CARBON AND NITROGEN IN CONIFEROUS FOREST SOILS AFTER CLEAR-FELLING AND HARVESTS OF DIFFERENT INTENSITY

Soil carbon and nitrogen contents were determined at four coniferous f orest sites in Sweden (Scots pine and Norway spruce in southern and no rthern 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 de tritus (i,e. dead branches and roots). When measured, 15-16 years foll owing clear-felling, the content of C and N in the humus layer had dec reased markedly at all sites, whereas increased amounts were frequentl y 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 si te. The corresponding figures for N were 22% and 13%, No changes in th e total pool were detected at the southern site with Scots pine, where as 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 ra tio 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 t his element, At the northern spruce site, whole-tree harvesting as wel l as removal of woody residues only (i.e, needles left on site) result ed in a greater reduction of N pools in the humus compared with conven tional harvest. In contrast, at the southern spruce site conventional harvesting resulted in a greater reduction in total N pools compared w ith the other treatments, Whole-tree harvesting increased C/N ratios i n the humus and 0-5 cm mineral soil layers compared with the other tre atments. This effect was detected in the humus layer only 8 years afte r 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.