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 drastically 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 layer and did not significantly change in the subsurface. These parameters were affected for the following 4 years, during which the average reductions in the soil surface and subsurface layers were, respectively, 60% and 50% for C-mic, 70% and 45% for N-mic, 60% and 40% for the ratio C-mic: organic C (C-org) and 70% and 30% for the ratio N-mic: total N (N-tot), while for C-ext the surface layer was the only zone consistently 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, it decreased in the surface layer (by up to 45%) while the effect on the 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 subsurface. During the time in which each parameter was affected by burning, the soil factor explained a high percentage of variance in the fluctuations 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 depended on both the soil and its depth. In the burned soils similar patterns of response were found between the following parameters listed in pairs: 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 relationships 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 effect of burning on the microbial biomass and the C-ext was not counteracted even under incubation conditions suitable for both microbial growth and C mineralization.