Methods for Estimating Soil Organic Carbon

Soil is the largest reservoirs of carbon, accounting for 2011 GtC, or 81% of the total carbon in the terrestrial biosphere (WBGU 1998). Flow of carbon between soil and the atmosphere is a continuous process, highly influenced by land use and management (Paustian et al. 1997). Organic carbon stored in soil is an important carbon pool for many land-use systems and projects, and even for national greenhouse gas inventories of different land-use categories. "Soil organic carbon" is also often referred to as "soil organic matter". Soil organic matter includes the whole non-mineral fraction of soil ranging from decayed plant and animal matter to brown to black material that bears no trace of the original anatomical structure of the material and is normally defined as "soil humus". Soil organic matter also includes living and dead microbial tissue, compounds synthesized by microorganisms and derivatives of these materials produced as a result of microbial decay. Soil organic carbon as defined by IPCC (2006) comprises "organic carbon in mineral soils to a specific depth chosen, also including live and dead fine roots within the soil". Although both organic and inorganic forms of carbon are found in soil, land use and management typically has a larger impact on the stocks of organic carbon. Therefore, this chapter focuses on soil organic carbon. Further, soil organic carbon is relevant to both mineral and organic soils. Organic soils contain a minimum of 12-20% organic matter by mass and are found under poorly drained conditions of wetlands (Brady and Weil 1999). All other soils are classified as mineral soils, which typically have relatively low amounts of organic matter. Mineral soils dominate most ecosystems except wetlands and are the focus of this chapter.

Stocks of organic carbon in soil vary with land-use systems. The share of soil organic carbon in the total carbon stock may vary from 50% to 84% in forests to 97% in grasslands (Bolin and Sukumar 2000). Soil organic carbon is likely to be a dominant pool in all land-use systems without trees such as grassland and cropland. Carbon stocks in soils are fairly stable under undisturbed conditions, such as natural forests or grasslands. Land-use change involving disturbance to topsoil leads to oxidation of organic matter and rapid loss of soil organic carbon. The concentration of organic carbon in soil is the highest in topsoil. Soil carbon dynamics is normally restricted to the top 15-45 cm, which is the zone of maximum microbial activity. Soil organic carbon is normally estimated to a depth of 0-30 cm since most of it is present in the top layers and root activity is also concentrated in this horizon.

Change in the stocks of soil organic carbon in mineral soils is estimated using the following formula:

where:

ASC = annual change in carbon stock in mineral soil, tonnes C/year

SC. = soil organic carbon stock at the beginning of the period t,, tonnes C/ha 1 1 SC = soil organic carbon stock at period t2, 5 or 10 years, tonnes C/ha.

13.1 Soil Carbon Inventory for Land-Use Projects and Greenhouse Gas Inventory

13.1.1 Soil Carbon Inventory for Mitigation Projects

An inventory of soil organic carbon (SOC) pool is necessary for carbon mitigation projects (such as afforestation, reforestation and agroforestry) as well as all land reclamation projects such as grassland reclamation, shelterbelts and watershed projects. Estimates of SOC are required for the following scenarios:

• Baseline scenario where SOC is estimated prior to initiation of the project activities as well as in the control plots by simulating baseline scenario conditions during the project monitoring phase if SOC is expected to change under the baseline scenario.

• Project scenario involving periodic estimation of SOC in land-use systems subjected to project activities, classified into homogeneous strata.

Most carbon mitigation as well as land reclamation projects increase the stocks of SOC. A carbon inventory should provide estimates and projections of SOC during the following phases:

• Project development phase, where SOC is estimated for all land-use systems and strata (see Section 10.3 for strata definition) before the implementation of project activities and projected into the future.

• Project monitoring phase, where SOC is measured and estimated periodically for land-use systems in which project activities are implemented, as well as for the baseline scenario land-use systems.

Soil organic carbon is routinely estimated in most land-based projects as an indicator of the impact of project activities on stock of soil carbon, soil fertility, moisture-holding capacity or soil erosion. Researchers and project managers in forestry, agriculture, grassland development and conservation projects estimate the stocks of SOC and changes in those stocks. Methods for estimating SOC are well documented and extensively used in all projects (IPCC 2003, 2006; Mac Dicken 1997; Hairiah et al. 2001).

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