Logging residue piles of Norway spruce, Scots pine and silver birch in a clear-cut: Effects on nitrous oxide emissions and soil percolate water nitrogen

We analysed how logging residue (LR) piles of common tree species in Finland, Norway spruce (Picea abies (L.) H. Karst.), Scots pine (Pinus sylvestris L.) and silver birch (Betula pendula Roth), affect nitrogen (N) losses in forest soil after final felling. A Norway spruce dominated stand was clear-cut and followed by two experimental setups to study the nitrous oxide (N₂O) emissions and leaching of carbon (C) and N. Experiments consisted of four treatments: tree species treatments consisting of 40 kg m^-2 of LR and a control treatment without residues. The C losses were monitored as dissolved organic carbon (DOC), the N losses as ammonium (NH₄-N), nitrate (NO₃-N) and dissolved organic nitrogen (DON) fluxes and concentrations in soil percolation waters and the N₂O emissions as fluxes from the forest soil to the atmosphere. In addition the soil temperatures, the molecular size distribution of the DOC from the soil percolation waters and the origin of the N₂O production were determined. The LR piles lowered the soil temperatures and, especially those of birch, increased the concentrations of NO₃-N in the soil percolation waters already 1 year after the establishment of the piles. The LR piles increased the NH₄-N concentrations. The smallest molecular size fraction (<1 kD) of DOC predominated in all treatments. The N₂O fluxes peaked under the piles during the second and third growing seasons; however, the inconsistent fluxes tended to be low. The production of N₂O was driven by both nitrification and denitrification processes, the proportion depending on the tree species. Our results indicate that LR piles accelerate N losses 1 year after the clear-cutting, especially NO₃-N, which predominates in the soil percolation waters under the birch residues, whereas spruce residues tend to stimulate N₂O emissions longer. These results have implications for sustainable forest management practices and nutrition of regrowing vegetation.

Keywords: Logging residue piles; N losses; N(2)O; Nitrogen; Soil percolation water; Tree species.