Microbial biomass estimated by phospholipid fatty acids (PLFA pattern) in a soil with different post-fire treatments (seeding, mulching) one year after the experimental fire

The soil microbial community in a field experiment with different post-fire treatments (seeding, mulching) was characterized by means of phospholipids fatty acids analyses (PLFA). The soil was a Leptosol developed over granite with a slope of 38-54%, located in the N.W. (Spain), within the Atlantic humid temperate zone. The total biomass and the biomass of specific microbial groups (fungi, bacteria, actinomycetes, gram-positive bacteria, gram-negative-bacteria) was assessed in soil samples taken from the A horizon (0-5 cm depth) at different sampling times over one year. The results showed that the total microbial biomass and the biomass of specific microbial groups in burnt soils were slightly lower than in the corresponding unburnt control. The fire decreased significantly the fungal:bacterial and gram-negative:gram-positive ratios. Differences in physiological state of the microbial communities were observed as consequence of the medium-term prescribed fire impact; the higher values in the ratios of saturated to unsaturated fatty acids and cyclopropyl fatty acids to monoicoc precursors exhibited by the burnt soils as compared with the unburnt soil suggested that the microbial communities were stressed by the experimental fire. The data also showed the absence of any medium-term response of microbial community to the seeding and mulching treatments.