Direct determination of organic carbon by dry combustion in soils with carbonates

In acid soils, where organic carbon (C) corresponds to total C, direct determination of organic C by dry combustion is possible, whereas in soils with carbonates also a separate measurement of inorganic C is required. In this case, direct quantification of organic C can be accomplished by the Walkley-Black method, which is time-consuming and involves greatly polluting by-products. Hence, a method able to determine directly organic C by dry combustion is strongly needed for soils with carbonates. This study proposes such a method, after it was found to be highly reliable in calcareous soils of a Mediterranean island. The correction factor to use in the Walkley-Black method to account for nonrecoverable C was calculated. It does not show any overall relationship with the contents of either organic C or inorganic C, and for all land uses examined in the island, it is not significantly different from the commonly suggested value 1.30.