Humic acid characteristics in podzol soil chronosequence

Humic acid (HA) characteristics were studied in a natural revegetation chronosequence of 10-, 20-, 60-year-old and mature podzol soils under a pine forest, along the spoil heaps of a sand quarry. The elemental composition of HAs showed a trend towards carbon increase and hydrogen decrease with soil age. Essential differences in the atomic ratios between humic acids extracted from the different soils may be due to the intensity of the humification process as related to soil age. Compared with HAs extracted from organic horizons, those from mineral horizons showed lower C/N and H/C ratios, whereas O/C ratios were higher. Electrophoretic data showed that the proportion of the more-mobile fraction (L-MS) was higher in the mineral horizons than in the organic horizons and, among organic layers, in the Oe and Oa horizons compared with Oi. Intensive transformation of pine remains may have occurred and led to HAs with an increasing degree of humification in only 60 years. Moreover, progressive accumulation of the L-MS fraction in the E and Bs horizons with soil age might be the result of the ongoing migration of the most-polar organic compounds down through the soil profiles.