Definition of Carbon Pools

The definitions of carbon pools according to IPCC (2006) are given in Table 4.3. Above-ground biomass (AGE) Above-ground biomass is expressed as tonnes of biomass or carbon per hectare. Above-ground biomass is the most important and visible carbon pool, and the dominant carbon pool in forests and plantations, although not in grasslands and croplands. Above-ground biomass is given the highest importance in carbon inventory and in most mitigation projects and is the most important pool for afforestation and reforestation CDM projects under the Kyoto Protocol as well as any inventory or mitigation project related to forest lands, agro-forestry and shelterbelts in croplands. The above-ground biomass is often the only carbon pool measured or estimated in roundwood production projects. The methods and models for measuring and projecting above-ground biomass are also the most

Table 4.3 Definition of carbon pools according to IPCC (2006)

Pool

Description

Above-ground biomass

Below-ground biomass

Deadwood

All biomass of living vegetation, both woody and herbaceous, above the soil including stems, stumps, branches, bark, seeds and foliage

All biomass of live roots. Fine roots of less than 2 mm diameter (the suggested minimum) are often excluded because these often cannot be distinguished empirically from soil organic matter All non-living woody biomass not contained in the litter, either standing, lying on the ground, or in the soil. Deadwood includes wood lying on the surface, dead roots, and stumps larger than or equal to 10 cm in diameter Litter All non-living biomass with a size greater than the limit for soil organic matter (the suggested minimum is 2 mm) and less than the minimum diameter chosen for deadwood (e.g. 10 cm) lying dead and in various states of decomposition above or within the mineral organic soil. This includes the litter layer as usually defined in soil typologies. Live fine roots above the mineral or organic soil (of less than the suggested minimum for below-ground biomass) are included whenever they cannot be empirically distinguished from the litter. Soil organic Organic carbon in mineral soils to a specified depth chosen and matter applied consistently through a time series. Live and dead fine roots within the soil (of less than the suggested minimum for below-ground biomass) are included wherever they cannot be empirically distinguished from the soil organic matter.

developed compared to other carbon pools. In non-forest land-use systems such as cropland and grassland, biomass predominantly consists of non-woody perennial and annual vegetation, which makes up a much smaller part of the total carbon stock in the ecosystem than that in forest lands. The non-woody biomass is part of the annual carbon cycle and is subjected to turnover every year or every few years and hence net biomass carbon stock may remain more or less constant, although stocks may diminish over time because of land degradation. Below-ground biomass (BGB) Below-ground or live root biomass is expressed as tonnes of biomass or carbon per hectare. Roots play an important role in the carbon cycle as they transfer considerable amounts of carbon to the ground, where it may be stored for a relatively long period of time. Although roots can extend to great depths, the greatest proportion of the total root mass is confined to the top 30 cm of the soil surface. Carbon loss and accumulation in the ground is intense in the top layer of the soil profile, which indicates that this should be the focus in sampling (Ponce-Hernandez et al. 2004). In many land-use systems such as grasslands and croplands, however, this pool may not be important. Further, below-ground biomass in grassland and cropland under annual crops is part of the annual carbon cycle, and need not be measured. Below-ground biomass is the least researched or measured carbon pool because of the difficulty in measuring or modelling of the stock or growth rates: estimating it requires uprooting of trees and grass and disturbs topsoil, which is destructive in normal circumstances, and most often, the quantity is estimated as a proportion of above-ground biomass.

Deadwood Deadwood is not a dominant carbon pool in any land-use system and usually accounts for only about 6% of the total carbon stock in forest and other wooded lands (Table 4.1). Deadwood includes naturally dead trees, both standing and fallen, and those killed as a result of pest attack, wind damage and human intervention, but excludes naturally falling woody and non-woody litter or biomass. The two fractions, namely deadwood and litter, are normally differentiated on the basis of size: a certain minimum diameter is usually stipulated as the cut-off point. Deadwood may occur largely in natural forests, but is rare in new plantations, agro-forestry systems, savannah, grasslands and croplands.

Litter The layer of organic debris, dead plant material fallen or removed and plant parts not attached to plants are considered litter. Build-up of litter is a natural process in which woody and non-woody parts of trees and shrubs dry up and fall to the ground (floor of the forest or of a plantation); the process is also part of the overall process of turnover of forest biomass. Litter is not a major carbon pool because it usually accounts for only 6-8% of plant biomass (Whittaker and Likens 1973; Bazilevich 1974) and sometimes even less (Table 4.1).

Soil carbon Soil organic matter is defined as organic carbon in mineral soils to a specified depth. The generic term for all organic compounds in the soil is particles that are not living roots or animals. As dead organic matter is fragmented and decomposed, it is transformed into soil organic matter. It includes a wide variety of materials that differ greatly in their residence time in soil: some of them are easily decomposed by microbial organisms and return the carbon to the atmosphere but some of the soil organic carbon is converted into recalcitrant compounds (e.g. oiganic-mineral complexes) that decompose slowly and may remain in soil for decades or centuries or even longer. Fires often result in the production of small amounts of so-called black carbon, a nearly inert carbon fraction with turnover times that may span several thousand years (IPCC 2006). Within a given land-use system, such as cropland and grassland, management practices can have a significant impact on storage of soil carbon. Management practices and other forms of disturbances can alter the net balance between carbon input and carbon losses from the soil. Input to soil carbon stock can come from higher plant production. When native grassland or forest land is converted into cropland, 20-40% of original soil carbon stock can be lost (Mann 1985; Davidson and Ackerman 1993; Ogle et al. 2005). Although both organic and inorganic forms of carbon are found in soil, land use and management typically has a larger impact on organic carbon stocks, and this handbook accordingly focuses only on organic form of carbon.

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