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�Measurements for Characterizing and Quantifying Soil Carbon Stocks

March 2023

Ben Kocar, Yoko Slowey and Mark Chappell

US Army Engineer Research and Development Center (ERDC)

DISCOVER | DEVELOP | DELIVER

US Army Corps of Engineers ∙ Engineer Research and Development Center

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Characterizing and Measuring Carbon in Soil – Why?

Physical Properties

Type and diversity of SOM affects diversity of soil community

Soil water holding characteristics-infiltration and retention

“Healthy” structure, aggregation (favorable for plants/other organisms)

Chemical Properties

Biological Effects

Extremely high CEC; 50-90% of cation “holding power” in soils

pH buffering

Low molecular weight organic acids-important ligands

Carbon Sequestration

Soil: Largest terrestrial reservoir of carbon (more than vegetation + atmosphere)

Beneficial Properties

Chemical “filtration”

Mollisol with calcic horizon (NRCS)

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Soil Carbon Fractions - Definitions

Organic C

Inorganic C

Mollisol with calcic horizon (NRCS)

Total Soil Carbon: All forms of carbon in a soil

Soil Inorganic Carbon (SIC): Dominated by calcite (CaCO₃) + other minor fractions. Most important above pH ~7.

Soil Organic Carbon (SOC): Carbon associated with SOM. Doesn’t include heteroatoms (O,N,S, etc.)

Soil Organic Matter (SOM): Milieu of soil biomolecules and their degraded and transformed products (also includes C coordinated with O, N, S). “refractory” vs. “labile”

SOM can be estimated as SOC multiplied by a conversion factor. A common conversion factor is SOM=SOC*1.72

Values typically reported in percent by weight

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Continuum of Soil Carbon Measurements and Methods of Quantification

Soli TOC® analyzer

Remote Sensing

Dry Combustion

Loss on Ignition (LOI)

Chemical Methods

Carbon “Speciation”

Laboratory

Conventional

Advanced

Field

VIS-NIR

MIR

Portable Spectrometers

Carbon Spectroscopy: (STXM, FTIR)

Hyperspectral Imaging + Computational Methods

Lab Equipment/Methods

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Characterizing and Measuring Carbon in Soil-Lab Conventional Methods

Other laboratory methods that may be viewed as more experimental: measurement of “active” versus “recalcitrant”, water-extractable carbon, soil respiration and enzyme activities.

Commercial Dry Combustion instruments increasingly capable of measuring SOC+TIC without pre-treatment, and provide an estimate for “labile” and “refractory” carbon

Method	Comments	Relative Cost
Dry Combustion	Measures Carbon Dioxide emitted during Soil Combustion, 300-600°C (needs carbonate pre-treatment, soil pH >7)	Moderate to High
Loss on Ignition	Measures loss in mass in a dried sample following combustion (needs carbonate pre-treatment, soil pH >7)	Low
Chemical (Walkley-Black)	Chemical oxidation of organic carbon using potassium dichromate. (no pretreatment for carbonates necessary). Many labs phasing this out	Moderate to High

E.G. DIN-19539 (DIN Standards Committee Water Practice, 2016):

1. Oxygenated conditions, 400°C (Organic C, TOC₄₀₀)

2. Oxygen-free conditions, 900°C (mostly inorganic C, TIC₉₀₀)

3. Oxygenated conditions, 900°C (Residual C, TOC₉₀₀ or ROC)

Soli TOC® analyzer

Note: Soil bulk density needed when using lab measured carbon to estimate field stocks

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Aromatic, Aliphatic, Carboxyl and Phenolic Carbon

Total Carbon

Lehmann et al, 2008. Nature

Fourier Transform Infrared Spectroscopy (FTIR)

Scanning Transmission X-ray Microscopy

Characterizing and Measuring Carbon in Soil-Advanced Laboratory Methods

Inferred carbon “lability” from FTIR spectra (Margenot et al, 2015. SSSAJ)

Why use advanced methods?

Understand chemical characteristics that influence “lability”, and/or examine specific soil biogeochemical processes

Largely used in research, but some techniques may (someday) progress into more applied use, e.g. field-portable FTIR

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FTIR subtraction spectra representing SOM of two organic tomato fields on similar soil type from a California landscape of organic management. The TSC content of the fields are 18 g C kg-1 (high) and 7 g C kg-1 (low). Spectra are calculated by difference between bulk soil and hypochlorite oxidized soil. Areas of bands representing chemically labile aliphatic moieties at 2930 cm-1 and 1420 cm-1 and chemically refractory aromatic moieties at 1620 cm-1 and 815 cm-1 indicate greater relative chemical lability of SOM in the higher TSC field. Aliphatic band areas show strong, positive relationships with the size of labile soil C and N pools such as MBC, POXC, and PMN, whereas aromatic band areas pools have negative relationships with these pools. The relative increase in chemically labile to refractory band areas associated with increasing TSC is greater at lower (<10 g C kg-1) compared to higher (>14 g C kg-1) TSC contents.

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The absorbance spectra at different depths with different contents in one soil core of shrub meadow in the VNIR and MIR regions. OH is the atomic group consisting of oxygen atoms and hydrogen atoms, and OC is the atomic group consisting of oxygen and carbon atoms. The SOC contents in the 65 cm, 45 cm and 25 cm depths were 1.59 g kg⁻¹, 4.83 g kg⁻¹, 25.45 g kg⁻¹, respectively.

Characterizing and Measuring Carbon in Soil- Field Methods

Trends and Variability in Soil Organic Carbon

Dr. Acree (ERDC) using an experimental MIR+XRF instrument (Olympus).

(not a part of study, below)

Field-portable MIR/VIS NIR

Capitalizes on specific spectral features for SOM
Field portability: high number of measurements possible
Issues with spectral overlap, reproducibility
Challenges associated with variable soil type

Chen et al, 2017. Scientific reports

US Army Corps of Engineers ∙ Engineer Research and Development Center

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Figure 2 provides the absorbance spectra in the VNIR and MIR regions at different depths of one representative soil profile of shrub meadow. The SOC concentrations usually decreased with depth, and the absorbance curves reduced overall with increasing soil depths. A comparison of the field-moist intact and air-dried ground spectra in the range of 400–2450 nm indicated that the soil moisture and structural integrity also affected the soil absorbance spectra; the process of air drying and soil grinding decreased the absorbance spectra and amplitude characteristic peaks. The spectra in the VNIR and MIR regions contained useful information associated with organic carbon, quartz, kaolin and montmorillonite contents (Fig. 2), which were used to establish prediction models of soil properties. The strong absorptions near 1400 and 1900 nm in the VNIR spectra were caused by the O–H functional group of free water, and the absorptions near 2200 nm were caused by the organic matter¹⁰^,¹¹. In the MIR region, the spectra information mainly responded to mineral properties, such as quartz, kaolin and montmorillonite.

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Characterizing and Measuring Carbon in Soil- Remote Sensing

Xiao et al. Remote Sensing of the Terrestrial Carbon Cycle: A Review of Advances over 50 years. Remote Sensing of Environment

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Remote Sensing of SOC:

Techniques often consist of (satellite/aerial) data + machine learning
Potential utility is enormous, but:

Challenges associated with ground cover, spectral response across soil types, etc.

Private sector companies using hybrid approaches for measuring SOC: remote sensing + selected ground measurements. Promising?

Unidentified manuscript

Characterizing and Measuring Carbon in Soil- Remote Sensing

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Soil Sampling Requirements: Highly Variable and Depend on Purpose

General Measure of soil fertility or “health” :

Loss on ignition has been used for decades (or longer)
“any” established method may be ok for a ballpark measurement

Measurements for Soil Enrichment Protocols (Carbon Credits)

Soil dried within 48 hours of arrival or kept in refrigeration

Aggregates broken, 2mm sieve

Dry combustion technique required. Walkley-Black or LOI not allowed

Possible future use of spectroscopy (e.g. Nir-Vis)

Experimental, e.g. Examining SOC Character to Estimate Carbon Turnover

Depending on purpose, special pre-treatments may be required

- preservation of redox conditions?

- thin sectioning or specialized sample mounts

Protocol Dependent. Climate Action Reserve, U.S. Soil Enrichment Protocol

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(no affiliation or endorsement by ERDC/USACE)

Useful Info on Soil Carbon Credits (links to sampling methods, etc.)

https://www.edf.org/sites/default/files/content/agricultural-soil-carbon-credits-protocol-synthesis.pdf

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Final Thoughts:

Many promising techniques for measuring SOC are in R&D phase. Ideally:

Reliable, high-throughput field measurements
Analysis of SOC across landscapes (hybrid methods, private sector?)
When in doubt, check specific protocols. Dry combustion techniques are likely the “safest” choice

Capturing field heterogeneity:

If possible: statistically relevant sampling across soil types (visual inspection, web soil survey, other resources)
Sampling depth
Composite sampling
Advanced sampling techniques, e.g. multi-increment sampling (MIS)
Consult soil testing lab, or appropriate protocols (e.g. for soil carbon capture and storage)

websoilsurvey.nrcs.usda.gov

Models: DayCent, others…

US Army Corps of Engineers ∙ Engineer Research and Development Center

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