Formation and mobility of soil organic carbon in a buried humic horizon of a volcanic ash soil

A buried humic horizon (¹⁴C age between 5.4 and 6.8 kyr BP) of a volcanic ash soil in Aomori, Japan, which was collected from the depth between 147 and 187 cm at 5 cm-interval (total eight sub-horizon samples), was investigated to clarify the degree of biological transformation and mobility of soil organic carbon (SOC) fractions. The SOC fractions were prepared from each sub-horizon sample by extraction and precipitation procedures with controlling pH of the extracted solution, resulting in humin, humic acid (HA), and four fulvic acid (FA) fractions (two hydrophilic FA fractions: FA₁ and FA₂, and two hydrophobic FA fractions: FA₃ and FA_IHSS). The prepared SOC fractions were characterized by ¹⁴C age and stable isotopic ratios of ¹³C (δ¹³C) and ¹⁵N (δ¹⁵N). The hydrophilic FA fractions showed the highest δ¹³C and δ¹⁵N values, indicating that these SOC fractions had been most enriched with ¹³C and ¹⁵N by biological metabolic processes. On the other hand, the HA fraction showed the lowest δ¹³C and δ¹⁵N values, therefore this fraction would have been less-metabolized, although HA fraction has been regarded as well-processed in general. The ¹⁴C age of the HA fraction was almost same as the deposition age of the corresponding sub-horizon, indicating that the C in the HA fraction would have been fixed in situ right after photosynthesis by plants at the early stage of soil formation and chemically stabilized at soil surface, by fire event, etc. The average rates of vertical translocation of the SOC fractions were low (humin and HA fractions: <1 mm per century, FA fractions: 1–4 mm per century), implying that the vertical translocation of SOC would not be the main mechanisms for forming thick humic horizons. The present study showed strong evidence of the in situ formation of SOC on soil surface, and successive up-building accumulation of soil particles containing SOC would contribute to the formation of the thick humic horizons.