Piling up Norway spruce logging residues alters soil organic matter in clear cuts

The aim of the study was to assess the impacts of piling up Norway spruce logging residues on the dynamics of forest soil organic matter composition in the years following final felling and to discuss possible consequences for soil C and N cycling. We focused on the content of dissolved organic carbon (DOC), phenolic compounds, and terpenes in the humus layer to determine whether they controlled changes in C- and N-cycling, as residue piles have previously been observed to be a hot spot for N-cycling and losses. We monitored the dynamics for four years in a logging residue experiment that included plots with residue piles (40 kg/m2) and control plots without residues (0 kg/m2). We also sampled two other field experiments six years after additions of different residue amounts (0, 10, 40 kg/m2). Overall, logging residues strongly affected humus layer properties, with the temporal variation depending on the compound group. Within months logging residues increased the concentrations of K2SO4-extractable DOC and condensed tannins, and both effects endured for years. There was a sharp increase in the concentration of volatile monoterpenes due to residues, but it lasted only for a year. The increase in di- and triterpenes occurred later, showing that diterpenes and triterpenes were enriched in time in the humus layer. The amount of microbial biomass C and rates of C and net N mineralization and nitrification were positively correlated with DOC concentration. There were also some strong and positive correlations of microbial biomass C and N and C mineralization with higher terpenes, especially with triterpenes. While, in the residue treatment net N mineralization and nitrification rates were negatively correlated with monoterpenes, indicating their important role in controlling N-cycling in situations with N excess as well. Logging residues caused similar changes in the two other field experiments, but the responses were not always proportional to the residue amounts added. In conclusion, piling up Norway spruce logging residues changes the dynamics of forest soil organic matter composition for years and creates hotspots for soil C- and N-cycling. The overall result of the uneven residue distribution is a strong spatial variation in soil, evident from the beginning to several years.