PCS TA 002 Soil Organic Carbon Tool_v1.0

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Document identification

• Document code: PCS-TA-002

• Title: Soil Organic Carbon Tool

• Scope: Quantification tool for estimating changes in soil organic carbon stocks within the project boundary under applicable land-based PCS methodologies.

• Application: Used to quantify SOC stock changes and associated uncertainty treatment where SOC is included as an eligible pool.

Version history and change log

Table DC-1. Revision history

Version	Date	Status	Summary of changes	Prepared by	Approved by
v1.0	TBD	Draft	Release for public consultation	PCS	TBD

Superseded versions

No superseded versions for v1.0.

Governance note on versioning and archiving

Only the latest approved version of this tool shall be used for new project registrations and for quantification/verification of monitoring periods unless PCS specifies otherwise. Superseded versions shall be archived and retained for traceability and audit purposes, including for projects assessed under earlier versions where applicable, consistent with PCS governance rules.

Chapter 1 - Introduction and Purpose

1. The Soil Organic Carbon (SOC) Estimation Tool provides standardized procedures for quantifying soil organic carbon stocks and stock changes in mineral soils within PCS-registered Nature-Based Solutions projects. SOC constitutes a significant portion of total carbon storage in many terrestrial ecosystems, and changes in SOC must be accounted for to ensure accurate estimation of greenhouse gas emission reductions or removals.

2. This tool defines the methodological steps required to estimate SOC at baseline and at subsequent monitoring periods, using field sampling, laboratory analysis, and scientifically credible calculation procedures. It integrates directly with PCS methodologies for forestry, agroforestry, grassland restoration, and other land management interventions where SOC dynamics are relevant. The tool ensures that SOC changes are quantified consistently across projects and that results are supported by verifiable data.

3. SOC is influenced by land-use history, vegetation composition, soil texture, management practices, hydrology, and organic matter inputs. Because these factors vary across project landscapes, SOC must be estimated through stratified sampling supported by laboratory measurement of carbon concentration and bulk density. This tool establishes the minimum sampling requirements, calculation procedures, and reporting standards needed to generate reliable SOC estimates.

4. The objective of the tool is to provide a transparent framework that preserves environmental integrity while remaining practical for project developers and verifiable by independent validation and verification bodies. The procedures comply with IPCC 2006 Guidelines and the Wetlands and Agriculture supplements, adapted to the PCS framework. The tool also specifies default values and conservative approaches to be applied when direct measurement is not feasible or when data gaps occur.

5. By standardizing SOC estimation across PCS projects, this tool ensures comparability of results, prevents overstatement of carbon benefits, and enables consistent long-term tracking of soil carbon changes in response to project interventions. It forms a mandatory component of GHG accounting whenever SOC is identified as a relevant pool under the applicable PCS methodology.

Chapter 2 - Scope and Applicability

1. This tool applies to all PCS Nature-Based Solutions project activities in which soil organic carbon in mineral soils is included as a carbon pool under the applicable methodology. The procedures outlined here must be used when SOC contributes materially to baseline emissions, project emissions, or carbon removals. The tool is intended for forestry, agroforestry, grassland restoration, sustainable land management, and other interventions where project activities directly influence soil carbon stocks.

2.1 Applicable Soil Types

1. This tool applies exclusively to mineral soils. Mineral soils contain mineral particles as the dominant fraction and accumulate organic carbon primarily through litter deposition, root turnover, and microbial processes. Projects with mineral soil strata must use this tool to estimate SOC at baseline and during monitoring.

2. This tool does not apply to:

• Peat soils

• Histosols

• Organic soils dominated by high organic matter layers

Such soils must be addressed using the PCS Peat Oxidation and Peat Burning Tool (PCS-TA-004).

2.2 Applicable Project Activities

1. The tool applies to project activities that may increase, decrease, or conserve SOC, including:

• Reforestation and afforestation on mineral soils

• Assisted natural regeneration

• Forest conservation practices that modify litter and root dynamics

• Grassland restoration or improved grazing management

• Agroforestry establishment or enhancement

• Soil management interventions that influence carbon inputs or decomposition rates

1. If SOC is identified as a relevant pool in the PCS methodology used by the project, the procedures in this tool must be applied.

2.3 Situations Where SOC Accounting Is Required

1. SOC accounting is required when:

• The applicable PCS methodology identifies SOC as a required pool for the land-use category.

• Project activities are expected to cause measurable SOC changes relative to baseline trends.

• Omitting SOC would lead to a material overestimation of net GHG benefits.

Materiality thresholds for SOC must follow the rules defined in the PCS methodology.

2.4 Situations Where SOC Accounting May Be Optional

1. SOC accounting may be optional when:

• The applicable methodology allows exclusion of SOC due to low expected change.

• Land-use and management conditions result in negligible SOC change under both baseline and project scenarios.

• The project area consists of shallow mineral soils with limited capacity for carbon accumulation.

1. Optional exclusion must be justified with empirical evidence or published literature. Conservativeness must be applied if uncertainty exists about potential SOC changes.

2.5 Geographical and Ecological Applicability

1. This tool is applicable across all climatic regions and ecological zones, including:

• Tropical and subtropical forests

• Temperate forests

• Dryland and semi-arid regions

• Grasslands and savannahs

• Agricultural landscapes transitioning to agroforestry

1. The tool’s equations follow IPCC general applicability rules and therefore accommodate a broad range of soil textures, depths, and organic carbon concentrations.

2.6 Activity Boundary and Sampling Domain

1. SOC estimation must correspond to the soil layers influenced by project activities. Unless otherwise justified, the standard sampling depth is 0–30 cm, as this layer captures most management-induced SOC change. Projects may sample deeper layers when interventions affect subsoil carbon or when required by the applicable PCS methodology.

2. Stratification applied in SOC sampling must match the project’s stratification for biomass or land-use classes, ensuring consistency in carbon accounting.

2.7 Integration with PCS Methodologies

1. This tool must be used together with the specific PCS methodology applicable to the project activity. The methodology determines:

• Whether SOC is required or optional,

• Monitoring frequency,

• Stratification requirements,

• Treatment of SOC in baseline and project scenarios,

• How SOC stock changes integrate into net GHG calculations.

1. All SOC results generated using this tool must feed directly into the project’s carbon accounting spreadsheets and Monitoring Report.

Chapter 3 - Key Concepts and Definitions

3.1 Soil Organic Carbon (SOC)

1. Soil Organic Carbon refers to the carbon stored in organic matter contained within mineral soils. It includes carbon derived from decomposed plant and animal residues, root biomass, microbial biomass, and humic substances. SOC is expressed as the mass of carbon per unit area and is typically measured from soil samples analyzed in a laboratory.

3.2 Mineral Soils

1. Mineral soils are soils in which organic matter constitutes less than the threshold defined for organic soils under PCS and IPCC criteria. They contain predominantly mineral particles and have limited organic horizons. This tool applies exclusively to mineral soils. Peat soils and highly organic substrates are addressed under separate PCS tools.

3.3 Soil Bulk Density

1. Bulk density is the mass of dry soil per unit volume, including pore space. It is essential for converting carbon concentration measured in soil samples into carbon stocks per hectare. Bulk density varies with soil texture, structure, compaction, and organic matter content, and must be determined for each sampled depth interval.

3.4 Carbon Concentration

1. Carbon concentration refers to the proportion of carbon by mass in a soil sample, typically expressed as a percentage. It is measured through laboratory analysis, most commonly using dry combustion methods. Carbon concentration multiplied by bulk density and depth provides the basis for SOC stock estimation.

3.5 Depth Intervals

1. Soil samples are collected in predefined depth increments, referred to as depth intervals. Each interval must be analyzed separately because carbon content and bulk density vary with depth. Depth intervals must remain consistent between baseline and monitoring periods to ensure comparability.

3.6 SOC Stock

1. SOC stock represents the total mass of soil organic carbon stored within a defined depth of soil per unit area. It is calculated for each depth interval and then summed across the soil profile. SOC stocks must be reported in tonnes of carbon per hectare (tC/ha).

3.7 SOC Stock Change

1. SOC stock change refers to the difference in SOC stocks between two time periods (e.g., baseline and monitoring period). Positive changes indicate an increase in carbon storage, while negative changes indicate carbon loss. SOC changes contribute directly to project emission reductions or removals under PCS methodologies.

3.8 Stratified Sampling

1. Stratified sampling divides the project area into homogeneous units (strata) based on soil type, land use, vegetation, or management conditions. SOC sampling must be designed to ensure that measurements within each stratum accurately represent its conditions. Stratification increases precision and reduces sampling error.

3.9 Laboratory Analysis

1. Laboratory analysis refers to the procedures used to determine carbon concentration and bulk density in soil samples. Samples must be analyzed using standardized methods, and laboratories must follow recognized quality control procedures. Analytical results must be traceable and documented.

3.10 SOC Accounting Depth

1. The accounting depth is the total depth of soil included in SOC stock estimation. The minimum depth must follow the applicable PCS methodology but is typically 30 cm or 1 meter, depending on ecosystem type and expected carbon dynamics. The chosen accounting depth must be applied consistently.

3.11 Dry Bulk Density vs. Field Moist Bulk Density

1. Dry bulk density is required for SOC calculations. If only field-moist bulk density is available, samples must be oven-dried to determine the dry equivalent. The Monitoring Report must document the procedure used to adjust for moisture.

Chapter 4 - Parameters and Symbols

1. This chapter defines the parameters and symbols used throughout the SOC estimation process. These values must be applied consistently and reported exactly as defined. Any deviation from the standard parameters must be justified and documented in the Monitoring Report.

2. SOC calculations require a clear distinction between measurable quantities (such as bulk density and carbon concentration) and derived quantities (such as SOC per depth increment or total SOC per hectare). The parameters listed below form the basis for all calculations presented in Chapter 5.

Table 1. Parameters and Symbols Used in SOC Estimation

Symbol	Description	Units	Notes
BD	Bulk density (dry)	g/cm³	Determined by laboratory drying of soil samples
C%	Carbon concentration	% by mass	Measured using approved laboratory methods
D	Depth interval thickness	cm	The vertical thickness of each sampled layer
SOC_layer	SOC stock for a single depth interval	tC/ha	Derived from BD, C%, and depth
SOC_total	Total SOC stock for the profile	tC/ha	Sum of SOC_layer across intervals
A	Area of the stratum	ha	Used when scaling SOC to the project area
n	Number of samples	—	Samples per depth interval or stratum
BD_adj	Moisture-adjusted bulk density	g/cm³	Used when field-moist samples require correction
CF_soil	Carbon fraction conversion factor	—	Typically C% / 100
ΔSOC	SOC stock change	tC/ha	Difference between baseline and monitoring SOC
ΔSOC_total	Total project SOC change	tC	ΔSOC × area of stratum
σ	Standard deviation	—	Used in uncertainty estimation
SE	Standard error	—	σ / √n
CI	Confidence interval	—	Applied for uncertainty assessment

1. Bulk density, carbon concentration, and depth intervals must be measured and documented for each sample. All derived quantities such as SOC_layer, SOC_total, and ΔSOC must be calculated using the equations provided in Chapter 5.

2. Where sampling is stratified, each parameter must be recorded separately for each stratum. This ensures that SOC estimates reflect spatial variability across the project area.

The Monitoring Report must include a complete parameter table for each sampling cycle, including all values used in calculations, their units, and their sources.

Chapter 5 - Calculation Procedures

1. This chapter sets out the calculation steps required to determine soil organic carbon stocks and stock changes within mineral soils. SOC must be estimated for each depth interval, for each sampling point, and for each stratum. The procedures outlined here must be applied consistently during baseline establishment and all subsequent monitoring periods.

2. SOC estimation consists of four primary steps: converting laboratory measurements into carbon mass per depth interval, scaling to a hectare basis, summing across all sampled intervals to determine total SOC for the profile, and calculating changes in SOC between sampling periods.

1

Convert laboratory measurements to SOC per depth interval

• Convert bulk density (BD), carbon concentration (C%), and depth interval thickness (D) into SOC_layer (tC/ha).

• BD = bulk density (g/cm³)

• C% = carbon concentration (% by mass)

• D = depth interval thickness (cm)

The Monitoring Report must show all input values and intermediate calculations for each sample.

2

Scale to a hectare basis

• Use standard conversion factors to express mass per unit area in tC/ha.

• Ensure units conversion (e.g., g cm⁻² to t ha⁻¹) is correctly applied.

3

Sum SOC_layer across sampled intervals

• Sum SOC_layer values across depth intervals to obtain SOC_total for the soil profile:

  • SOC_total = Σ SOC_layer (where n is number of depth intervals)

• Report SOC_total in tC/ha for each sampling point and average across samples within a stratum.

4

Calculate SOC stock changes between sampling periods

• ΔSOC (tC/ha) = SOC_monitoring − SOC_baseline for the same location, depth, and stratum.

• If stratified, calculate ΔSOC per stratum and scale by stratum area to obtain project-wide SOC change.

• Positive ΔSOC indicates removal; negative ΔSOC indicates an emission.

5.1 SOC for Each Depth Interval

1. SOC for a given depth interval is calculated using bulk density, carbon concentration, and the thickness of the sampled layer. SOC_layer represents the carbon mass in tonnes per hectare stored in the interval.

2. The general equation is:

Where:

• BD is bulk density (g/cm³),

• C% is carbon concentration (percent by mass),

• D is depth interval thickness (cm),

• The factor 100 converts g/cm² to t/ha.

The Monitoring Report must show all input values and intermediate calculations for each sample.

5.2 Total SOC Stock for the Soil Profile

1. Total SOC for the profile is obtained by summing the SOC_layer values across all sampled depth intervals:

Where n is the number of depth intervals sampled.

1. This value must be reported in tonnes of carbon per hectare (tC/ha) for each sampling point and then averaged across all samples within the stratum.

5.3 Stratified SOC Stock

1. For stratified sampling designs, total SOC for each stratum is calculated by averaging SOC_total values across all samples collected within that stratum:

This ensures that SOC reflects the specific soil and land-use conditions within each stratum.

5.4 Scaling SOC to the Project Area

1. SOC for each stratum must be scaled to the total area of that stratum:

Where A_stratum is the area in hectares.

To obtain project-wide SOC:

5.5 SOC Stock Change Between Periods

1. SOC stock change (ΔSOC) is calculated as the difference between SOC stocks at two points in time for the same location, depth, and stratum.

At the per-hectare level:

If stratification is used, ΔSOC must be calculated separately for each stratum, then scaled to the stratum area:

Project-wide SOC change is:

Positive values indicate an increase in carbon storage (removal). Negative values indicate a loss of carbon (emission).

5.6 Adjustments for Soil Moisture

1. If bulk density is measured under field-moist conditions, it must be corrected to dry bulk density. The Moisture Correction Factor (MCF) is:

Where w is the gravimetric moisture content.

Projects must document all moisture adjustments.

5.7 Consistency of Depth Intervals

1. Depth intervals must remain identical between baseline and monitoring periods. If changes occur (e.g., to increase resolution), the project must apply a conversion procedure ensuring comparability. Depth-normalized SOC values may be used to align intervals of different thicknesses.

5.8 Treatment of Outliers and Missing Data

1. If laboratory or field data are missing or invalid, the project must document the reason and apply conservative assumptions. Outliers may be excluded only when justified scientifically and documented in the Monitoring Report. All exclusions must follow PCS quality assurance rules.

Chapter 6 - Data and Measurement Requirements

1. Accurate SOC estimation requires field sampling, laboratory analysis, and supporting measurements that reflect soil conditions at the time of sampling. All data must be collected and recorded in accordance with PCS monitoring standards and must be traceable for verification.

6.1 Sampling Design and Stratification

1. SOC sampling must be carried out within a stratified sampling framework. Strata must reflect differences in soil type, land-use history, vegetation cover, management practices, and any other factors likely to affect SOC variability. Sampling locations must be selected to ensure that measurements represent the full range of conditions within each stratum. The sampling design, including number of samples and spatial layout, must be documented in the Monitoring Report.

6.2 Soil Sampling Procedures

1. Soil samples must be collected to the full accounting depth specified in the applicable PCS methodology. The sampling depth must be divided into consistent intervals, and each interval must be analyzed separately. Soil cores must be extracted using equipment suitable for maintaining the integrity of the sample. All tools and procedures must minimize disturbance and contamination. Samples must be properly labeled, stored, and transported to the laboratory to preserve their carbon content.

6.3 Bulk Density Determination

1. Bulk density must be determined for each depth interval. The volume of the sample must be measured accurately, and the sample must be oven-dried to determine dry mass. When field-moist bulk density is recorded, the project must apply a moisture correction using gravimetric moisture content. Bulk density results must be reported for each interval and location. If bulk density is not measured directly, conservative default values may be used only when allowed and clearly justified.

6.4 Carbon Concentration Measurement

1. Carbon concentration must be measured for each depth interval using laboratory procedures such as dry combustion or other validated analytical methods. Laboratories must follow quality control procedures, including the use of standards, duplicates, and blanks. The project must archive all laboratory reports, including raw data and metadata describing analytical conditions.

6.5 Depth Interval Consistency

1. Sampling intervals must be identical between baseline and monitoring periods. Any changes in interval thickness or sampling depth must be justified and must include a method for ensuring comparability of SOC estimates across sampling cycles. Depth intervals must be recorded in the field at the time of sampling.

6.6 Recording Soil and Site Characteristics

1. The project must document soil color, texture, structure, moisture conditions, and any signs of erosion or disturbance observed during sampling. Vegetation cover and land-use history must also be recorded, as they may influence SOC patterns. These observations support the interpretation of SOC results and help detect anomalies or trends.

6.7 Sample Handling and Laboratory Documentation

1. All soil samples must be labeled clearly with unique identifiers and accompanied by field notes detailing sampling location, depth, date, and conditions. Laboratories must provide documentation of analytical techniques, equipment calibration, and quality assurance procedures. The project must maintain a complete chain-of-custody record from field collection to laboratory analysis.

6.8 Data Traceability

1. All data used in SOC calculations must be traceable to original sources. Field sheets, GPS coordinates, photographs, laboratory reports, and calculation spreadsheets must be archived. Any assumptions, data exclusions, or adjustments must be disclosed. Traceability must allow a validation or verification body to replicate all SOC estimates.

Chapter 7 - Monitoring Requirements

1. Monitoring of soil organic carbon must ensure that SOC stock changes are measured consistently across monitoring periods and that sampling procedures remain aligned with the requirements of this tool. Monitoring must be designed to detect meaningful changes in SOC while maintaining comparability with baseline measurements.

7.1 Monitoring Frequency

1. SOC must be monitored at intervals defined in the applicable PCS methodology. Monitoring frequency must be sufficient to capture changes in soil carbon associated with project activities. When SOC is expected to change slowly, longer intervals may be allowed, but the sampling schedule must always align with PCS verification cycles. If significant management changes occur, additional SOC sampling may be required.

7.2 Consistency of Sampling Locations

1. SOC monitoring must maintain consistency in sampling locations whenever feasible. Permanent sampling points or GPS-referenced coordinates must be used to ensure comparability between baseline and monitoring measurements. If sampling points cannot be re-located due to disturbance or inaccessibility, new points may be established, provided that justification is documented and the replacement locations remain representative of the stratum.

7.3 Consistency of Sampling Depth and Intervals

1. The sampling depth and the number and thickness of depth intervals must remain identical between baseline and monitoring periods. Any change in sampling design must be justified and accompanied by a method to harmonize SOC estimates across periods. This requirement ensures that stock changes are attributed accurately to project activities rather than methodological variation.

7.4 Monitoring of Disturbances

1. The project must monitor disturbances that may affect SOC, including erosion, flooding, land degradation, excavation, or management changes. Disturbances must be documented, and their potential influence on SOC must be assessed. If a disturbance materially affects SOC within a specific stratum, additional sampling or revised calculations may be required.

7.5 Documentation of Field Procedures

1. All field procedures used during SOC monitoring must be documented in detail. This includes soil extraction technique, sample handling procedures, identification of depth intervals, and field conditions. Documentation must allow replication of field procedures by a validation or verification body.

7.6 Laboratory Monitoring Requirements

1. Laboratories analyzing SOC samples must comply with recognized quality assurance procedures, including equipment calibration and the use of reference standards. Duplicate analyses and blanks must be conducted to ensure data reliability. Laboratory records must be retained and submitted upon request.

7.7 Monitoring of Spatial Variability

1. Monitoring must assess whether SOC variability within each stratum remains consistent with the baseline or previous sampling cycle. If variability increases significantly due to ecological change, land-use patterns, or project activities, the project must reassess sampling intensity to maintain acceptable uncertainty levels.

7.8 Archiving Monitoring Records

1. All monitoring data, including field notes, GPS coordinates, laboratory results, chain-of-custody forms, and calculation sheets, must be archived and retained for verification. Monitoring records must allow complete reconstruction of sampling and analytical processes.

Chapter 8 - Uncertainty and Conservativeness

1. Uncertainty management is essential for ensuring that SOC estimates do not overstate carbon benefits. This chapter outlines how uncertainty must be identified, quantified when possible, and addressed through conservative assumptions to ensure environmental integrity.

8.1 Sources of Uncertainty

1. Uncertainty in SOC estimation may arise from several components of the sampling and analysis process. These include variability in soil characteristics within a stratum, sampling error due to limited sample numbers, laboratory measurement error, and differences in soil texture or moisture affecting bulk density. Additional uncertainty may arise from changes in sampling locations, disturbances affecting soil layers, or inconsistencies in depth interval collection.

8.2 Quantifying Uncertainty

1. Where feasible, uncertainty must be quantified using standard deviation, standard error, and confidence intervals for SOC measurements within each stratum. Projects must calculate the variability of SOC estimates based on sample data and must assess whether sampling intensity is adequate to achieve acceptable precision. If uncertainty exceeds acceptable thresholds, additional sampling may be required.

8.3 Conservative Treatment of Parameters

1. When uncertainty cannot be fully quantified, conservative assumptions must be applied. This includes selecting lower-bound SOC values when estimating increases in carbon storage and applying upper-bound values when estimating carbon losses. When default values for bulk density or carbon concentration are used, the most conservative applicable values must be selected unless laboratory measurements are available. All assumptions must be justified clearly.

8.4 Treatment of Missing or Invalid Data

1. If data gaps occur due to sample loss, laboratory errors, or field access limitations, the project must document the cause and apply conservative adjustments. SOC estimates may not be omitted due to missing data. When depth intervals cannot be sampled, conservative interpolation or exclusion may be applied, provided it does not overstate SOC gains.

8.5 Aggregation of Uncertainty at the Stratum Level

1. Uncertainty must be evaluated separately within each stratum and then aggregated following PCS guidance. Strata with high variability or insufficient sampling must be addressed through increased sample sizes, revised stratification, or conservative adjustments to SOC values. When uncertainty differs substantially between strata, results must reflect this variability rather than assuming uniform uncertainty across the project.

8.6 Reporting of Uncertainty Management

1. The Monitoring Report must describe all sources of uncertainty, the methods used to quantify or address them, and any conservative assumptions applied. Projects must show how uncertainty influenced final SOC estimates and must provide statistical outputs when available. Any deviations from standard uncertainty procedures must be justified.

Chapter 9 - Reporting Requirements

1. SOC estimation results must be reported in a manner that allows complete transparency and independent verification. This chapter defines the minimum reporting expectations for baseline and monitoring submissions.

9.1 Description of Sampling Design

1. The Monitoring Report must describe the sampling design used for SOC measurement, including stratification rationale, sampling frequency, number of samples per stratum, and the spatial arrangement of sample points. The report must also identify any deviations from the baseline sampling scheme and justify why these were necessary.

9.2 Field Sampling Documentation

1. The report must include documentation of field procedures, including tools used for soil extraction, techniques for depth interval separation, and the handling and labeling of samples. GPS coordinates for each sampling point must be provided, along with field notes detailing site conditions at the time of sampling.

9.3 Laboratory Analysis Records

1. For each sample and depth interval, the report must present laboratory results for carbon concentration and bulk density. Laboratory certificates, calibration records, and quality assurance documentation must be included or referenced. Any adjustments applied to laboratory data, such as moisture corrections, must be described.

9.4 SOC Calculations

1. The report must provide SOC calculations for each depth interval, each sampling point, and each stratum. These calculations must be shown in a transparent format that allows replication. The report must also include the equations used, intermediate values, and final SOC results in tonnes of carbon per hectare.

9.5 SOC Stock Change Reporting

1. SOC stock changes must be presented for each stratum and for the overall project. The report must specify SOC values for the baseline and monitoring periods, along with the difference between them. Negative changes must be reported clearly, as these represent emissions. All scaling calculations to project area must be shown.

9.6 Uncertainty Analysis Summary

1. The report must include a summary of uncertainty and conservativeness treatment. This includes statistical outputs, confidence intervals, justification of conservative assumptions, and explanation of any data exclusions. If additional sampling was undertaken to reduce uncertainty, the rationale and results must be documented.

9.7 Supporting Evidence and Archival Materials

1. All supporting evidence used to derive SOC estimates must be archived and referenced. This includes field sheets, photographs, GPS data, chain-of-custody forms, laboratory printouts, and any datasets used for stratification or soil classification. These materials must be retained for verification and future monitoring cycles.

Annex A - Default Values for SOC Estimation

1. Default values may be used only when direct field or laboratory measurements are not feasible and when doing so results in conservative estimates. All default values must be justified in the Monitoring Report.

A.1 Default Bulk Density Values

1. These values apply only when laboratory measurements cannot be obtained.

Table A-1. Default Bulk Density for Mineral Soils

Soil Type	Bulk Density (g/cm³)	Notes
Sandy soils	1.40	Coarse texture; low organic content
Loamy soils	1.30	Moderate organic matter
Clay soils	1.20	High fine-particle content
Degraded or compacted soils	1.50	Reflects reduced porosity
Humus-rich topsoil (0–10 cm)	1.10	Conservative upper-bound value

Bulk density defaults must be replaced with measured values in subsequent monitoring periods whenever feasible.

A.2 Default Carbon Concentration Values

1. Used only when laboratory analysis is not possible.

Table A-2. Default Carbon Concentration Values

Land Cover Type	Carbon Concentration (%)
Forest mineral soil	1.5–3.0
Grassland soil	0.8–2.0
Agricultural soil	0.5–1.5
Degraded soil	0.3–1.0

When applying ranges, the lower bound must be used to avoid overstating SOC gains.

A.3 Default Soil Depths for Accounting

1. When stratification or sampling limitations exist.

Table A-3. Standard SOC Accounting Depths

Ecosystem Type	Accounting Depth
Forest systems	0–30 cm or 0–100 cm
Grasslands	0–30 cm
Agroforestry	0–30 cm or deeper depending on root structure

Accounting depth must remain consistent across monitoring cycles.

A.4 Dry Bulk Density Conversion Factors

Table A-4. Moisture Correction Factors for BD

Soil Moisture Condition	Conversion Rule
Field moist	Multiply by (1 – gravimetric moisture fraction)
Saturated	Dry sample fully before BD calculation

Annex B - Worked Calculation Examples

1. These examples demonstrate the complete process of calculating SOC stocks and SOC stock changes.

B.1 Example 1 - Single Depth Interval (0–30 cm)

Measured values:

• Bulk density: 1.25 g/cm³

• Carbon concentration: 1.8%

• Interval depth: 30 cm

Step 1: SOC for the interval

1

Calculate SOC_layer for 0–30 cm

• Use BD, C%, and D to compute SOC_layer (tC/ha).

• Show intermediate calculations and unit conversions in the Monitoring Report.

B.2 Example 2 - Multiple Depth Intervals (0–30, 30–60, 60–100 cm)

Depth Interval	BD (g/cm³)	C%	D (cm)
0–30 cm	1.20	2.0	30
30–60 cm	1.35	1.0	30
60–100 cm	1.40	0.5	40

Calculate each:

1

0–30 cm

• Compute SOC_layer using BD = 1.20, C% = 2.0, D = 30 cm.

2

30–60 cm

• Compute SOC_layer using BD = 1.35, C% = 1.0, D = 30 cm.

3

60–100 cm

• Compute SOC_layer using BD = 1.40, C% = 0.5, D = 40 cm.

4

Total SOC

• Sum the SOC_layer values from each interval to obtain SOC_total for the profile.

B.3 Example 3 - SOC Stock Change Over Time

SOC_baseline = 140.5 tC/ha

SOC_monitoring = 155.0 tC/ha

If the stratum area is 250 ha:

• ΔSOC (tC/ha) = 155.0 − 140.5 = 14.5 tC/ha

• ΔSOC_total (tC) = 14.5 × 250 = 3,625 tC

Annex C - Data Recording Templates

1. Templates below must be used for field data collection and for documenting SOC calculations.

C.1 Field Sampling Template

Sample ID	Stratum	GPS Coordinates	Date	Depth Interval	Soil Texture	Notes

C.2 Laboratory Analysis Template

Sample ID	Depth Interval	Dry Mass (g)	Volume (cm³)	Bulk Density (g/cm³)	Carbon (%)	QC Comments

C.3 SOC Calculation Template

Sample ID	Interval	BD	C%	D (cm)	SOC Layer (tC/ha)	SOC Total (tC/ha)

C.4 Project-Level SOC Summary Table

Stratum	SOC Baseline (tC/ha)	SOC Monitoring (tC/ha)	ΔSOC (tC/ha)	Stratum Area (ha)	ΔSOC Total (tC)

Annex D - Soil Sampling and Core Handling Guidance

1. This annex provides operational guidance to ensure soil sampling is consistent, reproducible, and scientifically valid.

D.1 Sampling Equipment

1. Cores must be extracted using soil augers or corers designed to preserve sample integrity. Equipment must be cleaned between samples to avoid contamination.

D.2 Core Extraction Procedures

1. Cores must be removed vertically and with minimal disturbance. Each depth interval must be separated immediately and placed into labeled containers. Depth boundaries must be measured precisely.

D.3 Sample Storage and Transport

1. Samples must be stored in airtight bags or containers to avoid moisture loss or contamination. Samples must be transported promptly to the laboratory. Chain-of-custody documentation must accompany all samples.

D.4 Laboratory Preparation

1. Soil must be oven-dried at standard temperature (commonly 105°C unless otherwise required). Bulk density must be calculated using dry mass and known sample volume. Carbon concentration must be measured using validated analytical techniques.

D.5 Quality Assurance

1. Duplicate samples, laboratory blanks, and standard reference materials must be included to ensure analytical reliability. Any anomalies must be investigated and documented.