet (PP)) on eDNA immobilization. Characteristics varied widely across biochars, with pyrolysis temperature and feedstock controlling elemental composition, proximate analysis, surface area, functional group composition, surface area and pore geometries, translating to a range of eDNA removal efficiencies (30.4-84.2%) and Freundlich coefficients (0.16-2.06). Biochar aromaticity, polarity and volatile matter were parameters most related with eDNA adsorption. Pyrolysis temperature exerted greater influence on eDNA removal than feedstock, however, there were significant interactive effects between feedstock and pyrolysis temperatures. GC and MN biochars demonstrated temperature sensitivity for eDNA removal, with maximum adsorption observed at a minimum threshold of 500 degrees C, whereas WS and PP biochars yielded statistically equivalent removals across pyrolysis temperatures. Our results provide biochar production guidance for targeting eDNA removal based on available feedstocks and pyrolysis capabilities. These results will streamline biochar integration into wastewater treatment systems across scales to mitigate environmental risk of ARGs.

In agricultural contexts, the presence of per- and polyfluoroalkyl substances (PFASs) in recycled water may pose a threat to irrigated crops and the human food chain. Biochar potentially represents a low-cost and environmentally friendly approach to removing PFAS from recycled water via adsorption. Identifying key biochar properties responsible for successful adsorption of long- and short-chained PFAS as well as developing predictive models are crucial in identifying the potential of biochar as a scalable approach for remediating PFAS in agriculture. Here, a systematic and comprehensive evaluation of 17 physicochemical properties of 24 biochars was performed to determine the main factors influencing PFAS removal from water. Batch studies were conducted, and principal component analysis as well as correlation studies were used to determine factors influencing PFAS removal. Various parameters were influential in the removal of long- (carbon/nitrogen ratio, specific surface area) and short- ([nitrogen + oxygen]/carbon ratio, carbon/nitrogen ratio) chained PFAS. Using these 24 biochars as a training dataset, linear models were constructed to predict the removal of selected PFAS based on biochar properties. These models were used to select a commercial pine wood biochar (Rogue biochar), which performed effectively in removing PFOS, PFOA, PFBS, PFHxS, PFNA in different matrices. Post-pyrolysis thermal treatment facilitated maintenance of adsorption potential over subsequent cycles, while providing the additional benefit of increasing the removal of the short-chained sulfonate PFBS by two- to five times. Careful, evidence-based selection of biochars with optimal physiochemical characteristics can provide excellent removal of both long- and short-chained PFAS compounds from water.

Antibiotic-resistance genes (ARGs) in wastewater may promote antibiotic resistance in consumers of crops irrigated with wastewater. Removal of DNA from wastewater may thus mitigate potential environmental risks associated with irrigation and environmental release of recycled wastewater. Although biochar adsorbents are a potentially cost-effective strategy for removing DNA from water, biochar feedstock influence on performance has not been studied across a range of feedstock classes. Our objective was to produce biochar from 5 distinct feedstocks (manure (MN), black mustard (Brassica nigra) (MU), orange peel (OP), pine pellet (PP) and macadamia nutshell (MNS)) at a fixed pyrolysis temperature (500 °C), characterize biochars and relate characteristics to DNA adsorption. Adsorption reached equilibrium within two hours and kinetics fit the pseudo-second order model. Adsorption rates increased from MNS, PP, OP, MN to MU, with rates of 3.06 × 10−2, 5.65 × 10−2, 1.78 × 10−1, 4.00 × 10−1 and 5.05 × 10−1 mg g−1 min−1, respectively. Adsorption isotherms fit the Freundlich model, with affinities increasing from PP, MNS, OP, MN to MU (Kd = 1.30 × 10−2, 1.35 × 10−2, 1.27 × 10−1, 1.96 × 10−1 and 1.42, respectively). DNA adsorption on biochars increased with ionic strength from I = 0 – 0.10 M except for MN. Ca2+ addition increased adsorption for biochars at I = 0.01 and 0.10 M, except for MN, which increased only with I = 0.10 M. Lower sensitivity of adsorption on MN biochar to ionic conditions indicates a different mechanism may control adsorption. The high ash content of MN biochar may favor direct bonding of DNA to ash minerals compared to π-π interactions likely driving DNA adsorption to structural carbon. These findings help understand how feedstock-driven variability in biochars translates to DNA immobilization and will assist researchers and stakeholders in determining the most suitable feedstocks for this purpose.

The antibiotic cephalexin binds to DOM under environmentally-relevent solution conditions through π–π bonding. Under conditions where cephalexin–DOM binding occurs, cephalexin adsorption by biochar and clay is impeded by DOM addition.

This study implemented biochars produced from two arid agricultural byproducts, date palm leaves and pistachio shells, at pyrolysis temperatures from 400 to 800 degrees C to remove trimethoprim, sulfamethoxazole and sulfapyridine antibiotics from mixed solutions. By altering pyrolysis temperature and feedstock, produced biochars yielded a range of physicochemical properties resulting in distinct antibiotic adsorption. Antibiotic adsorption capacity generally decreased with increasing pyrolysis temperature, while adsorption affinities were temperature independent for trimethoprim and increased with pyrolysis temperature for sulfamethoxazole and sulfapyridine. Correlation against biochar properties suggested cation exchange capacity and functional group composition related well to adsorption capacity and polarity/hydrophobicity was linked to adsorption affinities. Antibiotic removal efficiencies by biochars from both feedstocks compared favorably against previous reports, with up to 97.6, 98.1 and 99.5 % of trimethoprim, sulfamethoxazole and sulfapyridine removed, respectively. This work relates biochar production conditions to properties and subsequent antibiotic adsorption, demonstrating application of these materials for removing antibiotics from wastewater.

The increased recycling of treated municipal wastewater (TMW) to agricultural land is potentially hampered by the presence of antibiotic compounds, which may lead to the spread of antibiotic resistance. Biochar-based adsorption has been widely assessed as a means of polishing TMW to remove antibiotics prior to use; however, relatively little is known about how acid and base modification of biochar impacts its antibiotic retention capabilities, especially under the dynamic (flow-through) conditions inherent in proposed polishing systems. Using small columns of sand mixed with dairy manure or rice husk biochars (initial pH = 8.48 and 8.91, respectively) modified with acid or base, the retention of trimethoprim (TMP), sulfamethoxazole (SMZ), and sulfapyridine (SPD) was assessed. For SMZ and SPD, retention by the columns was consistently markedly lower than that measured in batch studies, indicating that equilibrium data may overestimate the retention of more realistic (dynamic) systems. TMP was completely retained by all biochars under both conditions. In columns, SMZ retention was strongly controlled by its pH-dependent speciation (greater retention at low pH, i.e., in acid modified biochars). SPD retention was strongly controlled by biochar surface area, which was markedly increased in the acid-modified dairy manure biochar and base-modified rice husk biochar. The findings indicate that acid or base modification of certain biochars may improve retention of sulfonamide antibiotics (which are known to be particularly difficult to remove from TMW) even under dynamic conditions. Biochar-based polishing systems comprising such biochars may offer potential in mitigating the spread of antibiotics, and hence antibiotic resistance, in agricultural settings.

Livestock manure, dairy lagoon effluent, and treated wastewater are known reservoirs of antibiotic resistance genes (ARGs), antibiotic-resistant bacteria (ARB), and virulence factor genes (VFGs), and their application to agricultural farmland could be a serious public health threat. However, their dissemination to agricultural lands and impact on important geochemical pathways such as the nitrogen (N) cycle have not been jointly explored. In this study, shotgun metagenomic sequencing and analyses were performed to examine the diversity and composition of microbial communities, ARGs, VFGs, and N cycling genes in different livestock manure/lagoon and treated wastewater collected from concentrated animal feeding operations (CAFOs) and a municipal wastewater treatment plant along the west coast of the United States. Multivariate analysis showed that diversity indices of bacterial taxa from the different microbiomes were not significantly different based on InvSimpson (P = 0.05), but differences in ARG mechanisms

myo-Inositol hexakisphosphate (IHP6) is typically the most abundant form of organic phosphorus (P-o) in soils, and this species is highly reactive with Al and Fe (hydr)oxides because of the six phosphoryl groups in its structure. In this study, the effects of pH (4.5-6.5) and cations (Ca2+ and Mg2+) were investigated using in situ attenuated total reflectance Fourier transform infrared (ATR-FTIR) spectroscopy and Mg K-edge X-ray absorption spectroscopy (XAS). IHP6 has more complex infrared (IR) spectra than inorganic phosphate, even though both share certain bands of absorbances. Ca and Mg influenced the IHP6 adsorption process on the hematite surface, but this effect was less evident for phosphate. The variation in pH promoted a shift of several bands in the adsorbed IR spectra. As for adsorption kinetics, IHP6 was sensitive to a pH change from 4.5 to 6.5, with a reduction in the adsorption rate at the highest pH. Phosphate, on the other hand, presented more rapid adsorption kinetics than IHP6, with equilibrium reached in similar to 80 min. Two adsorption mechanisms were identified for IHP6 and phosphate kinetics, without the influence of the studied cations. However, differences in the shape of XAS spectra at the Mg K-edge revealed that there was a probable change in the atomic environment of Mg2+ caused by its association with phosphate. In other evaluated organic molecules, such as IHP6 and citrate, this effect was not observed as a result of the intense presence of MgCl2 in the linear combination fitting (LCF) analysis. In general, the results enhance the molecular-level understanding of oxyanion adsorption onto hematite surfaces that can predict IHP6 and phosphate behaviors in iron-oxide-rich soils.

Arctic soils are marked by cryoturbic features, which impact soil-atmosphere methane (CH4 ) dynamics vital to global climate regulation. Cryoturbic diapirism alters C/N chemistry within frost boils by introducing soluble organic carbon and nutrients, potentially influencing microbial CH4 oxidation. CH4 oxidation in soils, however, requires a spatio-temporal convergence of ecological factors to occur. Spatial delineation of microbial activity with respect to these key microbial and biogeochemical factors at relevant scales is experimentally challenging in inherently complex and heterogeneous natural soil matrices. This work aims to overcome this barrier by spatially linking microbial CH4 oxidation with C/N chemistry and metagenomic characteristics. This is achieved by using positron-emitting radiotracers to visualize millimeter-scale active CH4 uptake areas in Arctic soils with and without diapirism. X-ray absorption spectroscopic speciation of active and inactive areas shows CH4 uptake spatially associates with greater proportions of inorganic N in diapiric frost boils. Metagenomic analyses reveal Ralstonia pickettii associates with CH4 uptake across soils along with pertinent CH4 and inorganic N metabolism associated genes. This study highlights the critical relationship between CH4 and N cycles in Arctic soils, with potential implications for better understanding future climate. Furthermore, our experimental framework presents a novel, widely applicable strategy for unraveling ecological relationships underlying greenhouse gas dynamics under global change.

Boron is an essential plant micronutrient responsible for several important functions. Boron availability in soils may be influenced by binding with soil organic matter (SOM), particularly with aromatic diol and polyphenol groups on SOM. The mechanism by which aromatic diols bind boron, however, remains unclear. The objective of this work is to further investigate interaction between boric acid and varying concentrations of an aromatic, polyphenolic SOM analogue (tannic acid at 5, 10 and 20 g L-1) from pH = 5 -9. UV/Visible spectroscopy showed boric acid enhanced tannic acid deprotonation at pH = 7.0 and 9.0, resulting in singly deprotonated tannic acid subunits. Attenuated total reflectance-Fourier transform infrared spectroscopy (ATR-FTIR) showed boric acid/tannic acid binding for all concentrations at pH = 7 and 9, whereas binding at pH = 5.0 was observed only at 20 g L-1 tannic acid. Uncomplexed boron species were not evident at pH = 9.0, but were detectable at pH = 7.0 at lower tannic acid concentrations and prevalent at pH = 5.0, qualitatively indicating binding affinity increases from pH = 5.0 to 9.0. ATR-FTIR results indicated tetrahedral coordination of boron upon complexation to tannic acid with a monodentate mechanism. These results collectively highlight a transition of solution planar boric acid to a tetrahedral, monodentate coordination with a single phenol group in tannic acid polyphenols. This contrasts with previous spectroscopic studies, which indicated bidentate tetrahedral or monodentate trigonal planar orientations prevail at aromatic diol sites. This work presents a previously unobserved boric acid coordination mechanism to an SOM analogue and, therefore, may better inform prediction and modeling of boron behavior in soils. (c) 2021 Elsevier Ltd. All rights reserved.

Vanadium (V) has been a useful trace metal in describing Earth’s biogeochemical cycling and development of industrial processes; however, V has recently been recognized as a potential contaminant o...

Sulfate adsorbs on Fe-oxide minerals by both inner- and outer-sphere modes; however, the time dependence of coexisting surface species is not clear. Using in situ attenuated total reflectance-Fourier transform infrared spectroscopy, peak fitting, and multivariate curve resolution analyses, we quantify adsorption and desorption kinetics of inner- and outer-sphere sulfate species on hematite at two ionic strengths (I = 0.01 and 0.10 M) and background cations (K+ and Ca2+) at pH 4.5. We experimentally observed inner-sphere, bidentate bridging and outer-sphere species kinetics congruent with the stepwise Eigen-Werner-Wilkins mechanism, in which a rapid formation of outer-sphere association precedes a slower conversion of outer-sphere to inner-sphere sulfate complexes. The rate limitation imposed by inner-sphere complex formation is likely linked to the displacement of protonated surface hydroxyl groups on the oxide surface by the adsorbing oxyanion. Outer-sphere complexes are responsible for rapid adsorption and desorption at I = 0.10 M seen in total sulfate, whereas inner-sphere species desorb more slowly. At I = 0.01 M, outer-sphere complexes similarly adsorb rapidly relative to inner-sphere complexes but desorb more slowly than inner-sphere complexes, possibly because of ionic strength effects on surface charge density. This work presents a novel, direct spectroscopic quantification of mixed surface species adsorption kinetics on a model mineral surface, which may be used to confirm proposed molecular mechanisms for other oxyanion adsorption to mineral surfaces. Our results enhance molecular-level understanding of oxyanion adsorption on soils and help predict their behavior in soils.

Positron-emitting nuclides have long been used as imaging agents in medical science to spatially trace processes non-invasively, allowing for real-time molecular imaging using low tracer concentrations. This ability to non-destructively visualize processes in real time also makes positron imaging uniquely suitable for probing various processes in plants and porous environmental media, such as soils and sediments. Here, we provide an overview of historical and current applications of positron imaging in environmental research. We highlight plant physiological research, where positron imaging has been used extensively to image dynamics of macronutrients, signalling molecules, trace elements, and contaminant metals under various conditions and perturbations. We describe how positron imaging is used in porous soils and sediments to visualize transport, flow, and microbial metabolic processes. We also address the interface between positron imaging and other imaging approaches, and present accompanying chemical analysis of labelled compounds for reviewed topics, highlighting the bridge between positron imaging and complementary techniques across scales. Finally, we discuss possible future applications of positron imaging and its potential as a nexus of interdisciplinary biogeochemical research.

Functional groups in natural organic matter have the potential to interact with and stabilize iron (Fe) redox species (ferric, ferrous) through complexation reactions. In this study, iron complexes with cysteine, arginine and histidine are used to investigate the extent to which specific functional groups present in these amino acids might hinder the oxidation of Fe2+ or promote the reduction of Fe3+. Iron complexes are synthesized by addition of ferrous or ferric salts to cysteine, arginine and histidine solutions at pH 7. The Fe-amino acid complexes are analysed using X-ray Absorption Spectroscopy (XAS), theoretical CTM4XAS (Charge Transfer Multiplet for XAS) calculations, and vibrational spectroscopy (FTIR and Raman). In addition, oxidation of the amino acids is determined by linear sweep voltammetry and chemical equilibrium modeling is used to predict the speciation of Fe in complexes with the amino acids. It is observed the extent of Fe redox transformation is affected by the electron donating capability of the ligand with which Fe reacts. In particular, the extent of Fe3+ reduction is related to the oxidation of the ligand with 80, 14 and 0% Fe3+ reduction by cysteine, arginine and histidine, respectively. Conversely, Fe2+ is preserved by cysteine (80%) > arginine (77%) > histidine (62%). The chemical forms of Fe in these mixed Fe oxidation-state systems include Fe-organic complexes and Fe precipitates. XAS and vibrational spectroscopy indicate thiol-S and amino-N bind Fe in complexes with cysteine. While amino-N and guanidyl-N functional groups of arginine bind Fe, carboxylate-O binding and the formation of Fe precipitates are only observed when Fe3+ is the predominant redox species. Whereas Fe precipitates seem to prevail in Fe(III)-histidine, distinct organic (Fe-O/N-C) and mineral (Fe-O-Fe) or mixed organic-mineral (Fe(O/N-C) )(x)(O-Fe)(y)) coordination environments occur in Fe(II)-histidine with similar to 60% Fe2+; in either case, Fe-N(imidazole) binding is not evident. The ligand atoms to which Fe binds and the inherent reduction capacity of the ligand appear to mediate Fe redox transformations. The proportion of ferric iron (Fe3+) seems to determine whether a precipitate forms. Consistent with spectroscopic data, chemical equilibrium modeling using experimental solution conditions and fractional Fe2+ and Fe3+ concentrations obtained from CTM4XAS predict the formation of Fe(II)- and Fe(III)-amino acid complexes and hydrolyzed Fe species; these calculations however also predict the formation of Fe oxide precipitates in all Fe complexes. The results presented in this study help to explain the co-occurrence of Fe2+ and Fe3+ in natural environments where organic matter is present. They also highlight the role specific organic functional groups play on the formation and stabilization of Fe(II, III)-organic complexes and precipitates, and thus on the Fe redox cycle which affects Fe bioavailability and mobility in terrestrial and subsurface environments. (C) 2019 Elsevier Ltd. All rights reserved.

Here we study the precipitation of lead (Pb)-phosphate minerals over the pH range of 4.0 to 8.0 with and without oxalate, a ubiquitous and abundant low-molecular-weight organic acid derived from plants and microorganisms in environmental matrices. In the aqueous Pb-phosphate systems, phosphate precipitated Pb efficiently, reducing the dissolved Pb concentration below 1 μM at all the tested pH values, with the minimum solubility of about 0.1 μM measured at the intermediate pH of 6.0. The measured dissolved Pb and free Pb2+ ion activity were not in agreement with predictions from generally-accepted solubility products of the Pb phosphate minerals, particularly hydroxypyromorphite [Pb5(PO4)3OH]. Discrepancies between our measured Pb phosphate solubility products and older reported values are attributed to non-ideal behavior of these minerals (incongruent dissolution) as well as uncertainties in stability constants for soluble Pb-phosphate ion pairs. The presence of equimolar levels of oxalate and phosphate resulted in up to 250-fold increase in Pb solubility at acidic pH and about a 4-fold increase at pH 7.0, due to the strong suppression of Pb phosphate precipitation by oxalate and formation of soluble Pb-oxalate complexes. At pH 4.0 and 5.0, Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) identified a Pb-oxalate mineral phase as the only precipitate despite the presence of phosphate; in the absence of oxalate, Pb hydrogen phosphate, PbHPO4, stably formed under these acidic conditions. At pH 6.0 and greater, FTIR and XRD data revealed that Pb-phosphate [Pb3(PO4)2], and hydroxypyromorphite [Pb5(PO4)3OH] to a lesser extent, were the predominant precipitates both in the absence and presence of oxalate. Therefore, oxalate did not strongly interfere with Pb-phosphate mineral formation at aqueous pH greater than 6.0 but oxalate controlled Pb solubility at acidic pH values.

Dissolved organic matter (DOM) represents a fraction of soil organic matter essential to many biogeochemical processes in surface soils and subsoils. Given its dynamic nature, DOM is known to be sensitive to land management practices. This study investigates the influence of tillage on DOM dynamics, including amount and composition of DOM, through a soil profile up to 72 cm depth from moldboard plowed (MB) and no-till (NT) plots in a Mid-Atlantic agroecosystem. Composition of DOM is probed using UV/Visible spectrophotometry (UV/Vis), Fourier transform infrared spectroscopy (FTIR) and fluorescence excitation-emission matrices (EEMs). Tillage decreases extractable DOM at the topmost depth interval studied (0-12 cm), with similar amounts extracted at depths below this point from MB and NT soils. DOM content of MB and NT soils decreases with depth, following a gradual decline in NT soils and a sharp decrease below the plow layer (20 cm) in MB soils. In both soils, chemical composition of DOM becomes less aromatic and less polysaccharide-like in character with depth accompanied by a relative enrichment in carboxylate groups. EEMs demonstrate that species within DOM follow a corresponding transition from conjugated fluorophores to moieties attributable to simple phenol, protein, amino acid and nucleic acid-like DOM constituents. These depth trends in functional group composition follow a distribution similar to DOM content, with observed transitions more abrupt in the MB than NT soils and marked by the plow layer. The decrease in DOM content with depth and the aforementioned shifts in DOM characteristics point to sorptive fractionation of DOM by the soil matrix and/or a conversion from more plant-derived DOM near the surface to more microbially processed DOM in subsoils. Overall, this work further highlights the impact of tillage on DOM behavior below the zone of physical disruption, suggesting that tillage also influences downstream DOM dynamics.

•Fe speciation in redox-stratified peats is reported.•Fe2+ and Fe3+ co-occur irrespective of redox conditions.•Complexation of Fe with organic matter is the primary stabilization mechanism.•Fe (oxy)hydroxide is identified as a secondary Fe species.