Soil organic carbon and nitrogen stocks under pure and mixed stands of European beech, Douglas fir and Norway spruce

Numerous studies have addressed tree species effects on forest soil carbon (C) and nitrogen (N); however, knowledge of how and to what extent specific tree species and species mixtures impact forest soil C and N stocks is scarce and inconsistent across soil types. Therefore, we studied three forest sites in Southern Germany differing in parent material, soil properties as well as nutrient and water supply. Each site comprises adjacent groups of pure mature European beech (Fagus sylvatica), Douglas fir (Pseudotsuga menziesii) and Norway spruce (Picea abies) as well as single-tree mixtures of beech with Douglas fir or Norway spruce. To account for tree-species-specific spatial heterogeneity, we sampled the forest floor and mineral soil to a depth of 60 cm at different distances from the trees. Significant tree species and species mixing effects on soil organic carbon (OC) and N concentrations, C/N ratios and soil OC and N stocks were mainly found in the forest floor and in the uppermost (0-15 cm) mineral soil. Forest floor OC and N stocks and total soil OC stocks were higher under Douglas fir and Norway spruce compared with beech. While tree species effects on soil OC and N were present across sites, the influence of soil type induced variations in their magnitude. The forest floor C/N ratio under Douglas fir was low and comparable with beech in soils developed from nutrient-rich parent material, whereas it was higher and similar to spruce in the soil formed from sandstone. Tree species-specific differences in foliar nutrient concentrations between beech and conifer stands might influence litter decomposition rates among the species and thus modify soil OC and N stocks. Forest floor OC stocks were significantly higher in mixed beech conifer stands compared with pure beech, and most often smaller than or similar to pure conifer stands. Forest floor N stocks showed the same tendency, but differences were inconsistent and not always significant across sites. Admixture of beech with Douglas fir or Norway spruce reduced the share of OC and N stored in the forest floor compared with the pure conifer stands and significantly increased mineral topsoil (0-15 cm) OC stocks compared with pure beech stands. Hence, the vertical distribution of OC and N in the soil profile varied depending on the tree species composition. Total soil (forest floor + mineral soil) OC and N stocks of mixtures were similar to pure beech, pure conifers or intermediate depending on site and soil type. (C) 2016 Elsevier B.V. All rights reserved.