Why Soil Structure Matters
Soil structure—the arrangement of soil particles into aggregates and the pore spaces between them—is arguably the most important physical property of soil. It controls:
- Water infiltration and storage: Well-structured soils absorb and retain water effectively
- Root growth: Aggregated soils allow easier root penetration and exploration
- Gas exchange: Pore networks enable oxygen delivery to roots and microbes
- Carbon sequestration: Aggregates physically protect organic matter from decomposition
- Erosion resistance: Stable aggregates resist breakdown by water and wind
Yet soil structure is systematically omitted from Earth System Models and most pedotransfer functions, which rely primarily on texture (sand, silt, clay percentages). Our research demonstrates why this omission matters and how to account for structure in models and predictions.
Key Research Areas
1. Aggregate Formation & Stability
Soil aggregates form through complex interactions between:
- Biological agents: Roots, fungal hyphae, microbial polysaccharides
- Chemical bonds: Clay-organic matter interactions, polyvalent cations
- Physical processes: Wetting-drying cycles, freeze-thaw
We investigate how different factors contribute to aggregate stability across size classes:
- Macroaggregates (>250 μm): Primarily stabilized by roots and fungal hyphae
- Microaggregates (53-250 μm): More persistent, stabilized by organo-mineral complexes
- Silt and clay (<53 μm): Individual particles and micro-microaggregates
2. Carbon Distribution Across Aggregate Fractions
Soil organic carbon (SOC) is not uniformly distributed. Using physical fractionation techniques, we separate soils by:
- Size: Large vs. small aggregates
- Density: Light organic matter vs. dense mineral-associated carbon
Cover Crop Effects on Carbon Distribution
Our studies using stable isotopes (δ¹³C) and spectroscopy (NEXAFS, EXAFS) reveal:
- Cover crops rapidly incorporate new plant material into macroaggregates
- New carbon is more "plant-like" (phenolic resonance) compared to no-cover controls
- Cover crop carbon appears across all density fractions, not just light organic matter
- Iron oxide crystallinity may increase with cover cropping, affecting C stabilization
3. Pore Size Distribution & Hydraulic Implications
Soil structure creates a hierarchy of pore sizes:
- Macropores (>75 μm): Between aggregates; rapid water transmission and drainage
- Mesopores (0.2-75 μm): Plant-available water storage
- Micropores (<0.2 μm): Within aggregates; water held too tightly for plants
We measure pore size distribution using:
- Water retention curves analyzed via inverse modeling
- Mercury intrusion porosimetry
- X-ray computed tomography (collaborations)
Our findings show that conservation practices (no-till, cover crops) alter pore size distribution in ways that:
- Increase infiltration rates (more macropores)
- Marginally improve water storage
- Enhance gas diffusion
4. Management Effects on Structure
Long-term field experiments (24+ years) allow us to quantify how agricultural practices affect soil structure:
Tillage Effects:
- Standard tillage disrupts aggregates, reducing macroporosity
- No-till preserves or enhances aggregate stability over time
- Structural benefits take years to manifest (5-10+ years)
Cover Cropping:
- Increases root biomass and exudation, promoting aggregation
- More impact on macroaggregate formation than total SOC stocks (short-term)
- Effects visible after just one season in macroaggregate fraction
Irrigation Method:
- High-efficiency irrigation may reduce structural development compared to furrow irrigation
- Wetting-drying cycles important for aggregation; reduced cycles may limit structure formation
Soil Structure in Earth System Models
We've demonstrated that systematic omission of soil structure from Earth System Models (ESMs) has significant consequences:
Local vs. Global Effects
At local scales:
- Including structure significantly alters infiltration-runoff partitioning
- Affects recharge predictions in wet, vegetated regions
- Changes water availability for plants
At global scales:
- Coarse spatial resolution of ESMs masks local structure effects
- Current ESMs can't simulate intense, short rainfall where structure matters most
- Global climate implications remain elusive in current models
Advanced Characterization Methods
Spectroscopic Approaches
We use synchrotron-based techniques to probe carbon chemistry and iron speciation:
- Carbon NEXAFS: Identifies aromatic vs. aliphatic carbon, carboxylic groups
- Iron EXAFS: Determines iron oxide mineralogy and crystallinity
These techniques reveal that cover crops and conservation practices alter not just carbon quantity but also its chemistry and protection mechanisms.
Isotope Tracing
Using natural δ¹³C differences between C3 (most crops) and C4 (corn) plants, we trace:
- How quickly new carbon enters different aggregate fractions
- Whether old or new carbon is preferentially lost
- Turnover times for carbon in different pools
Practical Implications
Carbon Sequestration
While soil carbon credits and offsets generate interest, our research urges caution:
- Short-term (1 season) cover cropping may not increase bulk SOC stocks
- Carbon is redistributed among fractions rather than simply accumulated
- Long-term commitments (decades) needed for measurable sequestration
- Co-benefits (improved water infiltration, erosion control) may be more reliable than carbon credits
Resilience to Climate Extremes
Well-structured soils with stable aggregates better withstand:
- Intense rainfall without erosion or crusting
- Drought periods with maintained pore networks
- Freeze-thaw cycles without structural collapse
Recent Publications
- Soil Structure Changes Under Conservation Management Enhance Carbon Mineralization in Irrigated Croplands.AlvarezSagrero, J., Chacon, S. S., Mitchell, J., & Ghezzehei, T. A.Vadose Zone Journal. 2025.
Abstract
BibTeX
@article{p2025-Alvarez, title = {Soil Structure Changes Under Conservation Management Enhance Carbon Mineralization in Irrigated Croplands}, language = {en}, journal = {Vadose Zone Journal}, author = {AlvarezSagrero, Jennifer and Chacon, Stephany S and Mitchell, Jeffrey and Ghezzehei, Teamrat A.}, pages = {}, year = {2025}, research-theme = {soil-structure, rhizosphere, sustainable-agriculture} } - Impact of almond shell biochar properties and application rate on soil physical and hydraulic characteristics.Thao, T., Lopez, V. D., Gonzales, M., Berhe, A. A., Diaz, G., & Ghezzehei, T. A.Sustainable Environment, 11(1), 2485688. 2025.
Abstract
We conducted two 64-day incubation experiments to assess how locally produced almond-shell biochar influences soil physical and hydraulic properties. Biochar was created using slow pyrolysis at different temperatures (350 °C or 700 °C), separated into different particle sizes (<250 μm or 1–2 mm), and applied at 10 ton/ha or 60 ton/ha to a coarse-textured soil. While our analysis shows that biochar yielded greater cation exchange capacity (CEC) and specific surface area (SSA) with increasing pyrolysis temperature and finer particle size, its contributions to improving soil hydraulic properties were marginal. In the first experiment, the addition of biochar at high rates slightly improved water stable aggregate (WSA) (3.8%–5.3% increase) but has no effect on saturated hydraulic conductivity (Ksat). Soil respiration measured throughout the experiment were not significantly different among treatments. In the second experiment, the addition of biochar increased soil infiltration rate at the initial stage (8.18E–4 cm/s), but this effect diminished over time. WSA was lower for biochar amended soil and lowest at high application rates (5%–21% reduction). Cumulative carbon dioxide (CO2) flux varied between biochar particle sizes and rates. Additionally, a significant difference between the two experiments was also observed, with cumulative CO2 (38%–56% greater) and WSA (11%–40%) being inversely correlated. Our findings suggest that almond-shell derived biochar has a limited impact on arable loamy sand soil properties, specifically for water retention under short-term conditions.BibTeX
@article{Thao31122025, author = {Thao, Touyee and Lopez, Vivian D. and Gonzales, Melinda and Berhe, Asmeret A. and Diaz, Gerardo and Ghezzehei, Teamrat A.}, title = {Impact of almond shell biochar properties and application rate on soil physical and hydraulic characteristics}, journal = {Sustainable Environment}, volume = {11}, number = {1}, pages = {2485688}, year = {2025}, publisher = {Taylor \& Francis}, doi = {10.1080/27658511.2025.2485688}, pdf = {https://doi.org/10.1080/27658511.2025.2485688}, research-theme = {water-flow, soil-structure, sustainable-agriculture} } - Precipitation Disruption: When the Rhythm of the Rain Throws Soil Organic Matter Off-Beat.Min, K., Yang, Y., Wahab, L., Woo, S., Oh, M., Ghezzehei, T. A., & Berhe, A. A.New Phytologist. 2025.
Abstract
BibTeX
@article{p2025-Min, title = {Precipitation Disruption: When the Rhythm of the Rain Throws Soil Organic Matter Off-Beat}, language = {en}, journal = {New Phytologist}, year = {2025}, author = {Min, Kyungjin and Yang, Yang and Wahab, Leila and Woo, Sohyun and Oh, Minseung and Ghezzehei, Teamrat A. and Berhe, Asmeret Asefaw}, pages = {}, research-theme = {soil-structure} }
- Soil Organic Matter Stabilization by Polyvalent Cations in a Buried Alkaline Soil.Dolui, M., Nel, T., Chacon, S., Phillips, L. M., McMurtry, A. R., Moreland, K. C., … Berhe, A. A.JGR Biogeosciences (Under Review). 2025.
Abstract
Buried paleosols can store large quantities of organic carbon (C), much of which persists for millennia due to isolation from surface processes that promote decomposition. Subsoil organic matter (SOM) persistence is often enhanced by mineral associations and ionic conditions — particularly high clay content and polyvalent cations — that limit microbial degradation and leaching. However, the vulnerability of these deep C stocks under erosion or environmental change remains poorly understood. This study investigates controls on SOM stabilization in the Brady paleosol and overlying modern soils across contrasting geomorphic settings in the Great Plains of Nebraska, where Late Quaternary loess deposition and erosion created a sequence of buried and exposed paleosols. We sampled soils along burial and erosional toposequences and analyzed their physicochemical properties and radiocarbon-based persistence of occluded particulate organic matter (oPOM) and mineral fractions (MF). Brady Soil showed greater persistence (lower Fm) of oPOM and MF than modern soils, particularly under burial. This was linked to higher silt and clay content, elevated electrical conductivity, and increased exchangeable calcium and magnesium content, supporting roles for organo-mineral interactions, flocculation, and carbonate cementation. In modern soils, SOM persistence and C content were more strongly tied to pH and cation exchange capacity. Erosional exposure reduced SOM stability and promoted geochemical convergence toward modern surface soils. These findings show that burial enhances SOM persistence via multiple stabilization mechanisms, while erosion increases subsoil C vulnerability. Our results underscore the importance of geomorphic and geochemical context in predicting soil C stability under environmental change.BibTeX
@article{Dolui2025, author = {Dolui, Manisha and Nel, Teneille and Chacon, Stephanie and Phillips, Laura M. and McMurtry, Abbygail R. and Moreland, Kimber C. and McFarlane, Karis Jensen and Mason, Joseph A. and Marin-Spiotta, Erika and de Graaff, Marie-Anne and Ghezzehei, Teamrat A. and Berhe, Asmeret Asefaw}, title = {Soil Organic Matter Stabilization by Polyvalent Cations in a Buried Alkaline Soil}, journal = {JGR Biogeosciences (Under review)}, year = {2025}, doi = {10.22541/essoar.175181598.87921927/v1}, research-theme = {soil-structure} } - Environmental Variability and Moisture-Temperature Coupling Reveal Continental-Scale Controls on Soil Respiration.Rojas, Y. T. P., & Ghezzehei, T. A.JGR-Biogeosciences (under Review).
Abstract
Understanding how temporal patterns of moisture variability control biogeochemical responses remains a fundamental challenge in Earth system science. Jensen’s inequality provides a mathematical framework for quantifying when episodic environmental events dominate over mean conditions. We applied this framework to continental-scale AmeriFlux data (134.5 million hourly observations from 2,004 soil moisture sensors) to quantify how moisture distribution patterns control soil respiration responses across environmental gradients. Sensors in dry regions show large Jensen’s inequality effects (median temporal averaging difference of -63.6%) because they experience highly skewed moisture distributions where brief wet periods drive disproportionate respiratory responses. Wet regions show minimal effects (median -27.1%) because they have more uniform moisture distributions. Data density analysis reveals that sensors operating at θ≈0.05 exhibit severe temporal averaging effects, while sensors at θ≈0.45 show minimal effects, demonstrating the mechanistic basis for where ecosystems operate on the moisture-respiration relationship. Climate gradient analysis shows systematic transitions from severe effects in arid systems to moderate effects in humid systems. Depth analysis reveals that surface soils experience maximum episodic event importance while deeper soils show reduced effects due to environmental buffering. Moisture-temperature coupling demonstrates systematic negative correlations in water-limited systems, indicating that environmental co-variation modulates biogeochemical responses. Jensen’s inequality emerges as a diagnostic tool for identifying when moisture variability patterns dominate biogeochemical processes, with continental-scale patterns revealing fundamental controls on episodic event importance across ecosystems.BibTeX
@article{2025-PerezRojas, title = {Environmental Variability and Moisture-Temperature Coupling Reveal Continental-Scale Controls on Soil Respiration}, language = {en}, journal = {JGR-Biogeosciences (under review)}, author = {Rojas, Yulissa T. Perez and Ghezzehei, Teamrat A.}, pages = {}, research-theme = {water-flow, soil-structure} }
Related Research
Key Methods
- Wet sieving for aggregate fractionation
- Density separation (heavy liquids)
- Carbon NEXAFS spectroscopy
- Iron EXAFS spectroscopy
- Stable isotope analysis (δ¹³C)
- Water retention for pore size distribution
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