Charcoal as a habitat for microbes and its effect on the microbial community of the underlying humus

Wildfires produce a charcoal layer, which has an adsorbing capacity resembl ing activated carbon. After the fire a new litter layer starts to accumulat e on top of the charcoal layer, which liberates water-soluble compounds tha t percolate through the charcoal and the unburned humus layer. We first hyp othesized that since charcoal has the capacity to adsorb organic compounds it may form a new habitat for microbes, which decompose the adsorbed compou nds. Secondly, we hypothesized that the charcoal may cause depletion of dec omposable organic carbon in the underlying humus and thus reduce the microb ial biomass. To test our hypotheses we prepared microcosms, where we placed non-heated humus and on top one of the adsorbents: non-adsorptive pumice ( Pum), charcoal from Empetrum nigrum (EmpCh), charcoal from humus (HuCh) or activated carbon (ActC). We watered them with birch leaf litter extract. Th e adsorbing capacity increased in the order Pum < HuCh < EmpCh < ActC, the adsorbents bring capable of removing 0%, 26%, 42% and 51% of the dissolved C-org in the litter extract, respectively. After one month, all adsorbents harboured microbes, but their amount and basal respiration was largest in E mpCh and HuCh, and smallest in Pum. In addition, different kinds of microbi al communities with respect to their phospholipid fatty acid and substrate utilization patterns were formed in the adsorbents. The amount of microbial biomass and number of bacteria did not differ between humus under differen t adsorbents, although different microbial communities developed in humus u nder EmpCh compared with Pum, which is obviously related to the increased p H of the humus under EmpCh, and also ActC. We suggest that charcoal from bu rning can support microbial communities, which are small in size bur have a higher specific growth rate than those of the humus. Although the charcoal layer induces changes in the microbial community of the humus, it does not reduce the amount of humus microbes.