Irene Fernandez

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 CO2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.

Carbon mineralization dynamics in soils after wildfires in two Galician forests

The carbon mineralization dynamics of two Humic Cambisols, developed over granite, one under Pinus sylvestris L. (1740 m a.s.l.) and the other under Pinus pinaster Aiton (140 m a.s.l.), were determined in samples of 0‐5 and 5‐10 cm depth collected after high intensity wildfires. Burnt and unburnt soils were sampled five times over 2 yr after the wildfires to determine changes in C concentration and in potential mineralization activity of the soil organic matter. Soil samples from the same forests unaAected by the fires were used as controls. In both soils the fire resulted in a substantial decrease in the soil carbon concentration. Immediately after the fire, the C mineralization was decreased in the surface layer; however, the percentage of total C mineralized increased in both layers. The evolution of these variables over time depended on the soil and on the layer considered. During the first months after the burning the C mineralization presented values lower than those of the control in both layers of the soil located at higher altitude (M) and in the surface layer of the other soil (R), but values higher than those of the control in the subsurface layer of soil R. For the same period, the C mineralization coeAcient in the surface layer was similar to (M) or lower than (R) that of the corresponding control, whereas in the subsurface layer it was maintained above that of the control in both soils. Two years after the fire, the total C concentration had been recovered in the surface layer of both soils whereas in the subsurface layers its value was still 15‐19% lower than that of the same layer in the corresponding control. At the same time, the C mineralization and the percentage of the total C mineralized in the surface layer of the burnt soils were lower than those in the corresponding unburnt soils. In the subsurface layer, soil M exhibited values of these two mineralization indices higher than those of the control, whereas soil R presented values lower than those of the control from 1 yr after the fire. The cumulative CO2-C evolved by the samples during each incubation fits two kinetic models: a simple and a double exponential first order equation. In most cases the coeAcient of determination (R 2 ) was higher for the double exponential model. The fire aAected the kinetic parameters; the eAect was ephemeral on the labile C pool, which increased its content (C0) and its mineralization rate (k), but more persistent on the recalcitrant fraction, which shows a long-term decrease of its instantaneous mineralization rate (h). According to principal components analysis, the variability of the samples studied is mainly due to diAerences on their organic matter quality and, in a smaller proportion, to diAerences in organic matter concentration. The eAect of fire on these factors, which was more pronounced in the soil with the initially higher C concentration, persisted during the 2 yr study. # 1999 Elsevier Science Ltd. All rights reserved.

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

Abstract 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 Na 4 P 2 O 7 and NaOH and the percentage of humification of the remaining soil organic matter.