SOIL MICROBIAL AND EXTRACTABLE C AND N AFTER WILDFIRE

The effect of wildfire on soil microbes and extractable C (C-ext) and N (N-ext) changed with respect to the time from burning and soil depth . Initially, microbial biomass C (C-mic) and N (N-mic) were drasticall y reduced in the soil surface layer (0-5 cm) and reduced by 50% in the subsurface (5-10 cm), whereas C-ext increased by 62% in the surface l ayer and did not significantly change in the subsurface. These paramet ers were affected for the following 4 years, during which the average reductions in the soil surface and subsurface layers were, respectivel y, 60% and 50% for C-mic, 70% and 45% for N-mic, 60% and 40% for the r atio C-mic: organic C (C-org) and 70% and 30% for the ratio N-mic: tot al N (N-tot), while for C-ext the surface layer was the only zone cons istently affected and C-ext decreased by up to 59%. Immediately after a fire, the C-ext:C-org ratio increased by 3.5-fold and 2-fold in the surface and subsurface layers, respectively; thereafter for 2 years, i t decreased in the surface layer (by up to 45%) while the effect on th e subsurface layer was not consistent. The effect of burning on N-ext lasted 1 year, in which N-ext increased by up to 7- and 3-fold in the surface and subsurface layers, respectively, while the average N-ext:N -tot ratio doubled in the surface layer and increased by 34% in the su bsurface. During the time in which each parameter was affected by burn ing, the soil factor explained a high percentage of variance in the fl uctuations of C-mic, N-mic, C-mic: C-org and N-mic:N-tot, while those of N-ext and N-ext:N-tot, but not those of C-ext and C-ext:C-org depen ded on both the soil and its depth. In the burned soils similar patter ns of response were found between the following parameters listed in p airs: C-mic and N-mic; C-mic:C-org, and N-mic,:N-tot; C-ext and N-oxt; and C-ext:C-org and N-ext:N-tot. However, after the fire relationship s found previously between the parameters studied and many other soils properties were either no longer evident, or were inverted. Although the addition of cellulose to the burned soil favoured fungal mycelium development and increased C-mic and C-ext contents, the negative effec t of burning on the microbial biomass and the C-ext was not counteract ed even under incubation conditions suitable for both microbial growth and C mineralization.