Comparison of two soil organic matter extractants and determination of the “Walkley–Black” correction factors for organic fractions from a volcanic soil

Abstract

Two extractants (0.1 M NaOH and a mixture 0.1 M NaOH–0.1 M Na₄P₂O₇) were used separately to recover organic matter from a volcanic soil under Corsican pine (Pinus nigra Arn. ssp. laricio Maire). The presence of pyrophosphate did not considerably improve the recovery; on the contrary, in the topsoil, the aqueous NaOH was even slightly more efficient than the mixture. A comparison between dry combustion and dichromate oxidation (Walkley–Black method) to measure the organic carbon (C) in the extracted (fulvic and humic) fractions and in the soil residue was accomplished. Dichromate oxidation did not allow the detection of the entire (dry-burnt) C in the extracted fractions. The correction factor to account for undetected C varied according to horizon, organic fraction, and extractant. An apparent complete oxidation with the Walkley–Black method occurred for the soil residues, but it could be due to magnetite interference.

Acknowledgments

We are indebted with A Dodero for laboratory assistance. This research was supported by grants from the Italian Ministry of University and Scientific and Technological Research and the European Community (TMR Programme).