Is the effect of trees on soil properties mediated by soil fauna? A case study from post-mining sites

The effects of tree species differing in foliage and litter chemistry on the chemical, micro-morphological, and biological properties of soil were studied on post-mining sites afforested with one of six tree species (spruce, pine, larch, oak, lime, and alder) and also on sites left to natural succession (dominated by willow). The sites were located on a large colliery spoil heap that had been produced by the mining of coal in alkaline tertiary clays near the city of Sokolov, Czech Republic. Because no topsoil had been applied to the sites, soil development resulted from in situ interactions among the deposited overburden (spoil), trees, and soil biota. Soil formation differed markedly among sites afforested with different tree species. On sites with trees producing litter with a low C/N ratio (the deciduous species), the organic Oe layer was narrow or absent and a thick organomineral A layer was evident. On sites with trees producing litter with a high C/N ratio (the evergreen species), in contrast, a thick Oe layer and a thin A layer were evident. Besides C/N ratio, earthworm abundance and earthworm bioturbation activity (measured as the amount of earthworm casts in the topsoil) were the strongest predictors of A layer thickness and C accumulation in the mineral topsoil. Sites with higher C accumulation in mineral soil had higher microbial biomass and lower microbial respiration, which may have contributed to the higher C storage. The gradient of bioturbation was correlated with changes in the composition of the bacterial community and other soil biota, but partial correlation showed that the effects of litter quality and bioturbation were largely independent. Overall, the results indicate that the effect of tree species on soil development is substantially mediated by soil fauna activity and especially by earthworm bioturbation.

► Trees producing litter with low C/N ratio support higher fauna driven bioturbation.
► When bioturbation is small, litter remain on surface, soil respiration is high, and C storage low.
► Large bioturbation mix litter into soil A horizon, respiration is low, and C storage high.