Stimulating the anaerobic degradation of aromatic hydrocarbons in contaminated sediments by providing an electrode as the electron acceptor

The possibility that electrodes might serve as an electron acceptor to simulate the degradation of aromatic hydrocarbons in anaerobic contaminated sediments was investigated. Initial studies with Geobacter metallireducens demonstrated that although toluene was rapidly adsorbed onto the graphite electrodes it was rapidly oxidized to carbon dioxide with the electrode serving as the sole electron acceptor. Providing graphite electrodes as an electron acceptor in hydrocarbon‐contaminated sediments significantly stimulated the removal of added toluene and benzene. Rates of toluene and benzene removal accelerated with continued additions of toluene and benzene. [¹⁴C]‐Toluene and [¹⁴C]‐benzene were quantitatively recovered as [¹⁴C]‐CO₂, demonstrating that even though the graphite adsorbed toluene and benzene they were degraded. Introducing an electrode as an electron acceptor also accelerated the loss of added naphthalene and [¹⁴C]‐naphthalene was converted to [¹⁴C]‐CO₂. The results suggest that graphite electrodes can serve as an electron acceptor for the degradation of aromatic hydrocarbon contaminants in sediments, co‐localizing the contaminants, the degradative organisms and the electron acceptor. Once in position, they provide a permanent, low‐maintenance source of electron acceptor. Thus, graphite electrodes may offer an attractive alternative for enhancing contaminant degradation in anoxic environments.     

Biodegradation of atrazine in surface soils and subsurface sediments collected from an agricultural research farm

The purpose of the present study was to assess atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) mineralization by indigenous microbial communities and to investigate constraints associated with atrazine biodegradation in environmental samples collected from surface soil and subsurface zones at an agricultural site in Ohio. Atrazine mineralization in soil and sediment samples was monitored as ¹⁴CO₂ evolution in biometers which were amended with ¹⁴C-labeled atrazine. Variables of interest were the position of the label ([U-¹⁴C-ring]-atrazine and [2-¹⁴C-ethyl]-atrazine), incubation temperature (25°C and 10°C), inoculation with a previously characterized atrazine-mineralizing bacterial isolate (M91-3), and the effect of sterilization prior to inoculation. In uninoculated biometers, mineralization rate constants declined with increasing sample depth. First-order mineralization rate constants were somewhat lower for [2-¹⁴C-ethyl]-atrazine when compared to those of [U-¹⁴C-ring]-atrazine. Moreover, the total amount of ¹⁴CO₂ released was less with [2-¹⁴C-ethyl]-atrazine. Mineralization at 10°C was slow and linear. In inoculated biometers, less ¹⁴CO₂ was released in [2-¹⁴C-ethyl]-atrazine experiments as compared with [U-¹⁴C-ring]-atrazine probably as a result of assimilatory incorporation of ¹⁴C into biomass. The mineralization rate constants (k) and overall extents of mineralization (P _max ) were higher in biometers that were not sterilized prior to inoculation, suggesting that the native microbial populations in the sediments were contributing to the overall release of ¹⁴CO₂ from [U-¹⁴C-ring]-atrazine and [2-¹⁴C-ethyl]-atrazine. A positive correlation between k and aqueous phase atrazine concentrations (C _eq ) in the biometers was observed at 25°C, suggesting that sorption of atrazine influenced mineralization rates. The sorption effect on atrazine mineralization was greatly diminished at 10°C. It was concluded that sorption can limit biodegradation rates of weakly-sorbing solutes at high solid-to-solution ratios and at ambient surface temperatures if an active degrading population is present. Under vadose zone and subsurface aquifer conditions, however, low temperatures and the lack of degrading organisms are likely to be primary factors limiting the biodegradation of atrazine.     

Biodegradation of atrazine in surface soils and subsurface sediments collected from an agricultural research farm

The purpose of the present study was to assess atrazine (2-chloro-4-ethylamino-6-isopropylamino-s-triazine) mineralization by indigenous microbial communities and to investigate constraints associated with atrazine biodegradation in environmental samples collected from surface soil and subsurface zones at an agricultural site in Ohio. Atrazine mineralization in soil and sediment samples was monitored as ¹⁴CO₂ evolution in biometers which were amended with ¹⁴C-labeled atrazine. Variables of interest were the position of the label ([U-¹⁴C-ring]-atrazine and [2-¹⁴C-ethyl]-atrazine), incubation temperature (25°C and 10°C), inoculation with a previously characterized atrazine-mineralizing bacterial isolate (M91-3), and the effect of sterilization prior to inoculation. In uninoculated biometers, mineralization rate constants declined with increasing sample depth. First-order mineralization rate constants were somewhat lower for [2-¹⁴C-ethyl]-atrazine when compared to those of [U-¹⁴C-ring]-atrazine. Moreover, the total amount of ¹⁴CO₂ released was less with [2-¹⁴C-ethyl]-atrazine. Mineralization at 10°C was slow and linear. In inoculated biometers, less ¹⁴CO₂ was released in [2-¹⁴C-ethyl]-atrazine experiments as compared with [U-¹⁴C-ring]-atrazine probably as a result of assimilatory incorporation of ¹⁴C into biomass. The mineralization rate constants (k) and overall extents of mineralization (P _max ) were higher in biometers that were not sterilized prior to inoculation, suggesting that the native microbial populations in the sediments were contributing to the overall release of ¹⁴CO₂ from [U-¹⁴C-ring]-atrazine and [2-¹⁴C-ethyl]-atrazine. A positive correlation between k and aqueous phase atrazine concentrations (C _eq ) in the biometers was observed at 25°C, suggesting that sorption of atrazine influenced mineralization rates. The sorption effect on atrazine mineralization was greatly diminished at 10°C. It was concluded that sorption can limit biodegradation rates of weakly-sorbing solutes at high solid-to-solution ratios and at ambient surface temperatures if an active degrading population is present. Under vadose zone and subsurface aquifer conditions, however, low temperatures and the lack of degrading organisms are likely to be primary factors limiting the biodegradation of atrazine.Abbreviations C _eq solution phase atrazine concentration at equilibrium - C _s amount of atrazine sorbed - CLA [2-¹⁴C-ethyl]-atrazine - k first-order mineralization rate constant - K _d sorption coefficient - m slope - P _max maximum amount of CO₂ released - RLA [U-¹⁴C-ring]-atrazine     

Evaluating Biodegradation as a Primary and Secondary Treatment for Removing RDX (Hexahydro-1,3,5-trinitro-1,3,5-triazine) from a Perched Aquifer

Ground water beneath the U.S. Department of Energy (USDOE) Pantex Plant is contaminated with the high explosive RDX (hexahydro-1,3,5-trinitro-1,3,5-triazine). The authors evaluated biodegradation as a remedial option by measuring RDX mineralization in Pantex aquifer microcosms spiked with ¹⁴C-labeled RDX (75 g soil, 15 ml of 5 mg RDX/L). Under anaerobic conditions and constant temperature (16°C), cumulative ¹⁴CO₂ production ranged between 52% and 70% after 49 days, with nutrient-amended (C, N, P) microcosms yielding the greatest mineralization (70%). The authors also evaluated biodegradation as a secondary treatment for removing RDX degradates following oxidation by permanganate (KMnO₄) or reduction by dithionite-reduced aquifer solids (i.e., redox barriers). Under this coupled abiotic/biotic scenario, we found that although unconsumed permanganate initially inhibited biodegradation, > 48% of the initial ¹⁴C-RDX was recovered as ¹⁴CO₂ within 77 days. Following exposure to dithionite-reduced solids, RDX transformation products were also readily mineralized (> 47% in 98 days). When we seeded Pantex aquifer material into Ottawa Sand that had no prior exposure to RDX, mineralization increased 100%, indicating that the Pantex aquifer may have an adapted microbial community that could be exploited for remediation purposes. These results indicate that biodegradation effectively transformed and mineralized RDX in Pantex aquifer microcosms. Additionally, biodegradation may be an excellent secondary treatment for RDX degradates produced from in situ treatment with permanganate or redox barriers.     

Stimulation of Diesel Fuel Biodegradation by Indigenous Nitrogen Fixing Bacterial Consortia

Abstract Successful stimulation of N₂ fixation and petroleum hydrocarbon degradation in indigenous microbial consortia may decrease exogenous N requirements and reduce environmental impacts of bioremediation following petroleum pollution. This study explored the biodegradation of petroleum pollution by indigenous N₂ fixing marine microbial consortia. Particulate organic carbon (POC) in the form of ground, sterile corn-slash (post-harvest leaves and stems) was added to diesel fuel amended coastal water samples to stimulate biodegradation of petroleum hydrocarbons by native microorganisms capable of supplying a portion of their own N. It was hypothesized that addition of POC to petroleum amended water samples from N-limited coastal waters would promote the growth of N₂ fixing consortia and enhance biodegradation of petroleum. Manipulative experiments were conducted using samples from coastal waters (marinas and less polluted control site) to determine the effects of POC amendment on biodegradation of petroleum pollution by native microbial consortia. Structure and function of the microbial consortia were determined by measurement of N₂ fixation (acetylene reduction), hydrocarbon biodegradation (¹⁴C hexadecane mineralization), bacterial biomass (AODC), number of hydrocarbon degrading bacteria (MPN), and bacterial productivity (³H-thymidine incorporation). Throughout this study there was a consistent enhancement of petroleum hydrocarbon degradation in response to the addition of POC. Stimulation of diesel fuel biodegradation following the addition of POC was likely attributable to increases in bacterial N₂ fixation, diesel fuel bioavailability, bacterial biomass, and metabolic activity. Toxicity of the bulk phase water did not appear to be a factor affecting biodegradation of diesel fuel following POC addition. These results indicate that the addition of POC to diesel-fuel-polluted systems stimulated indigenous N₂ fixing microbial consortia to degrade petroleum hydrocarbons. Received: 29 December 1998; Accepted: 6 April 1999     

Microbial 13C utilization patterns via stable isotope probing of phospholipid biomarkers in Mojave Desert soils exposed to ambient and elevated atmospheric CO2

Changes in plant inputs under changing atmospheric CO₂ can be expected to alter the size and/or functional characteristics of soil microbial communities which can determine whether soils are a C sink or source. Stable isotope probing was used to trace autotrophically fixed ¹³C into phospholipid fatty acid (PLFA) biomarkers in Mojave Desert soils planted with the desert shrub, Larrea tridentata. Seedlings were pulse‐labeled with ¹³CO₂ under ambient and elevated CO₂ in controlled environmental growth chambers. The label was chased into the soil by extracting soil PLFAs after labeling at Days 0, 2, 10, 24, and 49. Eighteen of 29 PLFAs identified showed ¹³C enrichment relative to nonlabeled control soils. Patterns of PLFA enrichment varied temporally and were similar for various PLFAs found within a microbial functional group. Enrichment of PLFA ¹³C generally occurred within the first 2 days in general and fungal biomarkers, followed by increasingly greater enrichment in bacterial biomarkers as the study progressed (Gram‐negative, Gram‐positive, actinobacteria). While treatment CO₂ level did not affect total PLFA‐C concentrations, microbial functional group abundances and distribution responded to treatment CO₂ level and these shifts persisted throughout the study. Specifically, ratios of bacterial‐to‐total PLFA‐C decreased and fungal‐to‐bacterial PLFA‐C increased under elevated CO₂ compared with ambient conditions. Differences in the timing of ¹³C incorporation into lipid biomarkers coupled with changes in microbial functional groups indicate that microbial community characteristics in Mojave Desert soils have shifted in response to long‐term exposure to increased atmospheric CO₂.     

Applicability of radiotracer methods of measuring14CO2 assimilation for determining microbial activity in soil including a newin situ method

The applicability of two methods (pyrolysis gas chromatography and acidification-wet oxidation) for determining¹⁴CO₂ incorporation into soil microorganisms was investigated. Both methods were able to distinguish biologically incorporated¹⁴C from abiotically adsorbed¹⁴C, but to varying degrees, there being a larger carryover of abiotic¹⁴C into the organic fraction and a higher percentage of assimilated¹⁴C in the organic fraction with the wet oxidation method. Using¹⁴C assimilation as a measure, it was possible to determine microbial activities in soils of diverse properties under a variety of conditions, including polar soils under harsh environmental conditions. Both light and dark¹⁴CO₂ fixation was measurable.¹⁴CO₂ assimilation was not always proportional to the enumerable microorganisms. A new design for measurement of microbial activityin situ enabled measurement of total C influx (primary productivity) into soils with minimal perturbation to the natural soil ecosystem.     

Soil‐specific response functions of organic matter mineralization to the availability of labile carbon

Soil organic matter (SOM) mineralization processes are central to the functioning of soils in relation to feedbacks with atmospheric CO₂ concentration, to sustainable nutrient supply, to structural stability and in supporting biodiversity. Recognition that labile C‐inputs to soil (e.g. plant‐derived) can significantly affect mineralization of SOM (‘priming effects’) complicates prediction of environmental and land‐use change effects on SOM dynamics and soil C‐balance. The aim of this study is to construct response functions for SOM priming to labile C (glucose) addition rates, for four contrasting soils. Six rates of glucose (3 atm% ¹³C) addition (in the range 0–1 mg glucose g^?1 soil day^?1) were applied for 8 days. Soil CO₂ efflux was partitioned into SOM‐ and glucose‐derived components by isotopic mass balance, allowing quantification of SOM priming over time for each soil type. Priming effects resulting from pool substitution effects in the microbial biomass (‘apparent priming’) were accounted for by determining treatment effects on microbial biomass size and isotopic composition. In general, SOM priming increased with glucose addition rate, approaching maximum rates specific for each soil (up to 200%). Where glucose additions saturated microbial utilization capacity (>0.5 mg glucose g^?1 soil), priming was a soil‐specific function of glucose mineralization rate. At low to intermediate glucose addition rates, the magnitude (and direction) of priming effects was more variable. These results are consistent with the view that SOM priming is supported by the availability of labile C, that priming is not a ubiquitous function of all components of microbial communities and that soils differ in the extent to which labile C stimulates priming. That priming effects can be represented as response functions to labile C addition rates may be a means of their explicit representation in soil C‐models. However, these response functions are soil‐specific and may be affected by several interacting factors at lower addition rates.     

Methodological Modifications for Accurate and Efficient Determination of Contaminant Biodegradation in Unsaturated Calcareous Soils

Many techniques for quantifying microbial biodegradation of ¹⁴C-labeled compounds use soil-water slurries and trap mineralization-derived ¹⁴CO₂ in solution wells suspended within the incubation flasks. These methods are not satisfactory for studies of arid-region soils that are highly calcareous and unsaturated because (i) slurries do not simulate unsaturated conditions and (ii) the amount of CO₂ released from calcareous soils exceeds the capacity of the suspended well. This report describes simple, inexpensive methodological modifications for quantifying microbial degradation of [¹⁴C]benzene and 1,2-dichloro[U-¹⁴C]ethane in calcareous soils under unsaturated conditions. Soils at 50% water holding capacity were incubated with labeled contaminants for periods up to 10 weeks, followed by acidification of the soil and trapping of the evolved CO₂ in a separate container of 2 N NaOH. The CO₂ was transferred from the incubation flask to the trap solution by a gas transfer shunt containing activated charcoal to remove any volatilized labeled organics. The amount of ¹⁴CO₂ in the trap solution was measured by scintillation counting (disintegrations per minute). The method was tested by using two regional unamended surface soils, a sandy aridisol and a clay-rich riparian soil. The results demonstrated that both [¹⁴C]benzene and 1,2-dichloro[U-¹⁴C]ethane were mineralized to release substantial amounts of ¹⁴CO₂ within 10 weeks. Levels of mineralization varied with contaminant type, soil type, and aeration status (anaerobic vs. aerobic); no significant degradation was observed in abiotic control samples. Methodological refinements of this technique resulted in total ¹⁴CO₂ recovery efficiency of approximately 90%.     

Effects of Soil Water Content on Biodegradation of Phenanthrene in a Mixture of Organic Contaminants

Phenanthrene biodegradation was investigated at different soil water contents [0.11, 0.22, 0.33, 0.44 g H₂O (g soil)^?1] to determine the effects of water availability on biodegradation rate. A subsurface horizon of Kennebec silty loam soil was used in this study. [9-¹⁴C] phenanthrene was dissolved in a mixture of organic contaminants that consisted of 76% decane, 6% ρ-xylene, 6% phenanthrene, 6% pristane, and 6% naphthalene, and then added to the soil. The highest rate of mineralization, in which 0.23% of the [9-¹⁴C] phenanthrene degraded to ¹⁴CO₂ after 66 days of incubation, was observed at the soil water content of 0.44 g H₂O/g dry soil. Most of the ¹⁴C remained in the soil as the parent compound or as nonextractable compounds by acetonitrile at the highest water content. Concentrations of nonextractable compounds increased with water content, but residual extractable phenanthrene decreased significantly with increasing water content, which presumably indicates that bio-transformation occurred. The mineralization analysis of radiolabeled 9th carbon in phenanthrene underestimated phenanthrene biodegradation. The strong adsorption and low solubility of phenanthrene contributed to the low mineralization of phenanthrene 9th carbon. The other components were subject to higher biological and abiotic dissipation processes with increasing soil water content.     

Biodegradation process and the nature of metabolism of metalaxyl in soil

The enhanced biodegradation of metalaxyl was studied in tobacco, citrus, avocado and corn soils. The most rapid degradation of metalaxyl occurred in a tobacco soil in which the half-life (50% degradation) of metalaxyl was 6 days. The main breakdown product of metalaxyl in all soils was the acid metabolite. Ring labelled [¹⁴C]metalaxyl incubated for 4 wk in 6 soils demonstrated a low rate of ¹⁴CO₂ evolution ranging from 2.1% to 11.3% which was unrelated to the biodegradation properties of the soil. A relationship between the concentration of metalaxyl and the subsequent rate of biodegradation was found in the tobacco soils. Higher concentrations of metalaxyl resulted in faster biodegradation rates. A single exposure of tobacco and corn soils to metalaxyl (100 μg/ml or 200 μg/g dry weight of soil) significantly increased their subsequent capacity to degrade the fungicide. Addition of the fungicide thiram or the antibiotics streptomycin and chloramphenicol to an avocado soil resulted in 75% and 51% inhibition of metalaxyl degradation, respectively. A combination of the fungicide and antibiotics resulted in 89% inhibition. The results indicate that enhanced microbial degradation of metalaxyl can occur in a wide range of soils. Under experimental conditions using soil solutions or soil systems, a single application of the fungicide may trigger this event. A wide range of fungi and bacteria appear to take part in degrading metalaxyl.     

Effect of concentration and environmental form of tetradecenyl succinic acid on its mineralization in soil

Tetradecenyl succinic acid (TSA) is the major component of a detergent builder (C12-C14 alkenyl succinic acid), which is inherently biodegradable. ¹⁴C-TSA was dosed as a component of sewage sludge into a soil with a history of sludge amendment at final added concentrations of 1.5 and 30 mg (kg soil)^-1. In addition, it was dosed to the soil in an aqueous solution to a final added concentration of 30 mg (kg soil)^-1. Dose and form were found to have a pronouced effect on the mineralization kinetics. When dosed in a realistic form and concentration (i.e. 1.5 mg (kg soil)^-1 as a component of sludge), TSA was mineralized at its highest rate and to its greatest extent, and the mineralization half-life was 2.4 days. When dosed at 30 mg (kg soil)^-1 as a component of sludge, mineralization began immediately, and the half-life was 23 days. In contrast, when dosed at this concentration in aqueous solution, the onset of mineralization was preceded by a 13 day lag period and the mineralization half-life was 69 days. Primary biodegradation and mineralization rates of TSA were very similar. Approximately, half the radioactivity was evolved as ¹⁴CO₂, while the remaining radioactivity became non-extractable, having presumably been incorporated into biomass or natural soil organic matter (humics). This study demonstrated that TSA is effectively removed from sludge-amended soils as a result of biodegradation. Furthermore, it showed the effect that dose form and concentration have on the biodegradation kinetics and the importance of dosing a chemical not only at a relevant concentration but also in the environmental form in which it enters the soil environment.     

Carbon and nitrogen pools and mineralization in a grassland gley soil under elevated carbon dioxide at a natural CO2 spring

The growth and chemical composition of most plants are influenced by elevated CO₂, but accompanying effects on soil organic matter pools and mineralization are less clearly defined, partly because of the short‐term nature of most studies. Herein we describe soil properties from a naturally occurring cold CO₂ spring (Hakanoa) in Northland, New Zealand, at which the surrounding vegetation has been exposed to elevated CO₂ for at least several decades. The mean annual temperature at this site is ≈ 15.5 °C and rainfall ≈ 1550 mm. The site was unfertilized and ungrazed, with a vegetation of mainly C3 and C4 grasses, and had moderate levels of ‘available’ P. Two soils were present ? a gley soil and an organic soil – but only the gley soil is examined here. Average atmospheric CO₂ concentrations at 17 sampling locations in the gley soil area ranged from 372 to 670 ppmv. In samples at 0–5 cm depth, pH averaged 5.4; average values for organic C were 150 g, total N 11 g, microbial C 3.50 g, and microbial N 0.65 g kg^?1, respectively. Under standardized moisture conditions at 25 °C, average rates of CO₂‐C production (7–14 days) were 5.4 mg kg^?1 h^?1 and of net mineral‐N production (14 ?42 days) 0.40 mg kg^?1 h^?1. These properties were all correlated positively and significantly (P < 0.10) with atmospheric CO₂ concentrations, but not with soil moisture (except for CO₂‐C production) or with clay content; they were, however, correlated negatively and mainly significantly with soil pH. In spite of uncertainties associated with the uncontrolled environment of naturally occurring springs, we conclude that storage of C and N can increase under prolonged exposure to elevated CO₂, and may include an appreciable labile fraction in mineral soil with an adequate nutrient supply.     

High arctic heath soil respiration and biogeochemical dynamics during summer and autumn freeze‐in – effects of long‐term enhanced water and nutrient supply

In High Arctic NE Greenland, temperature and precipitation are predicted to increase during this century, however, relatively little information is available on the role of increased water supply on soil CO ₂ efflux in dry, high arctic ecosystems. We measured soil respiration (R_soil) in summer and autumn of 2009 in combination with microbial biomass and nutrient availability during autumn freeze‐in at a dry, open heath in Zackenberg, NE Greenland. This tundra site has been subject to fully factorial manipulation consisting of increased soil water supply for 14 years, and occasional nitrogen (N) addition in pulses. Summer watering enhanced R_soil during summer, but decreased R_soil in the following autumn. We speculate that this is due to intensified depletion of recently fixed plant carbon by soil organisms. Hence, autumn soil microbial activity seems tightly linked to growing season plant production through plant‐associated carbon pools. Nitrogen addition alone consistently increased R_soil, but when water and nitrogen were added in combination, autumn R_soil declined similarly to when water was added alone. Despite several freeze‐thaw events, the microbial biomass carbon (C) remained constant until finally being reduced by ~60% in late September. In spite of significantly reduced microbial biomass C and phosphorus (P), microbial N did not change. This suggests N released from dead microbes was quickly assimilated by surviving microbes. We observed no change in soil organic matter content after 14 years of environmental manipulations, suggesting high ecosystem resistance to environmental changes.     

Degradation of seed mucilage by soil microflora promotes early seedling growth of a desert sand dune plant

In contrast to the extensive understanding of seed mucilage biosynthesis, much less is known about how mucilage is biodegraded and what role it plays in the soil where seeds germinate. We studied seed mucilage biodegradation by a natural microbial community. High‐performance anion‐exchange chromatography (HPAEC) was used to determine monosaccharide composition in achene mucilage of Artemisia sphaerocephala. Mucilage degradation by the soil microbial community from natural habitats was examined by monosaccharide utilization tests using Biolog plates, chemical assays and phospholipid fatty acid (PLFA) analysis. Glucose (29.4%), mannose (20.3%) and arabinose (19.5%) were found to be the main components of achene mucilage. The mucilage was biodegraded to CO₂ and soluble sugars, and an increase in soil microbial biomass was observed during biodegradation. Fluorescence microscopy showed the presence of mucilage (or its derivatives) in seedling tissues after growth with fluorescein isothiocyanate (FITC)‐labelled mucilage. The biodegradation also promoted early seedling growth in barren sand dunes, which was associated with a large soil microbial community that supplies substances promoting seedling establishment. We conclude that biodegradation of seed mucilage can play an ecologically important role in the life cycles of plants especially in harsh desert environments to which A. sphaerocephala is well‐adapted.     

Coupling hydrocarbon degradation to anaerobic respiration and mineral diagenesis: theoretical constraints

The diagenetic mineral assemblages in petroleum reservoirs control the formation fluid pH and pCO₂. Anaerobic biodegradation of petroleum is controlled by the transfer of electrons from reduced organic species to inorganic, redox sensitive, aqueous and mineral species in many cases through intermediates such as H₂ and CH₃COO^?. The terminal electron accepting reactions induce the dissolution or precipitation of the same minerals that control the ambient pH and pCO₂ in petroleum reservoirs. In this study, we develop a model for anaerobic biodegradation of petroleum that couples the production of acetate and H₂ to ‘late stage’ diagenetic reactions. The model reveals that the principal terminal electron accepting process and electron donor control the type of diagenetic reaction, and that the petroleum biodegradation rate is controlled through thermodynamic restriction by the minimum ΔG required to support a specific microbial metabolism, the fluid flux and the mineral assemblage. These relationships are illustrated by modeling coupled microbial diagenesis and biodegradation of the Gullfaks oil reservoir. The results indicate that the complete dissolution of albite by acids generated during oil biodegradation and the corresponding elevated pCO₂ seen in the Gullfaks field are best explained by methanogenic respiration coupled to hydrocarbon degradation and that the biodegradation rate is likely controlled by the pCH₄. Biodegradation of Gullfaks oil by a consortium that includes either Fe³⁺‐reducing or ‐reducing bacteria cannot explain the observed diagenetic mineral assemblage or pCO₂. For octane, biodegradation, not water washing, was the principal agent for removal at fluid velocities <20 m Myr^?1.     

Enhanced Degradation of n-Hexadecane in Diesel Fuel Contaminated Soil by the Addition of Fermented Whey

The objective of this work has been to investigate the possibility of using fermented whey as an organic growth supplement in order to enhance the aerobic degradation of n-hexadecane in soil. Fermented whey was added at different dosages to nutrient amended soil microcosms contaminated with 5000 mg diesel fuel kg^?1 dry weight (dw). The target substance was ¹⁴C-labeled n-hexadecane, and the biodegradation was monitored by analysis of evolved ¹⁴CO₂. Biodegradation curves were fitted to a three-half-order kinetics model. Enhanced biodegradation was observed in sand at 7 and 22°C and in loamy sand at 22°C but the effect was most pronounced in the sand soil at 22°C. The addition of 6 or 60 ml fermented whey kg^{? 1} soil dw increased the degree of n-hexadecane biodegradation at the end of the experiment, 167 days, from 49% in the untreated sand to 60 or 67%, respectively. This increase in biodegradation was characterized by an increase in the amount of substrate biodegradation by first-order kinetics despite a decrease in the first order rate constant, k₁. The highest concentration of fermented whey, 60 ml kg^?1, gave rise to substrate competition, diauxie, which resulted in an extended lag phase.     

Elevated CO2 increases microbial carbon substrate use and nitrogen cycling in Mojave Desert soils

Identifying soil microbial responses to anthropogenically driven environmental changes is critically important as concerns intensify over the potential degradation of ecosystem function. We assessed the effects of elevated atmospheric CO₂ on microbial carbon (C) and nitrogen (N) cycling in Mojave Desert soils using extracellular enzyme activities (EEAs), community‐level physiological profiles (CLPPs), and gross N transformation rates. Soils were collected from unvegetated interspaces between plants and under the dominant shrub (Larrea tridentata) during the 2004–2005 growing season, an above‐average rainfall year. Because most measured variables responded strongly to soil water availability, all significant effects of soil water content were used as covariates to remove potential confounding effects of water availability on microbial responses to experimental treatment effects of cover type, CO₂, and sampling date. Microbial C and N activities were lower in interspace soils compared with soils under Larrea, and responses to date and CO₂ treatments were cover specific. Over the growing season, EEAs involved in cellulose (cellobiohydrolase) and orthophosphate (alkaline phosphatase) degradation decreased under ambient CO₂, but increased under elevated CO₂. Microbial C use and substrate use diversity in CLPPs decreased over time, and elevated CO₂ positively affected both. Elevated CO₂ also altered microbial C use patterns, suggesting changes in the quantity and/or quality of soil C inputs. In contrast, microbial biomass N was higher in interspace soils than soils under Larrea, and was lower in soils exposed to elevated CO₂. Gross rates of NH₄⁺ transformations increased over the growing season, and late‐season NH₄⁺ fluxes were negatively affected by elevated CO₂. Gross NO₃⁻ fluxes decreased over time, with early season interspace soils positively affected by elevated CO₂. General increases in microbial activities under elevated CO₂ are likely attributable to greater microbial biomass in interspace soils, and to increased microbial turnover rates and/or metabolic levels rather than pool size in soils under Larrea. Because soil water content and plant cover type dominates microbial C and N responses to CO₂, the ability of desert landscapes to mitigate or intensify the impacts of global change will ultimately depend on how changes in precipitation and increasing atmospheric CO₂ shift the spatial distribution of Mojave Desert plant communities.     

Linking temperature sensitivity of soil organic matter decomposition to its molecular structure,accessibility, and microbial physiology

Temperature sensitivity of soil organic matter (SOM) decomposition may have a significant impact on global warming. Enzyme‐kinetic hypothesis suggests that decomposition of low‐quality substrate (recalcitrant molecular structure) requires higher activation energy and thus has greater temperature sensitivity than that of high‐quality, labile substrate. Supporting evidence, however, relies largely on indirect indices of substrate quality. Furthermore, the enzyme‐substrate reactions that drive decomposition may be regulated by microbial physiology and/or constrained by protective effects of soil mineral matrix. We thus tested the kinetic hypothesis by directly assessing the carbon molecular structure of low‐density fraction (LF) which represents readily accessible, mineral‐free SOM pool. Using five mineral soil samples of contrasting SOM concentrations, we conducted 30‐days incubations (15, 25, and 35 °C) to measure microbial respiration and quantified easily soluble C as well as microbial biomass C pools before and after the incubations. Carbon structure of LFs (<1.6 and 1.6–1.8 g cm^?3) and bulk soil was measured by solid‐state ¹³C‐NMR. Decomposition Q₁₀ was significantly correlated with the abundance of aromatic plus alkyl‐C relative to O‐alkyl‐C groups in LFs but not in bulk soil fraction or with the indirect C quality indices based on microbial respiration or biomass. The warming did not significantly change the concentration of biomass C or the three types of soluble C despite two‐ to three‐fold increase in respiration. Thus, enhanced microbial maintenance respiration (reduced C‐use efficiency) especially in the soils rich in recalcitrant LF might lead to the apparent equilibrium between SOM solubilization and microbial C uptake. Our results showed physical fractionation coupled with direct assessment of molecular structure as an effective approach and supported the enzyme‐kinetic interpretation of widely observed C quality‐temperature relationship for short‐term decomposition. Factors controlling long‐term decomposition Q₁₀ are more complex due to protective effect of mineral matrix and thus remain as a central question.     

Factors affecting the distribution and dynamics of14C in two soils cropped to barley

The distribution of¹²C and¹⁴C and dynamics of¹⁴C in barley plots were studied after pulse labelling with¹⁴CO₂. Barley was grown in microplots on a Black soil at Ellerslie and a Gray Luvisol at Breton, Alberta and was sampled on four dates between July 31 and October 20. The quantity of¹²C in shoots, microbial biomass, and soil was greater at Ellerslie than Breton. Root¹²C did not differ between sites. There were no significant differences over time in the quantity of¹²C in any of the pools. On the first sampling date 18.59% of the¹⁴C was recovered in the soil-plant system at Ellerslie and 60.82% was recovered at Breton. At Ellerslie 20.35 MBq were recovered in shoots, 0.50 in roots, 0.08 in microbial C and 0.77 in soil while at Breton 27.92 MBq were recovered in shoots, 2.70 in roots, 0.22 in microbial C and 39.3 in soil. The difference between sites in soil¹⁴C was due to higher water filled porosity at Breton than at Ellerslie at the time of labelling. Soluble C (gg^?1 root C), used as a measure of root exuded C, increased above 70% water filled porosity. There were no significant differences over time in the quantity of¹⁴C in any of the pools or in the specific activity of¹⁴CO₂ released from soil during 10-day laboratory incubation. This indicated that the belowground system was in steady state with a continuous input of¹⁴C from labelled root matrial. Differences in specific activity of the various belowground pools revealed that an average of 17% of the microbial C was active at Ellerslie while 43% was active at Breton. Active microbial C (gm^?2) was the same at both sites because total microbial C was lower at Breton than at Ellerslie. At Breton some of the¹⁴C released under conditions of high water filled porosity at the time of labelling appeared to be stabilized against microbial turnover.