Can shrub species with higher litter quality mitigate soil acidification in pine and oak forests on poor sandy soils?

This study, aiming to unravel whether topsoil conditions under tree species with nutrient-poor leaf litter can be altered by admixing a shrub layer, was performed in 12 pine (mainly Pinus sylvestris) and 12 oak (Quercus robur) stands on sandy podzolic soils in north-east Belgium. We examined the effects of presence of a shrub layer on forest floor mass and topsoil chemical properties related to soil acidification. The shrub species included were European rowan (Sorbus aucuparia), alder buckthorn (Rhamnus frangula) and black cherry (Prunus serotina). For each tree species, 60-90 years old stands were selected containing shrubs present in varying cover classes: sparse (<20%), intermediate (20-70%) and dense (>70%). The oak stands were characterized by less but 'nutrient-richer' litterfall, compared to the pine stands. This was reflected in less humus build-up (FH-horizon) and higher pH, CEC, BS and lower C/N in the topsoil in the oak stands compared to the pine stands. However, despite the fact that the shrubs produced litter with significantly higher base cation and N concentrations than that of the studied tree species, we did not find any significant changes in topsoil conditions in the pine and oak stands under study, even under dense shrub layers (87-91% cover).