Thermal resistance to high temperatures of different organic fractions from soils under pine forests

Thermal resistance of the organic matter from two Humic Cambisols from the NW of Spain, developed over granite and under pine forest, was determined by heating samples of the upper 5 cm of soils at 220 and 350 °C for 30 min. Unheated samples of the same soils were used as control. Lignin, holocellulose, lipids and water-soluble compounds as well as humic substances were determined by different chemical fractionation methods. At 220 °C, the losses of holocellulose exceeded 75%, this being the most vulnerable organic fraction. At this temperature, the lipidic fraction exhibited different thermal resistance in both soils, showing losses of 73% in one case and only 11% in the other. Lignin exhibited the lowest losses, increasing proportionally. Fulvic and humic acids decreased at 220 °C but humin exhibited a net increase. At 350 °C, holocellulose practically disappeared and lignin losses were significant, whereas lipid loss slightly increased compared with the loss observed at 220 °C. All humic substances decreased at 350 °C, but humin was the most resistant humic fraction. Heating the soils at 220 and 350 °C increased the Fe and Al extracted with Na4P2O7 and NaOH and the percentage of humification of the remaining soil organic matter.