Progress 09/01/09 to 08/31/13
Outputs
Target Audience: The target audience for this project was the scientific community including students and professional scientists at all levels. Our efforts to reach these individuals included conference presentations and integration of our work into global communities of scientists working to understand soil processes. Details on these presentations are included in the Products section of this report. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? We have trained two PhD students and two post-docs on this project. One PhD has learned the experiment methods for implementing fluctuating redox experiments. This student has also learned how to numerically model the reactive network underlying dynamic biogeochemical processes. Both PhD are now trained in analyzing samples by 57Fe Mossbauer spectroscopy. We have also exposed a pure-system microbiologically oriented postdoctoral scholar to the complexities of natural system biogeochemistry and trained a lecturer from a local two-year collage in performing Fe isotope spike experiments and associated geochemical experiments. How have the results been disseminated to communities of interest? The scientific community was the main community of interest for this project, including students and professional scientists at all levels. Our efforts to reach these individuals included conference presentations and integration of our work into global communities of scientists working to understand soil processes. Details on these presentations are included in the Products section of this report. What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
This project aimed to advance understanding of the role of redox fluctuations on soil processes. Redox fluctuations are common during soil flooding and drying and can strongly influence the nutrient and carbon composition of agricultural and forestry lands. We focused our project on the iron (Fe) cycle, which responds strongly to redox fluctuations, and iron’s interactions with the phosphorus (P) and carbon (C) cycles. We conducted four types of experiments to address our objectives, integrating each of them with numerical modeling: (1) we performed molecular-scale characterization of the iron minerals at field sites with a long history of redox fluctuations; (2) we manipulated the redox status of soils in laboratory reactors and measured change in Fe, C, and P chemistry; (3) we conducted the iron atom exchange experiments in the presence of C and with natural soils; (4) and we evaluated the influence of soil storage conditions on iron reaction rates. We found the majority of the iron in these soils is short range ordered (SRO), indicating that this iron will be highly reactive toward C and P sorption and also highly susceptible to Fe reduction. We found that subjecting soils to fluctuating redox can change the reactivity of these Fe phases, with preliminary data suggesting that rapid oxidation drives higher reactivity. We found that exposing soils to oxygen-limited conditions can drive mobilization of colloids (particles < 450 nanometers) that contain substantial C and P. Thus the indirect effects of iron reduction on colloid mobilization can influence the biogeochemistry of these elements. Our work has also illustrated that iron atom exchange between aqueous ferrous iron (Fe2+) and solid phase Fe minerals occurs in natural soils and in the presence of most organic ligands. Finally, our work suggests that for tropical soils, air-drying or storage as an intact soil block maintains the soil’s capacity for FeIII reduction over long-term sample storage more effectively than cold-storage. Collectively, our findings illustrate redox fluctuations have the potential to substantially alter soil biogeochemical processes, but that the characteristics of the redox fluctuation are very important. We describe further details of some of these experiments below: Redox Oscillation Experiments We initiated our investigations using focused on 8-week oscillation experiments across three frequencies (3.5 days, 7 days, and 14 days). All treatments had the same ratio of time under oxic vs. anoxic conditions (1:6 oxidizing:reducing conditions), and all were exposed to full amplitude oscillations with 21% oxygen air used as the oxidant. Non-oscillating controls were also run. We found Fe reduction rates increased with successive oscillations such that by the end of the experiment newly precipitated Fe could be re-reduced in less than three days. In addition, a plateau in the maximum HCl-extractable Fe(II) concentration was consistent across all treatments. In terms of P behavior, we observe a decrease in the NaOH extractable P and an increase in P associated with the microbial pool. We developed a numerical model describing ferrous (Fe2+) iron behavior in the experiments, implementing several model formulations that differed in the number of distinct iron oxide pools, description of aeration kinetics during the oxidation phase, microbial growth rates and iron reduction rate laws. To identify the controlling parameters, we conducted additional oscillation experiments which revealed that the Fe(II) concentration plateau was independent of the oscillation frequency. Based on iterative model-experiment sequences, we refined our numerical model to include two iron oxide pools, a potentially reducible pool and a rapidly-reducible pool. This provided reasonable fits across all oscillation frequencies with the same microbial growth parameters for all treatments. Thus, the synthesis of our numerical and experimental data suggests that temporal dynamics of Fe(II) are best explained by progressive reduction of more recalcitrant Fe(III) solid phases and potentially minor increases in the population of Fe reducers. We further directly investigated P and C mobilization as colloids during Fe reduction in agricultural soils in Georgia as well as in basaltic soils in Hawaii. In laboratory incubations, GA soils were subjected to 7 days of anoxic conditions or equilibrated at pH 6 and 8 under oxic conditions and then the extract was size fractionated by differential centrifugation/ultrafiltration. Anoxic incubation stimulated colloidal P release with seasonally saturated soils releasing more colloidal P and Fe2+(aq) than well-drained soils; whereas, non-reductive particle dispersion, accomplished by raising the pH under oxic conditions, yielded no increase in colloidal P. This suggests Fe acts as a cementing agent in these soils, binding to the bulk soil P-bearing colloids that can be released during reducing conditions. Furthermore, it suggests prior periodic exposure to anoxic conditions increases susceptibility to redox-induced P mobilization. In the Hawaiian soils we found Fe reduction resulted in ~500% increases of colloidal C whereas much smaller amounts of colloidal C were mobilized under oxic conditions than in the Fe reduction treatments (p<0.05). Soil storage experiments We assessed the impact of moisture and temperature during laboratory storage of these soils from the Luquillo Experimental Forest, Puerto Rico on subsequent rates of Fe reduction over one year. Storage for a duration of 12 months at 4°C decreased FeII production following 7 days of anoxic incubation, while air-drying soils maintained similar FeII production levels. The observed FeII production and microbial cell counts in these tropical soils suggest that storage at cold temperatures, as well as homogenization when microbial activity is not suppressed during storage, reduced the resilience of the resident Fe reducing microbial communities, a finding consistent with studies on other microbial processes in tropical soils. Iron-atom Exchange Experiments One of the main consequences of fluctuating redox conditions is the production of soluble reduced iron (e.g., Fe2+) during the anoxic or oxygen-free part of the redox fluctuation. In the last decade scientists working with pure minerals have discovered that Fe2+(aq) can transfer electrons and actually exchange structural positions with solid-phase ferric (FeIII) atoms in many Fe minerals. In the soil, this could dramatically change the composition of soil iron phases with obvious feedbacks on the C and P cycles. We tested the Luquillo soils for their capacity to undergo iron atom exchange with a 28-day period. After accounting for sorption using a numerical model we found the aqueous and bulk pools both moved ~7% toward the isotopic equilibrium. We also computed that the rates of iron atom exchange were most rapid during Fe2+ sorption, and continued slowly thereafter. These rates of atom exchange are fast enough to impact ecological processes in the soil, but slow enough that changes soil redox—for instance when the soil dries out—will likely occur before complete Fe mineral turnover. We also conducted Fe2+ atom experiments with the soil minerals goethite and magnetite in the presence of various forms of organic C. Isotope tracer experiments with 57Fe Mössbauer spectroscopy reveal electron transfer and atom mixing occurs between Fe2+(aq) and structural Fe(III) in both goethite and magnetite in the presence of a suite of organic C compounds , including natural organic matter (NOM), extracellular polysaccharides (EPS), cell material, and electron shuttles. However, it was inhibited tin the presence of a long-chain (39 C) organic C containing phospholipid. These findings show that Fe2+(aq)-catalyzed Fe oxide recrystallization is a robust process that is likely to occur in a variety of C-rich biogeochemical environments.
Publications
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Ginn, B., Habteselassie, M.Y., Meile, C. and Thompson A. (2014) Effects of sample storage on Fe reduction in tropical soils. Soil Biology and Biochemistry 68, 44 51.
- Type:
Journal Articles
Status:
Under Review
Year Published:
2013
Citation:
Tishchenko, V., Meile, C., Scherer, M., Pasakarnis, T., and Thompson A. (In Review) Fe2+ catalyzed iron atom exchange and re-crystallization in tropical soils.
- Type:
Journal Articles
Status:
Accepted
Year Published:
2013
Citation:
Buettner, S., Kramer, M., Chadwick, O.A. and Thompson A. (In Press). Mobilization of colloidal carbon during iron reduction events in basaltic soils. Geoderma.
- Type:
Journal Articles
Status:
Published
Year Published:
2013
Citation:
Thompson, A., Amistadi, M.K., Chadwick, O.A., and J. Chorover. (2013). Fractionation of yttrium and holmium during basaltic soil weathering. Geochim. Cosmochim. Acta 119, 18-30.
- Type:
Journal Articles
Status:
Published
Year Published:
2012
Citation:
Henderson, R., Kabengi, N., Mantripragada, N., Cabrera, M., Hassan, S., and Thompson A., (2012). Anoxia-Induced Release of Colloid- and Nanoparticle-Bound Phosphorus in Grassland Soils. Environ. Sci. Technol. 46, 11727-11734
- Type:
Journal Articles
Status:
Other
Year Published:
2014
Citation:
Timothy Pasakarnis, Mike McCormick, Gene F. Parkin, Aaron Thompson, and Michelle M. Scherer. Fe(II)-catalyzed Fe oxide recrystallization:Effect of organic carbon. Invited article in preparation for a Research Front in Environmental Chemistry.
|
Progress 09/01/12 to 08/31/13
Outputs
Target Audience: The target audience for this project was the scientific community including students and professional scientists at all levels. Our efforts to reach these individuals included conference presentations and integration of our work into global communities of scientists working to understand soil processes. Details on these presentations are included in the Products section of this report. Changes/Problems:
Nothing Reported
What opportunities for training and professional development has the project provided? We have trained two PhD students and two post-docs on this project. One PhD has learned the experiment methods for implementing fluctuating redox experiments. This student has also learned how to numerically model the reactive network underlying dynamic biogeochemical processes. Both PhD are now trained in analyzing samples by 57Fe Mossbauer spectroscopy. We have also exposed a pure-system microbiologically oriented postdoctoral scholar to the complexities of natural system biogeochemistry and trained a lecturer from a local two-year collage in performing Fe isotope spike experiments and associated geochemical experiments. How have the results been disseminated to communities of interest?
Nothing Reported
What do you plan to do during the next reporting period to accomplish the goals?
Nothing Reported
Impacts
What was accomplished under these goals?
This project aimed to advance understanding of the role of redox fluctuations on soil processes. Redox fluctuations are common during soil flooding and drying and can strongly influence the nutrient and carbon composition of agricultural and forestry lands. We focused our project on the iron (Fe) cycle, which responds strongly to redox fluctuations, and iron’s interactions with the phosphorus (P) and carbon (C) cycles. We conducted four types of experiments to address our objectives, integrating each of them with numerical modeling: (1) we performed molecular-scale characterization of the iron minerals at field sites with a long history of redox fluctuations; (2) we manipulated the redox status of soils in laboratory reactors and measured change in Fe, C, and P chemistry; (3) we conducted the iron atom exchange experiments in the presence of C and with natural soils; (4) and we evaluated the influence of soil storage conditions on iron reaction rates. We found the majority of the iron in these soils is short range ordered (SRO), indicating that this iron will be highly reactive toward C and P sorption and also highly susceptible to Fe reduction. We found that subjecting soils to fluctuating redox can change the reactivity of these Fe phases, with preliminary data suggesting that rapid oxidation drives higher reactivity. We found that exposing soils to oxygen-limited conditions can drive mobilization of colloids (particles < 450 nanometers) that contain substantial C and P. Thus the indirect effects of iron reduction on colloid mobilization can influence the biogeochemistry of these elements. Our work has also illustrated that iron atom exchange between aqueous ferrous iron (Fe2+) and solid phase Fe minerals occurs in natural soils and in the presence of most organic ligands. Finally, our work suggests that for tropical soils, air-drying or storage as an intact soil block maintains the soil’s capacity for FeIII reduction over long-term sample storage more effectively than cold-storage. Collectively, our findings illustrate redox fluctuations have the potential to substantially alter soil biogeochemical processes, but that the characteristics of the redox fluctuation are very important. We describe further details of some of these experiments below: Redox Oscillation Experiments We initiated our investigations using focused on 8-week oscillation experiments across three frequencies (3.5 days, 7 days, and 14 days). All treatments had the same ratio of time under oxic vs. anoxic conditions (1:6 oxidizing:reducing conditions), and all were exposed to full amplitude oscillations with 21% oxygen air used as the oxidant. Non-oscillating controls were also run. We found Fe reduction rates increased with successive oscillations such that by the end of the experiment newly precipitated Fe could be re-reduced in less than three days. In addition, a plateau in the maximum HCl-extractable Fe(II) concentration was consistent across all treatments. In terms of P behavior, we observe a decrease in the NaOH extractable P and an increase in P associated with the microbial pool. We developed a numerical model describing ferrous (Fe2+) iron behavior in the experiments, implementing several model formulations that differed in the number of distinct iron oxide pools, description of aeration kinetics during the oxidation phase, microbial growth rates and iron reduction rate laws. To identify the controlling parameters, we conducted additional oscillation experiments which revealed that the Fe(II) concentration plateau was independent of the oscillation frequency. Based on iterative model-experiment sequences, we refined our numerical model to include two iron oxide pools, a potentially reducible pool and a rapidly-reducible pool. This provided reasonable fits across all oscillation frequencies with the same microbial growth parameters for all treatments. Thus, the synthesis of our numerical and experimental data suggests that temporal dynamics of Fe(II) are best explained by progressive reduction of more recalcitrant Fe(III) solid phases and potentially minor increases in the population of Fe reducers. We further directly investigated P and C mobilization as colloids during Fe reduction in agricultural soils in Georgia as well as in basaltic soils in Hawaii. In laboratory incubations, GA soils were subjected to 7 days of anoxic conditions or equilibrated at pH 6 and 8 under oxic conditions and then the extract was size fractionated by differential centrifugation/ultrafiltration. Anoxic incubation stimulated colloidal P release with seasonally saturated soils releasing more colloidal P and Fe2+(aq) than well-drained soils; whereas, non-reductive particle dispersion, accomplished by raising the pH under oxic conditions, yielded no increase in colloidal P. This suggests Fe acts as a cementing agent in these soils, binding to the bulk soil P-bearing colloids that can be released during reducing conditions. Furthermore, it suggests prior periodic exposure to anoxic conditions increases susceptibility to redox-induced P mobilization. In the Hawaiian soils we found Fe reduction resulted in ~500% increases of colloidal C whereas much smaller amounts of colloidal C were mobilized under oxic conditions than in the Fe reduction treatments (p<0.05). Soil storage experiments We assessed the impact of moisture and temperature during laboratory storage of these soils from the Luquillo Experimental Forest, Puerto Rico on subsequent rates of Fe reduction over one year. Storage for a duration of 12 months at 4°C decreased FeII production following 7 days of anoxic incubation, while air-drying soils maintained similar FeII production levels. The observed FeII production and microbial cell counts in these tropical soils suggest that storage at cold temperatures, as well as homogenization when microbial activity is not suppressed during storage, reduced the resilience of the resident Fe reducing microbial communities, a finding consistent with studies on other microbial processes in tropical soils. Iron-atom Exchange Experiments One of the main consequences of fluctuating redox conditions is the production of soluble reduced iron (e.g., Fe2+) during the anoxic or oxygen-free part of the redox fluctuation. In the last decade scientists working with pure minerals have discovered that Fe2+(aq) can transfer electrons and actually exchange structural positions with solid-phase ferric (FeIII) atoms in many Fe minerals. In the soil, this could dramatically change the composition of soil iron phases with obvious feedbacks on the C and P cycles. We tested the Luquillo soils for their capacity to undergo iron atom exchange with a 28-day period. After accounting for sorption using a numerical model we found the aqueous and bulk pools both moved ~7% toward the isotopic equilibrium. We also computed that the rates of iron atom exchange were most rapid during Fe2+ sorption, and continued slowly thereafter. These rates of atom exchange are fast enough to impact ecological processes in the soil, but slow enough that changes soil redox—for instance when the soil dries out—will likely occur before complete Fe mineral turnover. We also conducted Fe2+ atom experiments with the soil minerals goethite and magnetite in the presence of various forms of organic C. Isotope tracer experiments with 57Fe Mössbauer spectroscopy reveal electron transfer and atom mixing occurs between Fe2+(aq) and structural Fe(III) in both goethite and magnetite in the presence of a suite of organic C compounds , including natural organic matter (NOM), extracellular polysaccharides (EPS), cell material, and electron shuttles. However, it was inhibited tin the presence of a long-chain (39 C) organic C containing phospholipid. These findings show that Fe2+(aq)-catalyzed Fe oxide recrystallization is a robust process that is likely to occur in a variety of C-rich biogeochemical environments.
Publications
|
Progress 09/01/11 to 08/31/12
Outputs
OUTPUTS: Field Soil Characterization In our third year, we have collected additional soils from the Luquillo National Forest, Puerto Rico, this time at the ridge, slope and valley of a catena. We are using these soils for experiments exploring the rate of iron oxidation as a driving factor coupling iron and carbon biogeochemistry. Oscillation Experiments We have focused on testing several numerical models spanning a range of complexity against our experimental observations of iron dynamics during the oscillation treatments. A key observation is that iron reduction rates increased over the course of the experiment. We generated two competing hypotheses to explain this: (1) that the iron reducing population increased over the course of the experiment and (2) that the quantity of readily reducible iron increased over the course of the experiment. We used soil from the beginning and end of oscillation treatments to test these hypotheses by: (1) measuring iron reducer populations and supplying those soils as the sole source of iron for an iron reducing bacteria in pure culture. Soil storage experiments We have completed an assessment of the microbial community composition in our storage experiments and preformed statistical analysis of our data. We are preparing a manuscript on the Fe storage experiments with a planned submission by the end of the year. Iron-atom Exchange Experiments We have constructed several competing numerical model structures of Iron atom exchange in soils using coupled differential equations that treat each isotope of iron independently. We have tested these models against our observations of iron concentrations and isotopic signatures in the aqueous and solid phase pools. We have composed a manuscript reporting our findings that will be submitted by the end of the year. Our iron(II) atom experiments conducted with the soil mineral goethite in the presence of phosphate are continuing with emphasis placed on Mossbauer analysis of the resulting solids after the iron atom exchange has occurred. PARTICIPANTS: Aaron Thompson (PI/PD) Christof Meile (PI) Michelle Scherer (PI) Jared Wilmoth (PhD student) Tim Pasakarnis (PhD student) Brian Ginn (Post-doc) Viktor Tishchenko (Visiting Scholar) Kim Kaufman (Project Coordinator). TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
Field Soil Characterization Our characterization of soils from six different locations in the Valley field site of the Bisley Watershed, Luquillo National Forest, Puerto Rico will be included in a special issue of the journal Catena to be published in 2013. Oscillation Experiments Our numerical model containing two iron oxide pools was able to represent the experimental data across all oscillation frequencies with the same parameter assignments. In the model we partitioned all iron(III) oxides forming during the oxidation of iron(II) into a rapidly-reducible pool of iron that was small at the beginning of the experiment. This provided reasonable fits across all oscillation frequencies and highlights the impact of redox oscillations on the nature and reactivity of oxidized iron solid phases. Our bio-assay of the iron available for microbial reduction indicated a greater abundance of easily-reducible iron following oscillations than in the beginning of the experiment or in our continuously oxic controls. Soil Storage Experiments Microbial community analysis of the bacterial strains enriched from the cold-stored and air-dried soils had 16S rDNA nucleotide sequences with 99-100% similarity to a Pseudomonas strain and 97-98% similarity to Bacillus strain, respectively. The isolated strains did not comprise a significant portion of the extracted DNA from the one-year stored soils. We detected a weak band corresponding to the 4 degrees C isolate (Pseudomonas sp.) in the moist 4 degrees C soil, but the fingerprint bands for the air-dried isolate (Bacillus sp.) were below detection after one-year of storage. However, following a two-week anoxic incubation of these soils our isolates represented the brightest bands in the respective fingerprint profiles. These experiments highlighted the importance of storing soil under conditions that resemble periods of low microbial activity in the field. For instance, iron reduction rates in these tropical soils were more strongly impacted by cold storage than by air-drying. Iron-atom Exchange Experiments Our iron-atom exchange experiments indicate isotopically-labeled aqueous Iron(II) is exchanged for surface iron atoms as well as iron atoms in more recalcitrant pools. Our sterile treatments can be described as fast sorption/incorporation of Fe2+(aq) to the labile extractable phase (0.5 M HCl), occurring over timescales of minutes, followed by exchange between the labile layer and the bulk iron oxides. The characteristic timescale for desorption is on the order of a week, whereas release from the bulk phase (7 M HCl) to the labile phase (0.5 M HCl) is on the order of years. Mossbauer spectroscopy results indicate that the 57Fe label is incorporated preferentially into FeIII-oxyhydroxides of very low crystallinity (i.e., short-range-order, SRO), suggesting these phases participate disproportionally in Fe atom exchange reactions with Fe2+(aq) in natural soils. These results provide one of the first indications that ferrous catalyzed iron atom exchange occurs in soils and represents a new pathway for biogeochemical cycling of important elements, such as carbon and phosphorus.
Publications
- Wilmoth, J, Meile, C, Ginn, B, Pasakarnis, T, Hall, S, Scherer, M and Thompson, A. Fe-P-C and microbial Fe reduction dynamics examined by induced redox frequency and amplitude flux. Goldschmidt Conference, Montreal, Canada, June 24-29, 2012.
- Scherer, MM. Fe(II) catalyzed Fe oxide recrystallization: An Update . Telluride Workshop: Biogeochemistry and Redox Transformations of Iron, Telluride, CO, USA, August 7-10, 2012.
- Scherer, MM, Latta, DE, Pasakarnis T, Neumann, A, Barger, M, Rosso, K, Johnson, C. Fe electron transfer and atom exchange at mineral/water interfaces. Invited talk at 22nd Goldschmidt Geochemistry Conference, Montreal, Canada, June 24-29 2012.
- Thompson, A, Ginn, B, Tishchenko, V, Meile, C, Wilmoth, J, Pasakarnis, T and Scherer, M. Fe reduction and exchange rates during redox oscillation of the Luquillo CZO forest soils. Invited talk at 22nd Goldschmidt Geochemistry Conference, Montreal, Canada, June 24-29 2012.
- Thompson, A. High-amplitude redox fluctuations prime tropical forest soils for rapid iron reduction rates. Invited talk at the 97th Ecological Society of America Annual Meetings. Portland, OR, USA, August 5-9, 2012.
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Progress 09/01/10 to 08/31/11
Outputs
OUTPUTS: [Field Soil Characterization]: In our second year, we have collected additional soils from the Luquillo National Forest, Puerto Rico for use in our experiments. We have conducted a comprehensive analysis of the mineral composition of these soils including XRay diffraction, selective extractions and Mossbauer spectroscopy. [Oscillation Experiments]: We have conducted two additional redox oscillation experiments. The first was an 8-week oscillation across three frequencies (3.5 days, 7 days, and 14 days). All treatments had the same ratio of time under oxic vs. anoxic conditions (1/6 of the reaction time is spent under oxidizing conditions), and all were exposed to full amplitude oscillations with 21% oxygen air used as the oxidant. Non-oscillating controls were also run. Our 2nd oscillation experiment was conducted under the same conditions and then terminated after 4 weeks and iron (Fe) reduction rates were measured during a 7-d anoxic incubation across all treatments. We have analyzed the resulting aqueous and solid phase for Fe and phosphorus (P) and microbial abundance. Our development of a numerical model for these experiments has focused on process-based analysis of ferrous iron behavior. We have implemented several model formulations to describe the observed Fe dynamics spanning a range of complexity. These formulations include a range of iron oxide pools, accounting for gas exchange kinetics and lag phases in microbial growth, and several different formulations of the rate laws. We have compared these model simulations with the experimental data to help constrain our interpretations. [Soil storage experiments]: We have extended our assessment of the impact of soil storage on rates of Fe reduction to include measurements after one year. In addition, we have quantified the abundance of Fe reducers in these soils and we have begun an assessment of microbial community composition. [Fe-atom Exchange Experiments]: We have conducted experiments to measure the rate and extent of atom exchange between isotopically labeled Fe(II) and the same soil samples used in our oscillation experiments. These experiments were conducted under sterile and live conditions. We have conducted similar experiments with a separate set of soils from ridge, slope and valley sites in the Bisley watershed of the Luquillo Experimental Forest. These experiments have been conducted under sterile and live conditions. In addition, we are performing these Fe(II) atom experiments with the soil mineral goethite in the presence of phosphate. We have developed a conceptual model for this Fe(II) atom exchange between aqueous phase and a surface and bulk solid phase Fe pool. We have implemented this as a system of coupled differential equations and parameterized them using both measured concentrations and isotopic signatures in the aqueous and solid phase pools. PARTICIPANTS: Aaron Thompson (PI/PD) Christof Meile (PI) Michelle Scherer (PI) Jared Wilmoth (PhD student) Tim Pasakarnis (PhD student) Brian Ginn (Post-doc) Viktor Tishchenko (Visiting Scholar) TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
[Field Soil Characterization]: Our broader characterization of soils from six different locations in the Valley field site of the Bisley Watershed, Luquillo National Forest, Puerto Rico confirms that a nano-Fe phase is the most abundant Fe mineral component in the soil. Mossbauer parameters for this component are best approximated by nano-goethite similar to the soil used in our oscillation experiments. [Oscillation Experiments]: For full amplitude oscillations (21% oxygen air) with 1/6 of the reaction time spent under oxidizing conditions, we found Fe reduction rates increased with successive oscillations such that by the end of the experiment newly precipitated Fe could be re-reduced in less than three days. The observed increase in Fe reduction rates was independent of oscillation frequency. NaOH extractable P decreased over the course of the experiment. Our numerical model formulations suggest the observed temporal dynamics can be explained by an increasing population or activity of Fe reducers, yet no order-of-magnitude increases in the culturable Fe reducer population were detected over the course of the experiment. Similarly, Fe(II) reduction rates exhibit a low concentration plateau early in the experiment, suggesting Fe(II) production was limited by abiotic factors in addition to microbial growth. Thus, synthesis of our numerical and experimental data suggest temporal dynamics of Fe(II) are best explained by minor increases in the population of Fe reducers accompanied by progressive reduction of more recalcitrant Fe(III) solid phases. The plateau in the amount of reducible Fe coincides with the amount of Fe extractable by citrate-ascorbate and the spectral area of Fe that magnetically orders to form a Fe-oxyhydroxide sextet at 140 K in the Mossbauer spectra. [Soil storage experiments]: Rates of Fe reduction after one year were highest in the air-dried samples. Bacterial strains isolated enriched from the air-dried and cold-stored samples differed in morphology, microbial community analysis is underway. [Fe-atom Exchange Experiments]: Our Fe-atom exchange experiments indicate isotopically-labeled aqueous Fe(II) is exchanged for surface Fe atoms as well as Fe atoms in more recalcitrant pools. Our preliminary numerical simulations of this process quantified the dynamics of isotopic exchange, and led to the parameterization of sorption reactions, with initial turnover times in surface and bulk solid phase iron pools on the order of hours, and months, respectively.
Publications
- Kabengi, N, and Thompson, A. (2011) The emerging emphasis on nanometer scale processes in soil environments. Soil Sci. Soc. Am. J., 75, 1-2
- Scherer, M., Latta, D., Pasakarnis, T., Kaspar, M., Allman, D., Thompson, A., Tishchenko, V. (2011) Redox Driven Atom Exchange in Fe Oxides. Division of Colloid and Interface Science. ACS National Meeting in March, 2011 (Annaheim).
- Thompson, A., Wilmoth, J., Tishchenko, V., Meile, C., Scherer, M., and Pasakamis, T. (2011) Characterization of Nano-cystalline Fe in tropical soils using 57Fe Mossbauer and Fe atom-exchange experiments. Clay Minerals Society Annual Meeting September 25-30, 2011
- Thompson, A. Ginn, B., Tishchenko, V., Meile, C., Scherer, M., Wilmoth, J, and Pasakamis, T. (2011) Redox cycling and Fe atom exchange at the Bisley Site in Luquillo Exp. Forest. National Critical Zone Observatories Program All Hands Meeting May 8-13, 2011 Biosphere 2, University of Arizona, Tucson, Arizona
- Thompson, A. Ginn, B., Tishchenko, V., Meile, C., Scherer, M., Wilmoth, J, and Pasakamis, T. (2011) Fe atom exchange, and influence of redox oscillation frequency on Fe reduction at the Bisley site, Luquillo CZO. Global Soil Change Workshop NSF/Duke University Calhoun Experimental Forest, S.C. & Cowetta Hydrologic Lab, NC, June 13-16, 2011.
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Progress 09/01/09 to 08/31/10
Outputs
OUTPUTS: This USDA Soil Processes research project focuses on the importance of redox fluctuations (e.g., wetting-and-drying cycles) on coupled iron (Fe), carbon (C) and phosphorous (P) cycling. Our guiding hypothesis is that variations in redox conditions drive changes in Fe biogeochemistry that impact C degradation/sequestration and P availability. In this first year, we have collected soils from the Luquillo National Forest, Puerto Rico. Based on Mossbauer spectroscopic (MBS) analysis, we find the majority of the Fe in these soils is best represented by nano-goethite (>80 percent of the Fe mass) as illustrated by a MBS quadrapole splitting value of ca. -0.14 mm s-1 at 13 degrees Kelvin. We have conducted a 4-week redox oscillation experiment on this soil with a 6-day period in a O2-free atmosphere (reducing conditions) and a 1-day period in a 20 percent O2 atmosphere (oxidizing conditions) as well as non-oscillating controls. Fe reduction and oxidation rates showed a strong repeatable pattern. MBS analysis does not indicate a dramatic shift in the Fe mineral composition over the duration of the oscillation treatment, but indicates oxidation of Fe2+ in the clay minerals in the oxic, non-oscillating controls. However, wet chemical extractions using ammonium oxalate release much more Fe from the oscillating treatment. This may result from the rapid oxidation promoted by the 20 percent O2 atmosphere and indicates the importance of redox oscillations for the bioavailability and chemical reactivity of soil iron. To determine the environmental factor(s) controlling this phenomenon, we are currently in the progress of analyzing experimental data from incubations performed at three redox oscillation frequencies with or without the addition of P. We have also assessed the impact of moisture and temperature during laboratory storage of these soils on subsequent rates of Fe reduction. We found that after one week, Fe reduction rates are similar regardless of whether the soils are stored wet or air-dried, at 25 or 4 degrees C, and homogenized or unhomogenized. However, after six months of storage, Fe reductions rates decreased dramatically in the 4 degrees C treatments and only minimally in the air-dried soils. To assess if cold-storage of these tropical region soils altered the composition of resident Fe reducers to a greater extend than air-drying, we will be repeating these experiments at the one year mark and will also perform most probable number counts and DNA fingerprinting. Finally, we are using our data to construct a numerical model of the Fe behavior during these redox oscillations. Thus far, we have fit the Fe reduction behavior using a range of process descriptions including time-invariant iron reduction rates and have considered the impact of dynamic populations of iron reducing microbes. Preliminary results indicate that observed process rates are consistent with and sensitive to a dynamic population of Fe reducers. P and C dynamics will be incorporated in the coming year. PARTICIPANTS: Aaron Thompson (PI/PD) Christof Meile (PI) Michelle Scherer (PI) Jared Wilmoth (PhD student) Tim Pasakarnis (PhD student) Brian Ginn (Post-doc) Viktor Tishchenko (Visiting Scholar) TARGET AUDIENCES: Nothing significant to report during this reporting period. PROJECT MODIFICATIONS: Nothing significant to report during this reporting period.
Impacts
The gain in understanding of the coupled cycling of iron (Fe), carbon (C), and phosphorus (P) in soil systems ultimately aims at improving forestry and agricultural land management practices. Toward this end, we are constructing an integrated model that will provide predictions of the redistribution of Fe, C and P in the soil. In addition, we have begun the training of a PhD student on modeling the reactive network underlying the oscillation experiments and the training of an additional student on 57Fe Mossbauer spectroscopy. We have also exposed a postdoctoral scholar in the complexities of natural system biogeochemistry and trained a lecturer from a local two-year collage in Fe isotope analysis and geochemistry.
Publications
- Thompson, A., Rancourt D., Chadwick, O.A., and J. Chorover. (2011). Iron solid-phase differentiation along a redox gradient in basaltic soils. Geochim. Cosmochim. Acta. 75, 119 - 133.
- Ginn B, Meile C, Scherer M & Thompson A. The effect of redox cycles on the partitioning of Fe, C, and P within soil systems. Goldschmidt, Knoxville, TN, July 13 - 18, 2010.
- Abstract for oral presentation has been accepted by the 241st ACS National Meeting in Anaheim, CA, March 27-31, 2011. "Redox Driven Atom Exchange in Fe Oxides" by Michelle M. Scherer, Drew Latta, Tim Pasakarnis, Aaron Thompson, Victor Tishchenko.
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