Effects of pyrophosphate-extractable aluminum, iron, and calcium on organic carbon storage in buried humic horizons of a cumulative volcanic soil profile containing charred plant fragments, Miyakonojo, Miyazaki, Japan

Charred plant fragments (CPFs), having two primary types (carboxylic and phenolic functional groups) of metal-binding sites, are widely distributed in Japanese volcanic soils containing black humic acids. However, the effects of metal components on the storage of CPFs are still unknown. The aim of this study was to determine the direct and indirect effects of metal components on the CPF-carbon (CPF-C) and soil organic C (SOC) contents in four buried humic horizons (2A–5A, from 7.3 cal ka BP to AD 1471) of a cumulative volcanic soil profile (Miyakonojo, Miyazaki, Japan), employing correlation, and path analyses. Nineteen soil samples were collected from the humic horizons described above and used. Pyrophosphate-extractable aluminum (Alpy), iron (Fepy), and calcium (Capy), which are common in the soils, were selected as reactive metal components. The CPF-C content accounted for up to 15.1% of the SOC content (8.4% on average). The Alpy content was highest in all soils, mostly followed by the Fepy and then Capy contents. No consistent trends with soil age/depth were found for the accumulation levels of CPF-C, SOC, and metal components. However, there were significant correlations (p < 0.01) among the CPF-C, SOC, Alpy, Fepy, and Capy contents. The result of path analysis showed that a high correlation coefficient between the CPF-C and Fepy contents (r = 0.89, p < 0.001) was mainly explained by the direct effect of Fepy (0.84, p < 0.01) on the CPF-C content. On the other hand, in the case of SOC, a strong correlation coefficient between the SOC and Alpy contents (r = 0.93, p < 0.001) was primarily due to the direct effect of Alpy (0.74, p < 0.01) on the SOC content. The direct effects of Capy on these C contents were not important. Furthermore, significant indirect effects of the metal components on the CPF-C and SOC contents were not observed. From the results obtained, it is assumed that in Japanese volcanic soils, Fepy and Alpy contribute directly to the large accumulation and stabilization of CPF-C and SOC, respectively.