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 unaffected 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 coefficient 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 CO(2)-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 coefficient of determination (R(2)) was higher for the double exponential model. The fire affected the kinetic parameters; the effect was ephemeral on the labile C pool, which increased its content (C(0)) 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 differences on their organic matter quality and, in a smaller proportion, to differences in organic matter concentration. The effect of fire on these factors, which was more pronounced in the soil with the initially higher C concentration, persisted during the 2 yr study.