The term "agricultural soils" is used explicitly in the Protocol (Article 3.4). The term "forest soils" is absent but may be considered as part of the forest ecosystem. There are significant differences between agricultural and forest soils that may affect inventory practices.
Fully developed forest soils are natural bodies with a vertical sequence of layers (FAO/ISRIC, 1990). At the top is an organic surface layer or "forest floor" (O horizon) with subdivisions of fresh, undecomposed plant debris (Oi horizon, formerly called L); semi-decomposed, fragmented organic matter (Oe horizon, formerly called F) and humus; and amorphous organic matter without mineral material (Oa horizon, formerly called H). Below this surface layer is a mineral surface horizon (A); a subsurface mineral horizon often leached (E); a subsurface mineral horizon with features of accumulation (B horizon); a mineral horizon penetrable by roots (C); and locally hard bedrock (R). The E, B, C, and R horizon may be lacking, or the B horizon may be modified by groundwater or stagnant water.
Agricultural soils associated with rangelands and grasslands often have similar vertical sequences. However, if they are being cultivated (arable land)-or have been in the past-they may lack the O horizon (unless peat soils are being used), and the A horizon may have been mixed with parts of the E and even the B horizon, resulting in a plow layer (Ap horizon). The B and/or C horizons may have been broken up by deep cultivation. The soils may have been so degraded by past human actions that they are no longer cultivatable. Such soils may still be classified as agricultural soils and used, for example, for grazing or non-cropping production.
The thick organic layers of wetlands, which may have peaty horizons of more
than 30 cm up to several meters, are a special form of O horizon. These layers
are important stores of organic carbon, which may be released as CO2 and/or
CH4 if the land is drained and cultivated, artificially flooded, or subject
to wildfires in dry years.
Both the topsoil and the subsoil are relevant in the context of carbon sequestration in agricultural soils. The topsoil is the layer with accumulation of more labile soil organic matter ("nutrient humus"). More stable humus ("structural humus") occurs in both topsoil and subsoil. The activity of soil biota such as rodents, earthworms, termites, or leaf-cutting ants leads to a dynamic interaction between these two layers and the substratum.
These three terms-topsoil, subsoil, and substratum-are likely to be used for carbon accounting in relation to Articles 3.3 and 3.4. Therefore, their definitions merit attention.
The definition of topsoil varies according to the focus, as well as national tradition. From a soil science perspective, topsoil is the surface plus subsurface mineral horizons (A, as well as E if present). Agronomically, the topsoil coincides with the plow layer. FAO/UNEP (1999) does not make a distinction between these two views: "The topsoil is the upper part of the soil, with the lower limit at 30 cm, or shallower if a root growth inhibiting layer is present above that depth." The subsoil comprises all densely rooted layers below 30 cm. The substratum can extend down to 10 m or more in well-drained tropical soils, then may still have living rootlets (Nepstad et al., 1994). Agronomically, the substratum is the deeper layer not rooted by annual crops, below 100 cm.
There are many national soil classification systems, either soil science or land use oriented. The main internationally used systems are "Soil Taxonomy" (SSS, 1999) and the FAO/ Unesco terminology; the latter was recently updated as the World Reference Base for Soil Classification (WRB, 1998) and was recommended by the International Union of Soil Sciences and FAO. National soil scientists will be able to correlate their systems with the two international ones as far as required for carbon accounting.
Horizontally within a landscape, there may be large differences in soil organic matter and carbon storage at short distance, linked to differences in depth, texture, drainage condition, and slope position of the various components/facets of a landscape. Even if the landscape is homogeneously covered with high tropical forest, the total soil carbon stock can vary between 50 and 300 t ha-1 (Sombroek et al., 1999).
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