Clear-cutting and prescribed burning in coniferous forest: Comparison of effects on soil fungal and total microbial biomass, respiration activity and nitrification

Abstract The effects of clear-cutting (CC) and clear-cutting followed by prescribed burning (CC-B) on humus chemical and microbiological variables and quality were compared in a Norway spruce dominated stand in North-Eastern Finland. The pattern of chemical changes in humus was similar after both treatments but CC-B caused greater changes than CC. Treatments raised the pH, cation exchange capacity and base saturation compared to an untreated standing forest control (Ctr). Total microbial carbon (C mic ) measured by substrate-induced respiration (SIR) and fumigation-extraction (FE) methods decreased following treatments. CC caused a 21% reduction of C mic compared to Ctr (10,890 μg g −1 dry wt), as measured by SIR, and a 27% reduction compared to Ctr (7281 μg g −1 dry wt) as measured by FE. CC-B resulted in 53 and 67% lower C mic than Ctr as measured by SIR and FE, respectively. Reasons for this decline in C mic are proposed. Fungal biomass determined as humus ergosterol concentration fell even more steeply than total C mic . Humus quality was analysed by near infrared reflectance spectroscopy (NIR) which revealed differences in humus structure between treatments. The NIR data could be interpreted to explain 75–82% of the variation in C mic -FE, C mic -SIR and ergosterol concentration. CC and CC-B lowered soil basal respiration, but not proportionally with the reduction in C mic since the specific respiration rate (CO 2 -C evolved per unit C mic ) was clearly higher with CC-B than CC or Ctr. CC and CC-B both resulted in a higher concentration of NH 4 + but only the humus from CC-B showed nitrification during a 6 week laboratory incubation.