Carbon and nitrogen in coniferous forest soils after clear-felling and harvests of different intensity

Abstract

Soil carbon and nitrogen contents were determined at four coniferous forest sites in Sweden (Scots pine and Norway spruce in southern and northern Sweden, respectively) over a period of 15–16 years after clear-felling. Three levels of logging-residue harvesting had been applied: (i) conventional stem harvest (residues left on site); (ii) harvesting all above-ground tree parts except needles; (iii) above-ground whole-tree harvesting (no residues remaining). The forest floor and the top mineral soil layers (0–20 cm) were examined, excluding coarse woody detritus (i.e. dead branches and roots). When measured, 15–16 years following clear-felling, the content of C and N in the humus layer had decreased markedly at all sites, whereas increased amounts were frequently observed in the mineral soil. Total C pools decreased by 22% at the northern site with Norway spruce, and by 17% at the southern spruce site. The corresponding figures for N were 22% and 13%. No changes in the total pool were detected at the southern site with Scots pine, whereas at the northern pine site C decreased by 7% and N increased by 7%. Changes in C and N storage were accompanied by decreases in the C N ratio in the humus layer and increases in the C N ratio in the top 5 cm of the mineral soil. No general effect of harvest intensity on soil C or N pools was shown in analyses over all sites, but there was a site and treatment interaction for N, indicating site-specific effects on this element. At the northern spruce site, whole-tree harvesting as well as removal of woody residues only (i.e. needles left on site) resulted in a greater reduction of N pools in the humus compared with conventional harvest. In contrast, at the southern spruce site conventional harvesting resulted in a greater reduction in total N pools compared with the other treatments. Whole-tree harvesting increased C N ratios in the humus and 0–5 cm mineral soil layers compared with the other treatments. This effect was detected in the humus layer only 8 years after harvest at the southern sites, whereas at the northern sites it was observed in the mineral soil 16 years after felling. The implications of these results on N mineralization rates are discussed.