Tree neighbourhood-scale variation in topsoil chemistry depends on species identity effects related to litter quality

Trees have a large impact on the topsoil chemistry and the strength and direction of this impact depend on the identity of the tree species. Tree species with nutrient-poor litter have the potential to degrade soil fertility, which is characterized by high acidity, low availability of essential nutrients for plant growth, and high levels of available Al and Fe. In contrast, species with more easily decomposing, nutrient-rich litter can ameliorate the soil quality. In this study, we are investigating the effects of tree species identity on topsoil chemistry on a small spatial scale. Our study site, situated in Białowieża Forest in Poland, replicates mature monocultures and two-species mixtures, using a pool of four tree species. Soil was sampled at a metre distance to the base of a tree and in the centre along a transect between two trees. We found that the total C concentration, plant available P and base cation concentration, and C/N ratio were all larger close to the tree. The pH was unaffected by the distance. We identified clear species identity effects on this distance effect. In particular, we found that the pH and base cations were affected more negatively by the proximity to a tree nearby nutrient-poor trees in comparison with nutrient-rich trees. However, no non-additive diversity effects in mixtures could be distinguished. Our results highlight the ameliorating effects of admixing nutrient-rich species with nutrient-poor species and the importance of tree species choice in regard to the topsoil chemical composition on a small within-stand scale.