Microbial biomass and activity in the humus layer following burning: short-term effects of two different fires

During a 3-year study, soil microbial biomass C and N, length of the fungal hyphae, soil respiration, and the percent mass loss of needle litter were recorded in coniferous forest soil humus layers following a prescribed burning (PB) treatment or a forest fire simulation (FF) treatment (five plots per treatment). Unburned humus from adjacent plots served as controls (PC and FC, respectively). Prescribed burning was more intensive than the forest fire, and this was reflected in all the measurements taken. The amounts of microbial biomass C and N, length of fungal hyphae, and soil respiration in the PB area did not recover to their controls levels, whereas unchanged microbial biomass N and recovery of the length of the fungal hyphae to control levels were observed in the FF area. The mean microbial C/N ratio was approximately 7 in all the areas, which reflected the C/N ratio of the soil microbial community. Deviation from this mean value, as observed during the first three samplings from the PB area (3, 18, and 35 days after fire treatment), suggested a change in the composition of the microbial community. Of the two treated areas, the decrease in soil respiration (laboratory measurements) was much more pronounced in the PB area. However, when the humus samples from both areas were adjusted to 60% water holding capacity, no differences in respiration capacity were observed. The drier humus, due to higher soil temperatures, of the PB area is a likely explanation for the low soil respiration. Lower soil respiration was not reflected in lower litter decomposition rates of the PB area, since there was a significantly higher needle litter mass loss during the first year in the PB area followed by a decline to the control level during the second year. Consistently higher mass losses were recorded in the FC area than in the FF area.