Litter share and clay content determine soil restoration effects of rich litter tree species in forests on acidified sandy soils

Many West-European forests are located on degraded and acidified soils. Soil acidification has resulted in hampered ecosystem functioning and lower delivery of ecosystem services. Forest management, particularly the choice of tree species, can accelerate or counteract soil acidification by the quality of litter input. The positive impact of so called ‘rich litter’ on the soil nutrient status and belowground ecosystem functioning has already been evidenced in common gardens. Here, we evaluate the effect of the rich litter species black cherry (Prunus serotina Ehrh.) in mixed forest stands dominated by pedunculate oak (Quercus robur L.). We study the effects using a replicated set-up of 10 established forest stands (age 30 to 90) in Belgium, the Netherlands and Germany along an edaphic gradient in sandy soils on Pleistocene aeolian deposits. We hypothesize that black cherry has a positive effect on the soil nutrient status and aim to answer the following research questions: (i) does admixture of black cherry increase soil pH and base saturation? (ii) what proportion of rich litter admixture is needed in a poor litter matrix to observe significant improvement of the soil nutrient status? and (iii) does the magnitude of the rich litter effect interact with initial soil properties? The results of this study indicate that admixture of black cherry enhances the forest floor turnover and enriches topsoil chemical conditions significantly. Thickness of the litter layer decreases from a mean of 7 cm under oak to a mean of 4.5 cm under cherry and correspondingly base saturation increases to a maximum of 25%, NO3− concentration to 26 mg/mg and organic matter content to 8%. However, large shares of rich litter admixture (>30% basal area) are needed to improve topsoil conditions. Moreover, we find that rich litter effects are more pronounced on sandy soils with higher fine particle (loam + clay) content. This suggests that the actual impact of restoration efforts in acidified forest soils is a product of the trinity “litter quality – litter share – site quality”