Carbon mineralization dynamics in soils after wildfires in two Galician forests

The carbon mineralization dynamics of two Humic Cambisols, developed over g ranite, one under Pinus sylvestris L. (1740 m a.s.l.) and the other under P inus 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 s oils were sampled five times over 2 yr after the wildfires to determine cha nges in C concentration and in potential mineralization activity of the soi l organic matter. Soil samples from the same forests unaffected by the fire s were used as controls. In both soils the fire resulted in a substantial d ecrease 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 va riables over time depended on the soil and on the layer considered. During the first months after the burning the C mineralization presented values lo wer 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 hig her than those of the control in the subsurface layer of soil R. For the sa me period, the C mineralization coefficient in the surface layer was simila r to (M) or lower than (R) that of the corresponding control, whereas in th e subsurface layer it was maintained above that of the control in both soil s. Two years after the fire, the total C concentration had been recovered i n the surface layer of both soils whereas in the subsurface layers its valu e was still 15-19% lower than that of the same layer in the corresponding c ontrol. At the same time, the C mineralization and the percentage of the to tal C mineralized in the surface layer of the burnt soils were lower than t hose in the corresponding unburnt soils. In the subsurface layer, soil M ex hibited values of these two mineralization indices higher than those of the control, whereas soil R presented values lower than those of the control f rom 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 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 (Co) and its mineralization rate (k), but more persistent on t he recalcitrant fraction, which shows a long-term decrease of its instantan eous mineralization rate (h). According to principal components analysis, t he variability of the samples studied is mainly due to differences on their organic matter quality and, in a smaller proportion, to differences in org anic matter concentration. The effect of fire on these factors, which was m ore pronounced in the soil with the initially higher C concentration, persi sted during the 2 yr study. (C) 1999 Elsevier Science Ltd. All rights reser ved.