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Soil organic matter (SOM) is correlated with reactive iron (Fe) in humid soils, but Fe also promotes SOM decomposition when oxygen (O 2) becomes limited. Here we quantify Femediated OM protection vs. decomposition by adding 13 C dissolved organic matter (DOM) and 57 Fe II to soil slurries incubated under static or fluctuating O 2. We find Fe uniformly protects OM only under static oxic conditions, and only when Fe and DOM are added together: de novo reactive Fe III phases suppress DOM and SOM mineralization by 35 and 47%, respectively. Conversely, adding 57 Fe II alone increases SOM mineralization by 8% following oxidation to 57 Fe III. Under O 2 limitation, de novo reactive 57 Fe III phases are preferentially reduced, increasing anaerobic mineralization of DOM and SOM by 74% and 32-41%, respectively. Periodic O 2 limitation is common in humid soils, so Fe does not intrinsically protect OM; rather reactive Fe phases require their own physiochemical protection to contribute to OM persistence.

In redox-dynamic soils, iron reduction-oxidation events may initiate wide shifts in the concentration of colloidal and dissolved material because of either Fe mineral dissolution or pH shifts associated with Fe oxidation state changes. This can have profound effects on the mobilization of organic and metal constituents. We conducted laboratory studies of colloid dynamics in a Hawaiian soil subjected to four consecutive 14-day reduction-oxidation cycles across the "soil-Fe" (Fe(OH)3)/(Fe2+(aq)) equilibrium. Size fractionated samples were isolated by differential centrifugation and characterized based on analysis of the framework and trace elements (Si, C, Fe, Ti, Al, Zr, Nb, La, and U). Intracycle oscillations in all colloidal (3 kDa to 160 nm) elements peaked during the reduction half-cycles, mobilizing 10% of total soil Ti and from 1-5% of total soil Zr, Nb, La, and U at peak dispersion. Colloid dynamics were dependent on pH shifts accompanying the redox oscillations rather than the fluctuating solubility of Fe oxides. TEM/EDS and mass-balance calculations suggest a carbon-based colloid matrix with zones of metal enrichment. The cumulative effects of four redox cycles included an apparent increase in colloid stability. Proton production/consumption associated with Fe-redox cycling has important implications for mobilization colloid-borne trace elements and sorbed contaminants.

Models test our understanding of processes and can reach beyond the spatial and temporal scales of measurements. Multi-component Reactive Transport Models (RTMs), initially developed more than three decades ago, have been used extensively to explore the interactions of geothermal, hydrologic, geochemical, and geobiological processes in subsurface systems. Driven by extensive data sets now available from intensive measurements efforts, there is a pressing need to couple RTMs with other community models to explore non-linear interactions among the atmosphere, hydrosphere, biosphere, and geosphere. Here we briefly review the history of RTM development, summarize the current state of RTM approaches, and identify new research directions, opportunities, and infrastructure needs to broaden the use of RTMs. In particular, we envision the expanded use of RTMs in advancing process understanding in the Critical Zone, the veneer of the Earth that extends from the top of vegetation to the bottom of groundwater. We argue that, although parsimonious models are essential at larger scales, process-based models are the only way to explore the highly nonlinear coupling that characterizes natural systems. We present seven testable hypotheses that emphasize the unique capabilities of process-based RTMs for (1) elucidating chemical weathering and its physical and biogeochemical drivers; (2) understanding the interactions among roots, microorganisms , carbon, water, and minerals in the rhizosphere; (3) assessing the effects of heterogeneity across spatial and temporal scales; and (4) integrating the vast quantity of novel data, including-omics‖ data (genomics, transcriptomics, proteomics, metabolomics), elemental concentration and speciation data, and isotope data into our understanding of complex earth systems. With strong support from data-driven sciences, we are now in an exciting era where integration of RTM framework into other community models will facilitate process understanding across disciplines and across scales.

Iron oxides are important structural and biogeochemical components of soils that can be strongly altered by redox-driven processes. This study examined the influence of temporal oxygen variations on Fe speciation in soils from the Luquillo Critical Zone Observatory (Puerto Rico). We incubated soils under cycles of oxic-anoxic conditions (τ:τ = 1:6) at three frequencies with and without phosphate addition. Fe(II) production, P availability, and Fe mineral composition were monitored using batch analytical and spectroscopic techniques. The rate of soil Fe(II) production increased from ∼3 to >45 mmol Fe(II) kg d over the experiment with a concomitant increase of an Fe(II) concentration plateau within each anoxic period. The apparent maximum in Fe(II) produced is similar in all treatments, but was hastened by P-amendment. Numerical modeling suggests the Fe(II) dynamics can be explained by the formation of a rapidly reducible Fe(III) phases derived from the progressive dissolution and re-oxidation of native Fe(III) oxides accompanied by minor increases in Fe reducer populations. The shift in Fe(III) reactivity is evident from Fe-reducibility assays using Shewanella sp., however was undetectable by chemical extractions, Mössbauer or X-ray Absorption spectroscopies. More broadly, our findings suggest Fe reduction rates are strongly coupled to redox dynamics of the recent past, and that frequent shifts in redox conditions can prime a soil for rapid Fe-reduction.

Ferrihydrite is a common Fe mineral in soils and sediments that rapidly transforms to secondary minerals in the presence of Fe(II). Both the rate and products of Fe(II)-catalyzed ferrihydrite transformation have been shown to be significantly influenced by natural organic matter (NOM). Here, we used enriched Fe isotope experiments and Fe Mössbauer spectroscopy to track the formation of secondary minerals, as well as electron transfer and Fe mixing between aqueous Fe(II) and ferrihydrite coprecipitated with several types of NOM. Ferrihydrite coprecipitated with humic acids transformed primarily to goethite after reaction with Fe(II). In contrast, ferrihydrite coprecipitated with fulvic acids and Suwannee River NOM (SRNOM) resulted in no measurable formation of secondary minerals. Despite no secondary mineral transformation, Mössbauer spectra indicated electron transfer still occurred between Fe(II) and ferrihydrite coprecipitated with fulvic acid and SRNOM. In addition, isotope tracer experiments revealed that a significant fraction of structural Fe in the ferrihydrite mixed with the aqueous phase Fe(II) (∼85%). After reaction with Fe(II), Mössbauer spectroscopy indicated some subtle changes in the crystallinity, particle size, or particle interactions in the coprecipitate. Our observations suggest that ferrihydrite coprecipitated with fulvic acid and SRNOM remains a highly dynamic phase even without ferrihydrite transformation.

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