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In southwest France, thick humic acid soils have developed from Quaternary silty alluvial deposits. The objective of this study was to determine if specific patterns of carbon profiles occurred in these soils in order to evaluate total carbon storage using mathematical modeling and laboratory analysis on a small number of samples. Soil samples were collected from an oceanic zone of the French Pyrenean piedmont, ancient terraces of Pyrenean streams (southwest France), and from seven mature forests. The results demonstrated that these soils have accumulated large amounts of organic carbon. The pattern of C accumulation showed an exponential decrease with depth and appeared to be characteristic of these soils. This typical pattern allowed modeling of the carbon storage profile and reduction of the number of samples from 10 to 3 without modifying significantly the fitted model and the evaluation of the total amounts of C. This study also demonstrated that sampling to shallow depths fails to include a large proportion of the total C content in these soils. Therefore, studies restricted to shallow depths appear not to be relevant for studying the role of these soils in carbon storage. A regression predicting bulk density from organic carbon was used to calculate total C amounts down to a depth of 1 meter. The analysis used simple determinations of C concentration in three specific horizons.
Arrouays et al. (1994) studied this question.