Response of litter decomposition and soil C and N transformations in a Norway spruce thinning stand to removal of logging residue

Abstract

Increasing demand for production of bioenergy has led to an interest in forest management which uses logging residue from both clear-cuttings and thinning stands. The aim of this study was to investigate the effects of removal of logging residue in a thinning Norway spruce stand on (1) litter decomposition and (2) soil microbial processes in C and N cycling and the quality of soil organic matter. The study site was a 40-year-old Norway spruce stand growing on a relatively fertile site. During thinning, logging residue was either removed (whole-tree harvest) or left on the site (stem-only harvest). Different types of material in the logging residue, from main branches to needles, were weighed separately into mesh bags. The bags were placed above the moss layer in the whole-tree harvest treatment and in the logging residue layer in the stem-only harvest treatment, and decomposition was monitored for 5 years after treatment. From the humus layer, samples were taken 10 years after treatment. Harvest method affected the mass loss of the litter material very little but the C-to-N ratio of the remaining material was slightly higher in whole-tree harvest than in stem-only harvest, particularly in the needle material. In the humus layer samples, taken 10 years after treatment, the rate of C mineralization was lower in whole-tree harvest than in stem-only harvest; also the rate of net N mineralization and the amounts of C and N in the microbial biomass tended to be lower, although not statistically significantly. Removal of logging residue had no effect on pH ( p H H 2 O 3.9 in both treatments) or C-to-N ratio (28 in both treatments) in the humus layer. The concentrations of total water-soluble phenols and an important group of phenols, condensed tannins, were both lower in the humus layer of whole-tree harvest than in that of stem-only harvest. Concentrations of sesqui-, di- or triterpenes in the humus layer were similar in both treatments. In conclusion, 10 years after harvest, soil microbial activities and organic matter characteristics in whole-tree harvest differed from those in stem-only harvest.