Experimental assessment of ecological restoration options for compacted forest soils

It is ecologically undesirable to solve forest soil compaction due to mechanized harvesting at large spatial scales using agricultural mechanical soil loosening techniques. We therefore examined whether a stimulation of biological activity through litter manipulation, liming and/or inoculation of the anecic earthworm species Lumbricus terrestris could significantly contribute to the ecological restoration of compacted forest soils by comparing the impact of these treatments on the soil within and beside compacted wheel tracks. The replacement of native litter by litter with a better quality resulted in a faster litter decomposition. However, maximal decay rates were obtained only when litter manipulation, earthworm inoculation and liming were combined. Anecic earthworms were initially absent as soils were probably too acid. Liming as well as litter manipulation had a small positive influence on the numbers of retraced L. terrestris, inducing positive feedback mechanisms on soil pH and litter decomposition rates. None of the treatments, however, had a significant effect on the compaction degrees within or beside tracks within the small study period. L. terrestris realized only a small decrease of bulk density beside the tracks. Within the tracks a similar number of L. terrestris was retrieved, but effects on the compaction degree were negligible. Liming decreased penetration resistance, but only in the absence of anecic earthworms. Endogeic earthworms were overall more abundant than the anecics, especially within tracks where soil water contents and pH values were higher. However, endogeics only had a marginal effect on litter decomposition and although they positively influence soil structure, they could not realize a reduction of the compaction degree, quantified by bulk density and penetration resistance, due to different burrowing habits. Our results indicated that a positive impact of anecic earthworms on the structure of compacted forest soils can be obtained in the long-term, at least in case soil conditions (acidity, nutrient availability and moisture content) are favourable. This can be achieved by conversion of forests towards tree species with high quality litter.