Soil Biogeochemistry, Iron Cycling, and Microbial Carbon Dynamics

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Research Primer

Background

Beneath the meadows, floodplains, and forests of the Gunnison Basin, soils and sediments host a hidden chemistry that controls how carbon is stored, how nutrients move into rivers, and how greenhouse gases are released to the atmosphere. Soil biogeochemistry is the study of these coupled chemical, physical, and biological processes. In mountain watersheds like the East River and Slate River near Gothic, Colorado, these processes are especially dynamic because snowmelt, monsoon rains, and drought drive rapid shifts in moisture, oxygen, and temperature throughout the year. Understanding these dynamics matters for predicting water quality downstream, forecasting how much carbon montane soils can hold as the climate warms, and tracking how contaminants like lead move through floodplains.

Several core concepts tie this research together. Soil organic carbon (SOC) refers to the carbon stored in soils as decomposing plant material, microbial biomass, and stabilized organic compounds — a pool larger than all the carbon in the atmosphere and living vegetation combined. The fate of SOC is closely tied to redox potential, the chemistry that describes whether an environment is oxidizing (oxygen-rich) or reducing (oxygen-poor). Redox conditions in turn control iron speciation, the shuttling of iron between dissolved ferrous Fe(II) forms under low-oxygen conditions and solid ferric Fe(III) minerals under oxygen-rich conditions. Iron minerals are important because they bind organic matter and trace metals, and when they dissolve or reprecipitate, they can release or trap carbon, nutrients, and contaminants. Some of this material moves as colloids — tiny particles small enough to travel long distances through groundwater, carrying iron, organic matter, and nutrients across land-water boundaries.

Microorganisms are the engines of these transformations. Bacteria and archaea drive methane production and consumption, nitrogen cycling, and organic matter decomposition, and they exchange capabilities through horizontal gene transfer — the movement of DNA between unrelated microbes — often mediated by mobile genetic elements such as plasmids or newly discovered giant DNA structures. These genetic exchanges allow microbial communities to adapt rapidly to the shifting chemistry of mountain soils.

Foundational work

Early research in the Gunnison Basin built the empirical foundation for studying soil carbon and respiration in montane systems. Work in alpine and subalpine soils of the East River watershed showed that soil respiration rates varied nearly fourfold between early and late growing season, with strong spatial variability emerging as soils dried out (Carbon Dioxide Fluxes, 2016). Modeling studies soon followed: Liu and colleagues demonstrated that soil microbes alternate between dormant and active states as soils wet and dry, and that capturing this behavior was essential for accurate carbon respiration models in the upper soil profile (Liu et al., 2019).

Parallel foundational work in other settings established how microbes shape element cycles at rock and sediment surfaces. Studies of rock varnish by Northup and colleagues linked microbial communities to manganese and iron oxide precipitation (Northup et al., 2010), and follow-up work by Parchert and colleagues showed that microcolonial fungi also oxidize manganese and may participate in varnish formation (Parchert et al., 2012). Together these studies helped frame the field's central idea: microbes and minerals co-evolve, and their interactions leave measurable chemical signatures in soils and sediments.

Key findings

A consistent theme across Gunnison Basin research is that hydrology and plant phenology — not temperature alone — govern carbon dynamics. Winnick and colleagues found that in an East River subalpine meadow, plant phenology measured by satellite vegetation index was the strongest predictor of shallow soil CO2 production across three growing seasons, and that the respiration response to rainfall depended on whether plants were actively photosynthesizing (Winnick et al., 2020). Deep soils below about 165 cm alternated between being a carbon sink and source depending on how shallow layers responded to precipitation (Winnick et al., 2020). Molecular characterization added another layer: Hsu and colleagues showed that SOC composition varies dramatically with elevation across the East River catchment, with lower-elevation soils dominated by polysaccharides and higher-elevation soils containing more aromatic and phenolic carbon (Hsu et al., 2018). More recent work established that ancient rock-derived (petrogenic) carbon makes up 7-9% of SOC in shale-derived East River soils and must be accounted for when its fraction exceeds 0.125, or turnover estimates will be biased (Williams & Lawrence, 2025).

Iron and redox chemistry emerged as a second unifying theme. Dewey and colleagues showed that in contaminated floodplains, seasonal redox cycles repeatedly dissolve and reprecipitate iron and sulfide minerals, yet lead stays locked in particulate organic matter, keeping dissolved concentrations below 17 micrograms per liter (Dewey et al., 2021). Engel and colleagues then revealed how iron itself travels: up to 72% of dissolved iron in anoxic floodplain groundwater moves as tiny colloids made of silica-coated ferrihydrite nanoparticles and Fe(II)-organic complexes, which persist under both oxic and anoxic conditions because silica and organic coatings protect them (Engel et al., 2023). Babey and colleagues connected these chemistries to hydrology, showing that flooding events such as beaver ponding flush anoxic soil water downward into gravel beds, transferring reduced chemistry and altering downstream water quality, while snowmelt and drought restore oxic conditions (Babey et al., 2024).

Microbial discoveries have reshaped how researchers think about who is doing the biogeochemical work. Al-Shayeb and colleagues identified giant linear DNA elements called Borgs, up to one million base pairs long, associated with methane-oxidizing archaea in wetland soils and encoding genes for energy metabolism that were likely acquired through horizontal gene transfer (Al-Shayeb et al., 2022); similar elements were found at sites in Colorado and California (Dance, 2021). In Slate River floodplain sediments near Crested Butte, Rasmussen and colleagues reconstructed over 1,200 microbial genomes and found unconventional methanogens, methanotrophs, and methylotrophs, including exceptionally abundant Methanoperedens archaea in anoxic depths — suggesting a methane-cycling hotspot in these montane sediments (Rasmussen et al., 2024).

Current frontier

Early work in the 2000s and 2010s established the baseline chemistry and seasonal patterns of Gunnison Basin soils. Since 2020, research has shifted toward finer spatial, chemical, and genomic resolution. Noel and colleagues demonstrated that synchrotron X-ray fluorescence mapping can detect millimeter-scale anoxic microsites within otherwise oxic soils, opening a path toward incorporating redox heterogeneity into ecosystem models that currently treat soils as uniform (Noel et al., 2024). Metagenomic studies are expanding the catalog of locally relevant microbes: Rasmussen and colleagues recently recovered genomes of oligotrophic nitrifiers in Slate River sediments dominated by Nitrosotalea-like archaea and Palsa-1315 comammox bacteria, with no conventional ammonia-oxidizing bacteria detected, and showed these microbes can use alternative nitrogen sources such as urea and cyanate (Rasmussen et al., 2025).

The trajectory is toward integration: linking pore-scale chemistry measured by imaging techniques to watershed-scale solute exports, and connecting microbial genomic potential to measured gas and solute fluxes. Landscape-scale drivers are also entering the picture — Lowry and colleagues used satellite radar interferometry to detect slow-moving landslides across the Ragged Mountain hillslope in Gunnison County, reminding researchers that mass wasting continually refreshes the substrates on which soil biogeochemistry develops (Lowry et al., 2020).

Open questions

Several questions stand out for the coming decade. How will earlier snowmelt and longer dry periods reshape the phenology-driven respiration response that currently governs mountain carbon balance? What fraction of stored soil carbon is protected by iron minerals and organic coatings, and will that protection persist as floodplains experience more frequent redox swings? What roles do newly discovered genetic elements like Borgs, and uncultured microbial lineages like the comammox and unconventional methanotrophs of Slate River, play in real-world methane and nitrogen fluxes? And how do colloidal iron and associated nutrients move from hillslopes through gravel beds to rivers under changing hydrology? Answering these questions will require sustained pairing of high-resolution chemical imaging, long-term field monitoring, and genome-resolved microbiology at sites like the East River and Slate River floodplains.

References

Al-Shayeb, B. et al. (2022). Borgs are giant genetic elements with potential to expand metabolic capacity. Nature. →

Babey, T. et al. (2024). Mountainous floodplain connectivity in response to hydrological transitions. Water Resources Research. →

Carbon Dioxide Fluxes in Alpine and Subalpine Soils of the East River Watershed (2016). →

Dance, A. (2021). Massive DNA 'BORG' Structures Perplex Scientists. Nature. →

Dewey, C. et al. (2021). Porewater Lead Concentrations Limited by Particulate Organic Matter Coupled With Ephemeral Iron(III) and Sulfide Phases during Redox Cycles Within Contaminated Floodplain Soils. Environmental Science & Technology. →

Engel, M. et al. (2023). Structure and composition of natural ferrihydrite nano-colloids in anoxic groundwater. Water Research. →

Hsu, H. et al. (2018). A molecular investigation of soil organic carbon composition across a subalpine catchment. Soil Systems. →

Liu, Y. et al. (2019). Modeling transient soil moisture limitations on microbial carbon respiration. Journal of Geophysical Research: Biogeosciences. →

Lowry, B. et al. (2020). A Case Study of Novel Landslide Activity Recognition Using ALOS-1 InSAR within the Ragged Mountain Western Hillslope in Gunnison County, Colorado, USA. Remote Sensing. →

Noel, V. et al. (2024). X-ray chemical imaging for assessing redox microsites within soils and sediments. Frontiers in Environmental Chemistry. →

Northup, D.E. et al. (2010). Diversity of rock varnish bacterial communities from Black Canyon, New Mexico. Journal of Geophysical Research: Biogeosciences. →

Parchert, K. et al. (2012). Fungal Communities Associated with Rock Varnish in Black Canyon, New Mexico: Casual Inhabitants or Essential Partners? Geomicrobiology Journal. →

Rasmussen, M. et al. (2024). Diverse and unconventional methanogens, methanotrophs, and methylotrophs in metagenome-assembled genomes from subsurface sediments of the Slate River floodplain, Crested Butte, CO, USA. mSystems. →

Rasmussen, M. et al. (2025). Metagenome-assembled genomes for oligotrophic nitrifiers from a mountainous gravelbed floodplain. Environmental Microbiology. →

Williams, E. & Lawrence, C. (2025). Quantifying the effect of petrogenic carbon on SOC turnover for two Rocky Mountain soils: when are petrogenic carbon corrections required? Journal of Geophysical Research: Biogeosciences. →

Winnick, M. et al. (2020). Soil Respiration Response to Rainfall Modulated by Plant Phenology in a Montane Meadow, East River, Colorado, USA. Journal of Geophysical Research: Biogeosciences. →

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