The environmental controls on efficiency of enhanced rock weathering in soils.

Enhanced rock weathering (ERW) in soils is a promising carbon removal technology, but the realistically achievable efficiency, controlled primarily by in situ weathering rates of the applied rocks, is highly uncertain. Here we explored the impacts of coupled biogeochemical and transport processes and a set of primary environmental and operational controls, using forsterite as a proxy mineral in soils and a multiphase multi-component reactive transport model considering microbe-mediated reactions. For a onetime forsterite application of ~ 16 kg/m2, complete weathering within five years can be achieved, giving an equivalent carbon removal rate of ~ 2.3 kgCO2/m2/yr. However, the rate is highly variable based on site-specific conditions. We showed that the in situ weathering rate can be enhanced by conditions and operations that maintain high CO2 availability via effective transport of atmospheric CO2 (e.g. in well-drained soils) and/or sufficient biogenic CO2 supply (e.g. stimulated plant-microbe processes). Our results further highlight that the effect of increasing surface area on weathering rate can be significant-so that the energy penalty of reducing the grain size may be justified-only when CO2 supply is nonlimiting. Therefore, for ERW practices to be effective, siting and engineering design (e.g. optimal grain size) need to be co-optimized.

Role of Mineral-Organic Interactions in PFAS Retention by AFFF-Impacted Soil.

A comprehensive, generalized approach to predict the retention of per- and polyfluoroalkyl substances (PFAS) from aqueous film-forming foam (AFFF) by a soil matrix as a function of PFAS molecular and soil physiochemical properties was developed. An AFFF with 34 major PFAS (12 anions and 22 zwitterions) was added to uncontaminated soil in one-dimensional saturated column experiments and PFAS mass retained was measured. PFAS mass retention was described using an exhaustive statistical approach to generate a poly-parameter quantitative structure-property relationship (ppQSPR). The relevant predictive properties were PFAS molar mass, mass fluorine, number of nitrogens in the PFAS molecule, poorly crystalline Fe oxides, organic carbon, and specific (BET-N2) surface area. The retention of anionic PFAS was nearly independent of soil properties and largely a function of molecular hydrophobicity, with the size of the fluorinated side chain as the main predictor. Retention of nitrogen-containing zwitterionic PFAS was related to poorly crystalline metal oxides and organic carbon content. Knowledge of the extent to which a suite of PFAS may respond to variations in soil matrix properties, as developed here, paves the way for the development of reactive transport algorithms with the ability to capture PFAS dynamics in source zones over extended time frames.

Redox Properties of Pyrogenic Dissolved Organic Matter (pyDOM) from Biomass-Derived Chars.

Chars are ubiquitous in the environment and release significant amounts of redox-active pyrogenic dissolved organic matter (pyDOM). Yet, the redox properties of pyDOM remain poorly characterized. This work provides a systematic assessment of the quantity and redox properties of pyDOM released at circumneutral pH from a total of 14 chars pyrolyzed from wood and grass feedstocks from 200 to 700 °C. The amount of released pyDOM decreased with increasing pyrolysis temperature of chars, reflecting the increasing degree of condensation and decreasing char polarity. Using flow-injection analysis coupled to electrochemical detection, we demonstrated that electron-donating capacities (EDCpyDOM; up to 6.5 mmole-·gC-1) were higher than electron-accepting capacities (EACpyDOM; up to 1.2 mmole-·gC-1) for all pyDOM specimens. The optical properties and low metal contents of the pyDOM implicate phenols and quinones as the major redox-active moieties. Oxidation of a selected pyDOM by the oxidative enzyme laccase resulted in a 1.57 mmole-·gC-1 decrease in EDCpyDOM and a 0.25 mmole-·gC-1 increase in EACpyDOM, demonstrating a largely irreversible oxidation of presumably phenolic moieties. Non-mediated electrochemical reduction of the same pyDOM resulted in a 0.17 mmole-·gC-1 increase in EDCpyDOM and a 0.24 mmole-·gC-1 decrease in EACpyDOM, consistent with the largely reversible reduction of quinone moieties. Our results imply that pyDOM is an important dissolved redox-active phase in the environment and requires consideration in assessing and modeling biogeochemical redox processes and pollutant redox transformations, particularly in char-rich environments.

From pools to flow: The PROMISE framework for new insights on soil carbon cycling in a changing world.

Soils represent the largest terrestrial reservoir of organic carbon, and the balance between soil organic carbon (SOC) formation and loss will drive powerful carbon-climate feedbacks over the coming century. To date, efforts to predict SOC dynamics have rested on pool-based models, which assume classes of SOC with internally homogenous physicochemical properties. However, emerging evidence suggests that soil carbon turnover is not dominantly controlled by the chemistry of carbon inputs, but rather by restrictions on microbial access to organic matter in the spatially heterogeneous soil environment. The dynamic processes that control the physicochemical protection of carbon translate poorly to pool-based SOC models; as a result, we are challenged to mechanistically predict how environmental change will impact movement of carbon between soils and the atmosphere. Here, we propose a novel conceptual framework to explore controls on belowground carbon cycling: Probabilistic Representation of Organic Matter Interactions within the Soil Environment (PROMISE). In contrast to traditional model frameworks, PROMISE does not attempt to define carbon pools united by common thermodynamic or functional attributes. Rather, the PROMISE concept considers how SOC cycling rates are governed by the stochastic processes that influence the proximity between microbial decomposers and organic matter, with emphasis on their physical location in the soil matrix. We illustrate the applications of this framework with a new biogeochemical simulation model that traces the fate of individual carbon atoms as they interact with their environment, undergoing biochemical transformations and moving through the soil pore space. We also discuss how the PROMISE framework reshapes dialogue around issues related to SOC management in a changing world. We intend the PROMISE framework to spur the development of new hypotheses, analytical tools, and model structures across disciplines that will illuminate mechanistic controls on the flow of carbon between plant, soil, and atmospheric pools.

Macronutrient in soils and wheat from long-term agroexperiments reflects variations in residue and fertilizer inputs.

Previous studies in the long-term experiments at Pendleton, OR (USA), were focused on organic matter cycling, but the consequences of land management for nutrient status over time have received little attention. Soil and wheat (Triticum aestivum L.) tissue samples were analyzed to determine the macronutrient dynamics associated with residue management methods and fertilizer rate under a dryland winter wheat-fallow rotation. The treatments included: no burn residue incorporation with farmyard manure (FYM) or pea vines, no burn or spring burn with application of N fertilizer (0, 45, and 90 kg ha-1), and fall burn wheat residue incorporation. The results revealed no differences on the effect of residue burning on macronutrient concentration over time. After receiving the same treatments for 84 years, the concentrations of soil organic C, total N and S, and extractable Mg, K, P in the 0-10 cm depth significantly increased in FYM plots compared to the rest of the plots. The N fertilization rate of 90 kg ha-1 reduced the accumulations of P, K, and Ca in grain compared to the 0 and 45 kg N ha-1 applications. The results indicate that residue incorporation with FYM can play vital role in reducing the macronutrient decline over time.

Micronutrients decline under long-term tillage and nitrogen fertilization.

Tillage and nitrogen (N) fertilization can be expected to alter micronutrient dynamics in the soil and in plants over time. However, quantitative information regarding the effects of tillage and N application rates on micronutrient dynamics is limited. The objectives of this study were (a) to determine the long-term effect of different tillage methods as well as variation in N application rates on the distribution of Mehlich III extractable manganese, copper, zinc, boron, and iron in soils and (b) to assess accumulation of the same nutrients in wheat (Triticum aestivum L.) tissues. The system studied was under a dryland winter wheat-fallow (WW-F) rotation. Tillage methods included moldboard (MP), disk (DP) and sweep (SW), and the N application rates were 0, 45, 90, 135, and 180 kg ha-1. The concentration of soil manganese was greater under DP (131 mg kg-1) than under MP (111 mg kg-1). Inorganic N application reduced extractable soil copper while, it increased manganese accumulation in wheat grain over time. Comparison of micronutrients with adjacent long-term (since 1931) undisturbed grass pasture revealed that the WW-F plots had lost at least 43% and 53% of extractable zinc and copper, respectively, after 75 years of N fertilization and tillage. The results indicate that DP and inorganic N application could reduce the rate of micronutrient decline in soil and winter wheat grain over time compared to MP and no N fertilization.

Humic Substances Extracted by Alkali Are Invalid Proxies for the Dynamics and Functions of Organic Matter in Terrestrial and Aquatic Ecosystems.

The conceptual rigor of using organic materials extracted from soil by alkali, called , as proxies for soil organic matter has been questioned for almost 180 years. However, the humic substances paradigm, i.e., the understanding that alkali-extracted humic substances are newly synthesized materials with unique properties that are distinct from non-humic organic matter, continues to be propagated throughout the literature. Here, we revisit the mechanistic background of the extraction process to show that alkaline extraction is unable to discriminate for the chemical history of a compound. For this reason, alkaline extraction cannot distinguish between (i) materials that have undergone secondary synthesis or humification ("humic" substances), and (ii) materials that are decorated with ionizable functional groups for other reasons, such as oxidative depolymerization ("non-humic" substances). While this mechanistic consideration alone renders invalid a distinction between humic substances and non-humic substances based on alkaline extraction, we further show that the evidence available to date does not support the assumption that processes of secondary synthesis create quantitatively significant proportions of "chemically reactive, yet recalcitrant" materials in natural environments. Any definition of humic substances that invokes both alkaline extraction and secondary synthesis is thus flawed on at least two accounts: (i) alkaline extraction is unable to achieve its purpose of separating humic from non-humic substances, and (ii) the assertion that the extracted materials have unique molecular properties as a consequence of secondary synthesis cannot be proven because alkaline extraction cannot separate materials created by secondary synthesis from other, ionizable organic compounds. Finally, we point out that since the definition of humic substances is tied to the alkaline-extraction procedure, neither the existence of operationally defined humic substances in the environment nor their chemical integrity during the course of alkaline extraction can be independently verified. We conclude that organic materials extracted by alkali require appropriate nomenclature and rigorous definition to merit consideration in teaching, research, and application.

Mineral Surfaces as Agents of Environmental Proteolysis: Mechanisms and Controls.

We investigated the extent to which contact with mineral surfaces affected the molecular integrity of a model protein, with an emphasis on identifying the mechanisms (hydrolysis, oxidation) and conditions leading to protein alteration. To this end, we studied the ability of four mineral surface archetypes (negatively charged, positively charged, neutral, redox-active) to abiotically fragment a well-characterized protein (GB1) as a function of pH and contact time. GB1 was exposed to the soil minerals montmorillonite, goethite, kaolinite, and birnessite at pH 5 and pH 7 for 1, 8, 24, and 168 h and the supernatant was screened for peptide fragments using Tandem Mass Spectrometry. To distinguish between products of oxidative and hydrolytic cleavage, we combined results from the SEQUEST algorithm, which identifies protein fragments that were cleaved hydrolytically, with the output of a deconvolution algorithm (DECON-Routine) designed to identify oxidation fragments. All four minerals were able to induce protein cleavage. Manganese oxide was effective at both hydrolytic and oxidative cleavage. The fact that phyllosilicates-which are not redox active-induced oxidative cleavage indicates that surfaces acted as catalysts and not as reactants. Our results extend previous observations of proteolytic capabilities in soil minerals to the groups of phyllosilicates and Fe-oxides. We identified structural regions of the protein with particularly high susceptibility to cleavage (loops and ß strands) as well as regions that were entirely unaffected (α helix).

Carbohydrates protect protein against abiotic fragmentation by soil minerals.

The degradation and turnover of soil organic matter is an important part of global carbon cycling and of particular importance with respect to attempts to predict the response of ecosystems to global climate change. Thus, it is important to mechanistically understand the processes by which organic matter can be degraded in the soil environment, including contact with reactive or catalytic mineral surfaces. We have characterized the outcome of the interaction of two minerals, birnessite and kaolinite, with two disaccharides, cellobiose and trehalose. These results show that birnessite reacts with and degrades the carbohydrates, while kaolinite does not. The reaction of disaccharides with birnessite produces Mn(II), indicating that degradation of the disaccharides is the result of their oxidation by birnessite. Furthermore, we find that both sugars can inhibit the degradation of a model protein by birnessite, demonstrating that the presence of one organic constituent can impact abiotic degradation of another. Therefore, both the reactivity of the mineral matrix and the presence of certain organic constituents influence the outcomes of abiotic degradation. These results suggest the possibility that microorganisms may be able to control the functionality of exoenzymes through the concomitant excretion of protective organic substances, such as those found in biofilms.

Anaerobic microsites have an unaccounted role in soil carbon stabilization.

Soils represent the largest carbon reservoir within terrestrial ecosystems. The mechanisms controlling the amount of carbon stored and its feedback to the climate system, however, remain poorly resolved. Global carbon models assume that carbon cycling in upland soils is entirely driven by aerobic respiration; the impact of anaerobic microsites prevalent even within well-drained soils is missed within this conception. Here, we show that anaerobic microsites are important regulators of soil carbon persistence, shifting microbial metabolism to less efficient anaerobic respiration, and selectively protecting otherwise bioavailable, reduced organic compounds such as lipids and waxes from decomposition. Further, shifting from anaerobic to aerobic conditions leads to a 10-fold increase in volume-specific mineralization rate, illustrating the sensitivity of anaerobically protected carbon to disturbance. The vulnerability of anaerobically protected carbon to future climate or land use change thus constitutes a yet unrecognized soil carbon-climate feedback that should be incorporated into terrestrial ecosystem models.

Sorption of Fluorotelomer Sulfonates, Fluorotelomer Sulfonamido Betaines, and a Fluorotelomer Sulfonamido Amine in National Foam Aqueous Film-Forming Foam to Soil.

During fire-fighter training, equipment testing, and emergency responses with aqueous film-forming foams (AFFFs), milligrams per liter concentrations of anionic, zwitterionic, and cationic per- and polyfluoroalkyl substances (PFASs) enter the environment. Because the behavior of zwitterionic and cationic PFASs in the subsurface is unknown, batch sorption experiments were conducted using National Foam AFFF, which contains anionic fluorotelomer sulfonates (FtSs), zwitterionic fluorotelomer sulfonamido betaines (FtSaBs), and cationic 6:2 fluorotelomer sulfonamido amine (FtSaAm). Sorption of the FtSs, FtSaBs, and 6:2 FtSaAm to six soils with varying organic carbon, effective cation-exchange capacity, and anion-exchange capacity was evaluated to determine sorption mechanisms. Due to the poor recovery of the FtSaBs and 6:2 FtSaAm with published PFAS soil extraction methods, a new soil extraction method was developed to achieve good (90-100%) recoveries. The 6:2 FtSaAm was depleted from the aqueous phase in all but one soil, which is attributed to electrostatic and hydrophobic interactions. Sorption of the FtSs was driven by hydrophobic interactions, while the FtSaBs behave more like cations that strongly associate with the solid phase relative to groundwater. Thus, the sorption mechanisms of the FtSs, FtSaBs, and 6:2 FtSaAm are more complex than expected and cannot be predicted by bulk soil properties.

Can Biochar Covers Reduce Emissions from Manure Lagoons While Capturing Nutrients?

The unique physical and chemical properties of biochars make them promising materials for odor, gas, and nutrient sorption. Floating covers made from organic materials (biocovers) are one option for reducing odor and gas emissions from livestock manure lagoons. This study evaluated the potential of floating biochar covers to reduce odor and gas emissions while simultaneously sorbing nutrients from liquid dairy manure. This new approach has the potential to mitigate multiple environmental problems. Two biochars were tested: one made via gasification of Douglas fir chips at 650°C (FC650), and the other made from a mixture of Douglas fir [ (Mirb.) Franco] bark and center wood pyrolyzed at 600°C (HF600). The HF600 biocover reduced mean headspace ammonia concentration by 72 to 80%. No significant reduction was found with the FC650 biocover. Nutrient uptake ranged from 0.21 to 4.88 mg N g biochar and 0.64 to 2.70 mg P g biochar for the HF600 and FC650 biochars, respectively. Potassium ranged from a loss of 4.52 to a gain of 2.65 mg g biochar for the FC650 and HF600 biochars, respectively. The biochars also sorbed Ca, Mg, Na, Fe, Al, and Si. In a separate sensory evaluation, judges assessed odor offensiveness and odor threshold of five biocover treatments including four biochars applied over dairy manure. Reductions in mean odor offensiveness and mean odor threshold were observed in three treatments compared with the control. These results show that biochar covers hold promise as an effective practice for reducing odor and gas emissions while sorbing nutrients from liquid dairy manure.

Protein-Mineral Interactions: Molecular Dynamics Simulations Capture Importance of Variations in Mineral Surface Composition and Structure.

Molecular dynamics simulations, conventional and metadynamics, were performed to determine the interaction of model protein Gb1 over kaolinite (001), Na(+)-montmorillonite (001), Ca(2+)-montmorillonite (001), goethite (100), and Na(+)-birnessite (001) mineral surfaces. Gb1, a small (56 residue) protein with a well-characterized solution-state nuclear magnetic resonance (NMR) structure and having α-helix, 4-fold ß-sheet, and hydrophobic core features, is used as a model protein to study protein soil mineral interactions and gain insights on structural changes and potential degradation of protein. From our simulations, we observe little change to the hydrated Gb1 structure over the kaolinite, montmorillonite, and goethite surfaces relative to its solvated structure without these mineral surfaces present. Over the Na(+)-birnessite basal surface, however, the Gb1 structure is highly disturbed as a result of interaction with this birnessite surface. Unraveling of the Gb1 ß-sheet at specific turns and a partial unraveling of the α-helix is observed over birnessite, which suggests specific vulnerable residue sites for oxidation or hydrolysis possibly leading to fragmentation.

Abiotic Protein Fragmentation by Manganese Oxide: Implications for a Mechanism to Supply Soil Biota with Oligopeptides.

The ability of plants and microorganisms to take up organic nitrogen in the form of free amino acids and oligopeptides has received increasing attention over the last two decades, yet the mechanisms for the formation of such compounds in soil environments remain poorly understood. We used Nuclear Magnetic Resonance (NMR) and Electron Paramagnetic Resonance (EPR) spectroscopies to distinguish the reaction of a model protein with a pedogenic oxide (Birnessite, MnO2) from its response to a phyllosilicate (Kaolinite). Our data demonstrate that birnessite fragments the model protein while kaolinite does not, resulting in soluble peptides that would be available to soil biota and confirming the existence of an abiotic pathway for the formation of organic nitrogen compounds for direct uptake by plants and microorganisms. The absence of reduced Mn(II) in the solution suggests that birnessite acts as a catalyst rather than an oxidant in this reaction. NMR and EPR spectroscopies are shown to be valuable tools to observe these reactions and capture the extent of protein transformation together with the extent of mineral response.

The contentious nature of soil organic matter.

The exchange of nutrients, energy and carbon between soil organic matter, the soil environment, aquatic systems and the atmosphere is important for agricultural productivity, water quality and climate. Long-standing theory suggests that soil organic matter is composed of inherently stable and chemically unique compounds. Here we argue that the available evidence does not support the formation of large-molecular-size and persistent 'humic substances' in soils. Instead, soil organic matter is a continuum of progressively decomposing organic compounds. We discuss implications of this view of the nature of soil organic matter for aquatic health, soil carbon-climate interactions and land management.

Long-term litter decomposition controlled by manganese redox cycling.

Litter decomposition is a keystone ecosystem process impacting nutrient cycling and productivity, soil properties, and the terrestrial carbon (C) balance, but the factors regulating decomposition rate are still poorly understood. Traditional models assume that the rate is controlled by litter quality, relying on parameters such as lignin content as predictors. However, a strong correlation has been observed between the manganese (Mn) content of litter and decomposition rates across a variety of forest ecosystems. Here, we show that long-term litter decomposition in forest ecosystems is tightly coupled to Mn redox cycling. Over 7 years of litter decomposition, microbial transformation of litter was paralleled by variations in Mn oxidation state and concentration. A detailed chemical imaging analysis of the litter revealed that fungi recruit and redistribute unreactive Mn(2+) provided by fresh plant litter to produce oxidative Mn(3+) species at sites of active decay, with Mn eventually accumulating as insoluble Mn(3+/4+) oxides. Formation of reactive Mn(3+) species coincided with the generation of aromatic oxidation products, providing direct proof of the previously posited role of Mn(3+)-based oxidizers in the breakdown of litter. Our results suggest that the litter-decomposing machinery at our coniferous forest site depends on the ability of plants and microbes to supply, accumulate, and regenerate short-lived Mn(3+) species in the litter layer. This observation indicates that biogeochemical constraints on bioavailability, mobility, and reactivity of Mn in the plant-soil system may have a profound impact on litter decomposition rates.

Redox properties of plant biomass-derived black carbon (biochar).

Soils and sediments worldwide contain appreciable amounts of thermally altered organic matter (chars). Chars contain electroactive quinoid functional groups and polycondensed aromatic sheets that were recently shown to be of biogeochemical and envirotechnical relevance. However, so far no systematic investigation of the redox properties of chars formed under different pyrolysis conditions has been performed. Here, using mediated electrochemical analysis, we show that chars made from different feedstock and over a range of pyrolysis conditions are redox-active and reversibly accept and donate up to 2 mmol electrons per gram of char. The analysis of two thermosequences revealed that chars produced at intermediate to high heat treatment temperatures (HTTs) (400-700 °C) show the highest capacities to accept and donate electrons. Combined electrochemical, elemental, and spectroscopic analyses of the thermosequence chars provide evidence that the pool of redox-active moieties is dominated by electron-donating, phenolic moieties in the low-HTT chars, by newly formed electron accepting quinone moieties in intermediate-HTT chars, and by electron accepting quinones and possibly condensed aromatics in the high-HTT chars. We propose to consider chars in environmental engineering applications that require controlled electron transfer reactions. Electroactive char components may also contribute to the redox properties of traditionally defined "humic substances".