Soil organic matter changes in a spruce ecosystem 25 years after disturbance

Long-term effects of forest dieback on soil organic matter (SOM) pools in an unmanaged high-elevation Norway spruce [Picea abies (L.) H. Karst] forest were investigated by comparing three dieback sites with three adjacent sites with healthy spruce. The aim was to assess the long-term biogeochemical impact of a major disturbance in unmanaged forest ecosystems due to potential modification of their soil organic carbon (OC) pool. The spruces at the dieback sites had been killed by a bark beetle (Ips typographus) infestation 25 yr ago. Total OC stocks in the topsoil of all sites were determined; different OC pools were quantified by density/particle-saze fractionation and ¹³C cross polarization magic angle spinning nuclear magnetic resonance (CPMAS NMR) spectroscopy. Additionally, the abovegronnd and belowground plant-derived OC input was quantified and the OC species were analyzed. The forest floor at the dieback sites showed significantly lower OC stocks, storing <70% of the respective amount at the sites with healthy forest In the forest floor and the light fraction of the Ah horizons at the dieback sites, the C/N ratio and the concentration of O-alkyl C in the OM were significantly lower, whereas the OM of the other fractions and of the bulk SOM in Ah horizons showed no changes. The observed SOM changes were caused primarily by an accelerated SOM turnover and only secondarily caused by changed litter input Particularly the OC pool with a low degree of humification was reduced in its amount and increased in its decomposition status while the more stable C fractions showed no significant changes.