The amount of SOC stock at any given time is controlled by factors of soil formation. The soil forming factors are climate, topography, parent material, biota, time, and human activity (Amundson and Jenny, 1997). Two general approaches to determine rates of SOC accumulation and cycling are: (a) the “chronosequence” approach—which monitors SOC in soils of different ages but similar environment and bedrock, and (b) a “mass balance” approach in which C cycling rates are inferred for soils near or at steady state (Amundson, 2001). The average C atom in atmospheric CO2 passes through soil organic matter (SOM) somewhere in the world approximately every 12 years. In recent decades, the most notable factor that influences the global SOC dynamics in space and time is human induced land use/cover change (IPCC, 2003).

To understand the impact of projects in the capturing carbon, the SOC stock needs to be traced though time. Soil monitoring assesses the changes in soil carbon status with reference to the soil carbon stock at the beginning of the project. The Marrakesh Accords specify that all emissions from sources and removal by sinks caused by Article 3.3 and elected Article 3.4 activities be reported annually (IPCC, 2003). However, the inter-annual variability in SOC stock is often very low. Moreover, the cost of detecting a change in SOC stock using field and laboratory measurements is expensive. Hence, the cost of detecting SOC change might cost more than the actual value of carbon sequestered, even though soil-monitoring schemes may serve a number of other purposes. Although the change in SOC stock varies with factors that influence the rate of production and decomposition of carbon, a five-year monitoring cycle is recommended (IPCC, 2003), whereas UNFCCC (2006) recommend a monitoring interval of between 10 and 20 years. A fine temporal resolution in SOC monitoring can be also achieved using modeling of SOC using remote sensing and other easily available data. Soil monitoring assesses the changes in soil carbon status with reference to the soil carbon stock at the beginning of the project.

Tracking changes in soil carbon over time requires that the same equivalent mass of soil be measured from one monitoring event to another. Sampling to a fixed depth (equal volume) can underestimate carbon gains via forestation. As the bulk density can change due to land use, the same sampled volume contains less of the original soil-mass equivalent. Therefore, rates of accrual estimated from sampling to a fixed depth should be considered conservative estimates of soil-carbon accretion (Pearson et al. 2007). The changes in SOC stock can be converted to tonnes CO2 equivalent by multiplying by 3.67, which is the ratio of the molecular weights between carbon (12) and carbon dioxide (44).

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Amundson, R., Jenny, H. 1997. On a state factor model of ecosystems. Bioscience 47:536-543.

Amundson, R. 2001. The carbon budget in soils. Ann. Rev. Earth Planet. Sci. 29:535-562.

IPCC. 2003. Good Practice Guidance for Land Use, Land-Use Change and Forestry, In Penman, J., et al., eds. Institute for Global Environmental Strategies (IGES), Japan.

Pearson, T.R.H., Brown, S., Birdsey R.A. 2007. Measurement Guidelines for sequestration of forest carbon. USDA forest service, Delware.

UNFCCC. 2006. Approved afforestation and reforestation baselines methodology AR-AM0002: "Restoration of degraded lands through afforestation/reforestation".