Peter S. Nico

Effect of dissolved CO2 on a shallow groundwater system: a controlled release field experiment.

Capturing carbon dioxide (CO(2)) emissions from industrial sources and injecting the emissions deep underground in geologic formations is one method being considered to control CO(2) concentrations in the atmosphere. Sequestering CO(2) underground has its own set of environmental risks, including the potential migration of CO(2) out of the storage reservoir and resulting acidification and release of trace constituents in shallow groundwater. A field study involving the controlled release of groundwater containing dissolved CO(2) was initiated to investigate potential groundwater impacts. Dissolution of CO(2) in the groundwater resulted in a sustained and easily detected decrease of ~3 pH units. Several trace constituents, including As and Pb, remained below their respective detections limits and/or at background levels. Other constituents (Ba, Ca, Cr, Sr, Mg, Mn, and Fe) displayed a pulse response, consisting of an initial increase in concentration followed by either a return to background levels or slightly greater than background. This suggests a fast-release mechanism (desorption, exchange, and/or fast dissolution of small finite amounts of metals) concomitant in some cases with a slower release potentially involving different solid phases or mechanisms. Inorganic constituents regulated by the U.S. Environmental Protection Agency remained below their respective maximum contaminant levels throughout the experiment.

NanoSIMS Study of Organic Matter Associated with Soil Aggregates: Advantages, Limitations, and Combination with STXM

Direct observations of processes occurring at the mineral-organic interface are increasingly seen as relevant for the cycling of both natural soil organic matter and organic contaminants in soils and sediments. Advanced analytical tools with the capability to visualize and characterize organic matter at the submicrometer scale, such as Nano Secondary Ion Mass Spectrometry (NanoSIMS) and Scanning Transmission X-ray Microscopy (STXM) coupled to Near Edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS), may be combined to locate and characterize mineral-associated organic matter. Taking advantage of samples collected from a decadal (15)N litter labeling experiment in a temperate forest, we demonstrate the potential of NanoSIMS to image intact soil particles and to detect spots of isotopic enrichment even at low levels of (15)N application. We show how microsites of isotopic enrichment detected by NanoSIMS can be speciated by STXM-NEXAFS performed on the same particle. Finally, by showing how (15)N enrichment at one microsite could be linked to the presence of microbial metabolites, we emphasize the potential of this combined imaging and spectroscopic approach to link microenvironment with geochemical process and/or location with ecological function.

Are oxygen limitations under recognized regulators of organic carbon turnover in upland soils

Understanding the processes controlling organic matter (OM) stocks in upland soils, and the ability to management them, is crucial for maintaining soil fertility and carbon (C) storage as well as projecting change with time. OM inputs are balanced by the mineralization (oxidation) rate, with the difference determining whether the system is aggrading, degrading or at equilibrium with reference to its C storage. In upland soils, it is well recognized that the rate and extent of OM mineralization is affected by climatic factors (particularly temperature and rainfall) in combination with OM chemistry, mineral–organic associations, and physical protection. Here we examine evidence for the existence of persistent anaerobic microsites in upland soils and their effect on microbially mediated OM mineralization rates. We corroborate long-standing assumptions that residence times of OM tend to be greater in soil domains with limited oxygen supply (aggregates or peds). Moreover, the particularly long residence times of reduced organic compounds (e.g., aliphatics) are consistent with thermodynamic constraints on their oxidation under anaerobic conditions. Incorporating (i) pore length and connectivity governing oxygen diffusion rates (and thus oxygen supply) with (ii) ‘hot spots’ of microbial OM decomposition (and thus oxygen consumption), and (iii) kinetic and thermodynamic constraints on OM metabolism under anaerobic conditions will thus improve conceptual and numerical models of C cycling in upland soils. We conclude that constraints on microbial metabolism induced by oxygen limitations act as a largely unrecognized and greatly underestimated control on overall rates of C oxidation in upland soils.

Long-term litter decomposition controlled by manganese redox cycling

Significance The rate-controlling mechanisms of litter decomposition are of fundamental importance for ecosystem nutrient cycling, productivity, and net carbon (C) balance. Current C cycling models rely primarily on climatic factors and lignin content as the main predictors of litter decomposition rates. Here, we show how the ability of the integrated plant–soil system to promote active redox cycling of manganese (Mn) regulates litter decomposition. Our work suggests that incorporating the coupling of litter decomposition and other elemental cycles, such as the Mn cycle, into conceptual and numerical models may significantly improve our mechanistic understanding and predictions of C cycling in terrestrial ecosystems. 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 Mn2+ provided by fresh plant litter to produce oxidative Mn3+ species at sites of active decay, with Mn eventually accumulating as insoluble Mn3+/4+ oxides. Formation of reactive Mn3+ species coincided with the generation of aromatic oxidation products, providing direct proof of the previously posited role of Mn3+-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 Mn3+ 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.

Arsenic Chemistry in Soils and Sediments

Publisher Summary This chapter explores and describes the biological and chemical processes that control the partitioning of As between the solid and aqueous phase. Strong partitioning of As on soil solids is mostly disrupted by the onset of anaerobic conditions (anaerobiosis), leading to increased aqueous concentrations of As. Variations in As chemistry, compounded by biogeochemical transformations of the soil matrix, upon the onset of anaerobic conditions transpire to produce a convolution of reactions that have varying impacts on As retention. Arsenic desorption upon anaerobiosis is ascribed to both the reduction of As, from arsenate to arsenite, and iron(III)—the latter leading to the reductive dissolution of ferric (hydr)oxides that act as principal sinks of As. The two modes of As reduction, respiratory or detoxification, both require As to be released from the solid phase. The primary reductases for respiratory reduction, ArrA and ArrB, both reside interior to the outer membrane of bacteria, and thus it is likely that As(V) must desorb from the surface, cross the outer membrane, and then undergo reduction.

Influence of uranyl speciation and iron oxides on uranium biogeochemical redox reactions

Uranium is a pollutant of concern to both human and ecosystem health. Uraniums redox state often dictates its partitioning between the aqueous- and solid-phases, and thus controls its dissolved concentration and, coupled with groundwater flow, its migration within the environment. In anaerobic environments, the more oxidized and mobile form of uranium (UO2 2+ and associated species) may be reduced, directly or indirectly, by microorganisms to U(IV) with subsequent precipitation of UO2. However, various factors within soils and sediments may limit biological reduction of U(VI), inclusive of alterations in U(VI) speciation and competitive electron acceptors. Here we elucidate the impact of U(VI) speciation on the extent and rate of reduction with specific emphasis on speciation changes induced by dissolved Ca, and we examine the impact of Fe(III) (hydr)oxides (ferrihydrite, goethite and hematite) varying in free energies of formation on U reduction. The amount of uranium removed from solution during 100 h of incubation with S. putrefaciens was 77% with no Ca or ferrihydrite present but only 24% (with ferrihydrite) and 14% (no ferrihydrite) were removed for systems with 0.8 mM Ca. Imparting an important criterion on uranium reduction, goethite and hematite decrease the dissolved concentration of calcium through adsorption and thus tend to diminish the effect of calcium on uranium reduction. Dissimilatory reduction of Fe(III) and U(VI) can proceed through different enzyme pathways, even within a single organism, thus providing a potential second means by which Fe(III) bearing minerals may impact U(VI) reduction. We quantify rate coefficients for simultaneous dissimilatory reduction of Fe(III) and U(VI) in systems varying in Ca concentration (0 to 0.8 mM), and using a mathematical construct implemented with the reactive transport code MIN3P, we reveal the predominant influence of uranyl speciation, specifically the formation of uranyl-calcium-carbonato complexes, and ferrihydrite on the rate and extent of uranium reduction in complex geochemical systems.

Surface Enhanced Raman Spectroscopy of Organic Molecules on Magnetite (Fe3O4) Nanoparticles.

Surface-enhanced Raman spectroscopy (SERS) of species bound to environmentally relevant oxide nanoparticles is largely limited to organic molecules structurally related to catechol that facilitate a chemical enhancement of the Raman signal. Here, we report that magnetite (Fe3O4) nanoparticles provide a SERS signal from oxalic acid and cysteine via an electric field enhancement. Magnetite thus likely provides an oxide substrate for SERS study of any adsorbed organic molecule. This substrate combines benefits from both metal-based and chemical SERS by providing an oxide surface for studies of environmentally and catalytically relevant detailed chemical bonding information with fewer restrictions of molecular structure or binding mechanisms. Therefore, the magnetite-based SERS demonstrated here provides a new approach to establishing the surface interactions of environmentally relevant organic ligands and mineral surfaces.

Laboratory study of simulated atmospheric transformations of chromium in ultrafine combustion aerosol particles

While atmospheric particles can have adverse health effects, the reasons for this toxicity are largely unclear. One possible reason is that the particles can contain toxic metals such as chromium. Chromium exists in the environment in two major oxidation states: III, which is an essential nutrient, and VI, which is highly toxic and carcinogenic. Currently little is known about the speciation of chromium in airborne particles or how this speciation is altered by atmospheric reactions. To investigate the potential impacts of atmospheric aging on the speciation and toxicity of chromium-containing particles, we collected chromium and chromium-iron combustion ultrafine particles on Teflon filters and exposed the particles to a combination of light, ozone, water vapor, and, in some cases, basic or acidic conditions. After the aging process, the aged and not-aged samples were analyzed for Cr oxidation state using X-ray Absorption Near Edge Spectroscopy (XANES). We found that the aging process reduced Cr(VI) by as much as 20% in chromium particles that had high initial Cr(VI)/Cr(total) ratios. This reduction of Cr(VI) to Cr(III) appears to be due to reactions primarily with light and hydroperoxyl radical (HO 2 ) in the chamber. Particles that had low initial Cr(VI)/Cr(total) ratios experienced no significant change in Cr oxidation states after aging. Compared to particles containing only Cr, the addition of Fe to the flame decreased the Cr(VI)/Cr(total) ratio in fresh Cr-Fe particles by ∼60%. Aging of these Cr-Fe particles had no additional effects on the Cr(VI)/Cr(total) ratio.

Belowground Response to Drought in a Tropical Forest Soil. I. Changes in Microbial Functional Potential and Metabolism

Global climate models predict a future of increased severity of drought in many tropical forests. Soil microbes are central to the balance of these systems as sources or sinks of atmospheric carbon (C), yet how they respond metabolically to drought is not well-understood. We simulated drought in the typically aseasonal Luquillo Experimental Forest, Puerto Rico, by intercepting precipitation falling through the forest canopy. This approach reduced soil moisture by 13% and water potential by 0.14 MPa (from -0.2 to -0.34). Previous results from this experiment have demonstrated that the diversity and composition of these soil microbial communities are sensitive to even small changes in soil water. Here, we show prolonged drought significantly alters the functional potential of the community and provokes a clear osmotic stress response, including the production of compatible solutes that increase intracellular C demand. Subsequently, a microbial population emerges with a greater capacity for extracellular enzyme production targeting macromolecular carbon. Significantly, some of these drought-induced functional shifts in the soil microbiota are attenuated by prior exposure to a short-term drought suggesting that acclimation may occur despite a lack of longer-term drought history.

Effects of Fulvic Acid on Uranium(VI) Sorption Kinetics

This study focuses on the effects of fulvic acid (FA) on uranium(VI) sorption kinetics to a silica sand. Using a tritium-labeled FA in batch experiments made it possible to investigate sorption rates over a wide range of environmentally relevant FA concentrations (0.37-23 mg L(-1) TOC). Equilibrium speciation calculations were coupled with an evaluation of U(VI) and FA sorption rates based on characteristic times. This allowed us to suggest plausible sorption mechanisms as a function of solution conditions (e.g., pH, U(VI)/FA/surface site ratios). Our results indicate that U(VI) sorption onto silica sand can be either slower or faster in the presence of FA compared to a ligand-free system. This suggests a shift in the underlying mechanisms of FA effects on U(VI) sorption, from competitive sorption to influences of U(VI)-FA complexes, in the same system. Changes in metal sorption rates depend on the relative concentrations of metals, organic ligands, and mineral surface sites. Hence, these results elucidate the sometimes conflicting information in the literature about the influence of organic matter on metal sorption rates. Furthermore, they provide guidance for the selection of appropriate sorption equilibration times for experiments that are designed to determine metal distribution coefficients (Kd values) under equilibrium conditions.