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�What is soil carbon? �A synopsis of properties and factors affecting carbon storage, turnover, and persistence within soils

March 2023

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Yoko Slowey, Ben Kocar and Mark Chappell

US Army Engineer Research and Development Center (ERDC)

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DISCOVER | DEVELOP | DELIVER

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Soils – Integral to Resilient Environments

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Ecosystem services of soils:

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  • Carbon and nutrient storage
  • Flood mitigation
  • Buffering of heat island effect
  • Physical support for infrastructure

https://www.noaa.gov/education/resource-collections/climate/carbon-cycle

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Soil - The Largest Terrestrial Carbon Pool

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Play a critical role in regulating atmospheric CO2

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Soil Organic Matter

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Role of Soil Organic Matter on Soil Quality

  • Physical
    • Dark color
    • Aggregate stability
    • Water retention and infiltration

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  • Chemical
    • High cationic exchange capacity (CEC)
    • pH buffering capacity

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  • Biological
    • Microbial community structure

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Organic Matter Composition

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Biomass

(living)

Detritus

(dead, identifiable)

Humus (non-identifiable)

Nonhumic Material

(proteins, carbohydrates, etc.)

Humin: insoluble in acid or base; highly condensed, high MW, OM

Humic Acids: soluble in base, but insoluble in acid; medium MW (300,000 da)

Fulvic Acids:soluble in acid and base; low MW (<50,000 da)

Soil Organic Matter Compositions (Mostly “Traditional” View)

Humic Material

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Soil Organic Matter Composition (“Emergent” View)

  • Considers:

    • Microbial Access
    • Temperature Dependence of Enzymes
    • Organic Matter Transport and Adsorption

- Need to understand carbon flows rather than carbon stocks

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Soil Carbon Cycling

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Soil carbon

Carbon Inputs

Carbon Outputs

Vast Majority is Plant Tissue, also some Animal Tissue

Decomposition and oxidation of Soil Organic Matter (SOM) and evolution of CO2 (Microbially-driven)

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  • Weathering of carbonate minerals
  • Erosion
  • Leaching/translocation of dissolved organic matter

Surficial Detritus, Root Mass

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Soil Carbon Cycling

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Soil carbon

Carbon Inputs

Carbon Outputs

Vast Majority is Plant Tissue, also some Animal Tissue

Decomposition and oxidation of Soil Organic Matter (SOM) and evolution of CO2 (Microbially-driven)

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  • Weathering of carbonate minerals
  • Erosion
  • Leaching/translocation of dissolved organic matter

Surficial Detritus, Root Mass

Soil Carbon Cycling is Bounded and Governed by Ecosystem Properties

- Biota

- Climate

- Parent Material

- Topography

- Time

Ecosystem-Dependent Carbon Fluxes Control Soil Carbon Stocks

“Natural” Soil Carbon Equilibrium

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Soil Carbon “Saturation”

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Soil Carbon Stabilization Mechanisms

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SOC

oxidation

chemical

protection

biochemical

protection

physical

protection

Redox

(O2 availability)

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Factors that Promote SOC Stabilization & Accumulation

  • Soil mineralogy
    • High contents of clays and poorly crystalline (high surface area) solids
      • Chemically bound SOC is blocked from microbial attack
  • Physical structure
    • Highly structured/aggregated soil
      • Physical protection of SOC
      • Decline in oxygen diffusion leading to slow microbial metabolism
  • Water content
    • High water content
      • Limits transport of oxygen into soils

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Impact of tilling on SOC

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File Name

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File Name

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Torn et al., Nature 1997

Substrate age (kyr)

Carbon (Kg m-2)

∆14C (‰)

Poorly crystalline Al+Fe (Kg m-2)

Highest C content

  • Mid weathering stage
  • Slowest C oxidation (oldest C based on 14C)
  • Highest content of poorly crystalline phase

Five Soil Forming Factors:

Constant: Climate, Biota, Topography, Parent Material

Variable: Time (age of parent material)

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Case Study: Hawaiian Archipelago as a Model System

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Summary

  • SOC is a large carbon stock, and plays a key role in global carbon cycling

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  • SOC is a central component of “soil health”

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  • SOC stocks are controlled by carbon fluxes, which are governed by ecosystem properties

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  • The carbon “carrying capacity” or “saturation point” in soil is an emergent property of ecosystem functions and perturbations that control carbon fluxes

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Yoko Slowey, PhD (Yoko.M.Slowey@erdc.dren.mil)

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Ben Kocar, PhD, CPSS (Benjamin.D.Kocar@erdc.dren.mil)

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Mark Chappell, PhD (Mark.A.Chappell@erdc.dren.mil)

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https://www.noaa.gov/education/resource-collections/climate/carbon-cycle

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Questions?

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