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

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.