Dynamics of soil nitrogen cycling and losses under Norway spruce logging residues on a clear-cut

This study aimed to determine the effects of Norway spruce logging residues on the early-stage dynamics of N cycling processes on a clear cut and how this is related to N losses via leaching or nitrous oxide emissions. We compared N cycling processes in soil under spruce logging residues (fresh residues 40 kg m−2) and in corresponding soil without the residues for four growing seasons on a clear-cut in southern Finland. In addition, two other sites were studied in the same region six years after clear-cutting in plots with three different amounts of spruce logging residues (0, 10, 40 kg m−2). Logging residues strongly stimulated net nitrification within the first three months after the residue treatment, and this stimulation continued over the whole study period, although NO3-N concentrations in the soil decreased. Residues increased the pH, rate of net N mineralization and microbial C-to-N ratio. N2O production in the laboratory and fluxes in the field were both studied after four growing seasons. N2O production was higher in the humus layer under the residues and originated from both autotrophic nitrification and denitrification. N2O fluxes in the field were low but were also higher in the residue-containing plots. N concentrations in percolate water were monitored for four years at the same site. Logging residues increased NO3-N and NH4-N concentrations; the highest concentrations were found during the second and third years after clear-cutting. After that the concentrations of dissolved organic N increased. Both the percolate water study at this site and the N cycling studies at the two other sites six years after clear-cutting showed that the largest amount of residues resulted in the highest rates of net N mineralization and net nitrification and the highest concentrations of mineral N forms in the percolate water. The amount and decomposition stage of the logging residues as well as the development of ground vegetation may together determine the dynamics of soil N cycling processes and losses.