Organic matter changes immediately after a wildfire in an atlantic forest soil and comparison with laboratory soil heating

The quantity, chemical composition and mineralization kinetics of the organic matter of an acid Humic Cambisol, developed over granite, under Pinus sylvestris L. were determined in 0–5 and 5–10 cm samples collected immediately after a high-intensity wildfire, and compared with those of an unaffected site nearby. Organic matter was characterized by different chemical fractionation methods, and the C mineralization was determined by aerobic incubation. A similar unburnt soil located in the same area was heated at the laboratory at 150, 220, 350 and 490°C to measure the losses of C content; the samples heated at 220 and 350°C were selected to determine chemical changes in organic matter composition. Surface and subsurface soil layers lost about 50% of their C content during wildfire. The C mineralized decreased in the surface layer; however, the percentage of total C mineralized increased in both layers. The cumulative CO 2C mineralized fitted a double exponential first-order kinetic model, but the fire affected the kinetic parameters, increasing both the labile pool of the potentially mineralizable C and the mineralization rate constants of the recalcitrant and labile pools. Cellulose + hemicelluloses declined significantly after the burning, whereas lipids did not vary. The fire decreased the amount of unhumified organic matter and the alkali-soluble compounds, particularly humic acids, but there was a net increase of humin. The organic matter bound to Fe and especially to A1 was much higher after the burning. In the soil heated under laboratory conditions the changes observed at 150°C were very low, whereas at 490°C almost all the organic matter disappeared. The changes exhibited by the samples heated at 220°C were the most similar to those observed in the samples from the wildfire. At 220 and 350°C the humification and metal complexation percentages of the organic matter increased, similar to the trend observed in the samples from the wildfire.