Soil organic matter mobilization by re-compaction of old forest skid trails

Abstract

Wood harvesting is restricted to a system of permanent skid trails in order to minimize the soil disturbance and damage. Therefore, it is not the compaction of previously undisturbed forest soil, but the re-compaction of already existing skid trails that is of practical relevance when investigating machinery-induced wood harvest effects on soil properties. This study investigated the effects of machinery-induced re-compaction on soil physical, chemical, and microbiological properties of an old skid trail in a spruce forest by wheeling 8 times with a maximum total load of 32.2 Mg, using a genuine old skid trail as a control. Re-compaction significantly increased the bulk density and reduced the porosity, whereas the soil organic carbon (SOC) and total nitrogen (N) contents were not significantly affected. However, re-compaction reduced the SOC/total N ratio by 10%, suggesting considerable SOC mineralization after re-compaction. K2SO4 extractable C contents were increased by 94% at 0–3 cm and 67% at 7–10 cm depth after re-compaction. This led to 20% and 90% increased microbial biomass C/soil organic C (MBC/SOC) ratios at 0–3 cm and at 7–10 cm depth, respectively. In contrast, the ergosterol/MBC ratio was significantly decreased by 10% at 0–3 cm and by about 30% at 7–10 cm depth by re-compaction, apparently due to the promotion of bacteria and mobilization of soil organic matter.