CDM AR-Tool16: Soil Organic Carbon Change

Updated 2026-11-02 · 3 min read · By the akar.earth specialist team

AR-Tool16 soil carbon explained: how the CDM tool estimates soil organic carbon gains in A/R projects from IPCC defaults, its limits, and use in Indonesia.

AR-Tool16 is the Clean Development Mechanism (CDM) tool for estimating the change in soil organic carbon (SOC) caused by afforestation and reforestation projects. Instead of repeated soil sampling, it uses IPCC default values to estimate how much soil carbon rises when degraded land is returned to tree cover.

It is called by CDM A/R methodologies such as AR-ACM0003. For many ARR projects it was the simplest way to add the soil pool. Under VM0047, soil carbon is handled under that methodology’s own rules, but the AR-Tool16 approach is still a useful reference.

The logic of AR-Tool16

Degraded cropland or overgrazed grassland usually holds less soil carbon than the same soil under natural vegetation. When trees return, litter and roots rebuild organic matter. AR-Tool16 models this recovery with the IPCC Tier 1 approach described in the IPCC AFOLU guidelines:

• a reference SOC stock for the soil type and climate zone under native vegetation;
• stock change factors for land use, management and inputs that describe how far the pre-project land has fallen below the reference;
• an assumption that SOC rises in a straight line from the initial stock back toward the reference over a fixed period.

The tool caps the annual rate of increase so that credited gains remain conservative. Check the current version for the cap and the transition period.

Applicability conditions

AR-Tool16 applies only when conditions keep the default approach credible. In general:

• the soil is mineral, not organic (peat) or wetland;
• the land was in a degraded or low-carbon state before the project, such as cropland or grassland;
• litter stays on site rather than being removed;
• soil disturbance from site preparation and planting is limited and not repeated.

Read the exact conditions in the tool version your methodology references. If any condition fails, the soil pool must be excluded or estimated by another approved method.

How AR-Tool16 works step by step

1. Classify soils and climate. Identify the soil type and IPCC climate zone for each stratum.
2. Find the reference stock. Look up the IPCC default SOC stock for that soil and climate, or use local data if the methodology allows.
3. Characterise pre-project land. Record land use, tillage and input level using land-use history and interviews.
4. Apply stock change factors to estimate the initial SOC stock.
5. Calculate annual gain as the gap between reference and initial stock, spread over the transition period, subject to the cap.
6. Check conditions each monitoring period. Confirm that disturbance and litter management rules still hold.
7. Report removals as CO2 by multiplying carbon by 44/12.

Indonesia and Southeast Asia application

Many Indonesian ARR sites are former cropland, alang-alang grassland or scrub on mineral soils. These are the conditions where a default-based soil gain is plausible. Points to watch:

• Peat is excluded. Large parts of coastal Sumatra and Kalimantan are on peat. Any planting there needs peat-specific accounting. See peatland REDD+.
• Volcanic soils differ. Andosols in Java, Bali and parts of Sumatra can hold high SOC already. The gap to reference may be small.
• Land history is hard to document. Smallholder plots change between cassava, maize and fallow. Interview records and historical imagery help.
• Site preparation. Mechanical land clearing can break the disturbance condition. Manual planting usually keeps within it.

For soil carbon outside ARR, such as improved farm management, see soil carbon for smallholders and the Gold Standard SOC framework.

Pitfalls verifiers flag

• Wrong IPCC climate zone or soil class.
• Land-use history assumed rather than documented.
• Tool applied to organic soils or wetland margins.
• Litter removed for fuel or fodder during the project.
• Repeated soil disturbance from replanting.

Evidence to collect for AR-Tool16

Keep a land-use history for each stratum, based on historical imagery and interviews, a soil map or field soil descriptions, the IPCC climate zone used, and records of site preparation methods. Photos of litter on the ground at each monitoring event help show the litter condition is met.

Where doubt exists about a stratum, exclude it from the soil pool rather than risk a finding.

Should you include soil carbon?

Model it first. In many ARR projects soil adds a modest share of total removals compared with trees. It brings extra conditions to monitor. Read carbon pools for how to decide which pools to include. Our carbon modelling service tests both options before you lock the design. More methodology pages are on the standards hub.