Distribution of organic carbon in soil profile data

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Soil organic carbon (SOC) content has been estimated at pan-European scale for the soil layer from 0 to a depth of 30 cm (Jones et al., 2005). The methodology used to generate the data layer relied on a combination of a pedo-transfer rule (PTR) and pedo-transfer functions (PTF). The PTR has been developed based on PTR No. 21 of the PTR database of the Soil Geographic Database of Eurasia (SGDBE). The original conditions of the rule system have been revised and amended to accommodate organic soils and peat. The revised PTR for topsoil SOC content comprises 120 ordered conditions of combinations of 5 soil and environmental parameters with an output to one of 6 classes of SOC content. Differences in SOC content due to temperature differences are allowed for by the PTF on temperature variations with a continuous output. The methodology has been verified using measured data from soil profiles from sites across Europe, but the conditions and function parameters are only applicable to the topsoil layer.

Analyses of measured soil profiles suggests that the subsoil layer contains significant quantities of OC. The 30-100 cm depth layer is estimated to contain as much OC as the topsoil layer (Batjes, 1996; FAO, 2001; Jobbagy & Jackson, 2000). An approach was therefore explored how the existing methodology could be advanced to allow extending the SOC content to a depth of 100 cm. Rather then developing a PTR for subsoil SOC content it was investigated whether the rule-based system could be substituted by a function linking the SOC content of the topsoil to the subsoil. Where the influencing factors are discrete parameters, e.g. land cover classes, a function can be defined based on the statistical analysis of soil profiles for each factor class. Statistical methods have already been used to provide estimates of SOC content to a depth of 100 cm in large-scale databases, such as the maps on soil-water holding capacities from Reynolds, et al., 1999.

For the development of a PTF to estimate subsoil SOC content from the topsoil the factors influencing the relationship and the characteristics of the relationship depending on the factors will have to be determined. For this purpose three databases with measurements on soil profiles across Europe and one national profile database have been investigated. The main parameters influencing the change of SOC content with depth were taken from the literature. The study then evaluated the potential of the parameters to formulate a function relating topsoil to subsoil SOC content at any depth within the subsoil up to 100 cm.

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