Why measuring for change?

To understand the impact of projects in the capturing carbon, the SOC stock needs to be tracked 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, 2006).

Approaches in measuring for change

The 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 parent material, 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, 2006).

While monitoring changes in SOC between treatments, over time periods, we should consider changes in bulk density caused, erosion, deposition, compaction, decomposition, tillage and expanding clays. To express changes in soil carbon stocks on an equal mass basis requires that the change in the soil bulk density. Estimates of soil carbon stocks to a fixed depth using single depth bulk density are mostly biased due to the spatial and temporal variability in bulk density (Lee et al., 2009; VandenBygaart and Angers, 2006). A management that leads to a decrease in bulk density will under estimates soil organic carbon stock and vice versa (Ellert and Bettany, 1995). 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, Lee et al., 2009; VandenBygaart and Angers, 2006). 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).

Frequency of measuring for change

A project that off-sets CO₂ by carbon sequestration should be able to prove that significant carbon gains have occurred following a given land management practice compared to the pre-treatment baseline (Olson, 2013). Intensity of measurements depends on the type ecosystem and type of management. For instance crop residue additions and animal manure applications in paddy field sequesters significant carbon over periods of 20 and 40 years (Rui and Zhang, 2010). The SOC pool for different ecosystems is in the order swamps and marsh > boreal forest > tundra and alpine meadow > temperate grassland and pastures > temperate evergreen forest > temperate deciduous forest > tropical evergreen forest > tropical season forest (Lalet al., 1997). 

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. 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 also more expensive than measuring carbon stock in above ground woody biomass. 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.

A fine temporal resolution in SOC monitoring can be also achieved using modeling of SOC using remote sensing and other easily available data sources. Soil monitoring assesses the changes in soil carbon status with reference to the soil carbon stock at the beginning of the project.

 

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Ellert, B. H. &Bettany, J. R. 1995. Calculation of organic matter and nutrients stored in soils under contrasting management regimes. Can. J. Soil Sci. 75:529-538.

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.

IPCC. 2006. IPCC Guidelines for National Greenhouse Gas Inventories. Volume 4: Agriculture, Forestry and Other Land Use. Eggleston, H.S., Buendia, L., Ngara, T., and Tanabe, K. (eds). IGES, Japan.

Lal, R. 1997. Residue management, conservation tillage and soil restoration for mitigating greenhouse effect by CO2–enrichment. Soil Tillage Res. 43:81–107.

Lee, J., Wopmans, J.W., Rolston, D.E., Baer, S.G., and Six, J. 2009. Determining soil carbon stock changes: Simple bulk density corrections fail. Agriculture, Ecosystems and Environment 134:251-256.

Olson, K. R. 2003. Soil organic carbon sequestration, storage, retention and loss in U.S. croplands: issues paper for protocol development. Geoderma 195-196: 201–206.

Rui, W.Y. & Zhang, W.J. 2010.Effect size and duration of recommended management practices on carbon sequestration in paddy field in Yangtze Delta Plain of China: A meta-analysis

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

Vandenbygaart, A.J. & Angers, D.A. (2006) Towards accurate measurements of soil organic carbon stock change in agroecosystems. Canadian Journal of Soil Scince, 86(3): 465-471.