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

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 (Cmic) measured by substrate-induced respiration (SIR) and fumigation-extraction (FE) methods decreased following treatments. CC caused a 21% reduction of Cmic 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 Cmic than Ctr as measured by SIR and FE, respectively. Reasons for this decline in Cmic are proposed. Fungal biomass determined as humus ergosterol concentration fell even more steeply than total Cmic. 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 Cmic-FE, Cmic-SIR and ergosterol concentration. CC and CC-B lowered soil basal respiration, but not proportionally with the reduction in Cmic since the specific respiration rate (CO2-C evolved per unit Cmic) was clearly higher with CC-B than CC or Ctr. CC and CC-B both resulted in a higher concentration of NH4+ but only the humus from CC-B showed nitrification during a 6 week laboratory incubation.