Changes in soil organic C and N stocks after forest transformation from Norway spruce and Scots pine into Douglas fir, Douglas fir/spruce, or European beech stands at different sites in Southern Germany

At 18 sites in Bavaria (Germany) with substantial difference in elevation, parent material, climate, and stand age, which encompass most variance of site properties to be found in Central Europe, the effects of a replacement of Norway spruce (Picea abies L. Karst.; 16 sites) or Scots pine (Pinus sylvestris L.; two sites) by Douglas fir (Pseudotsuga menziesii, spp. menziesii) and European beech (Fagus sylvatica) stands on humus form as well as soil organic carbon (OC) and nitrogen (N) concentrations and stocks were investigated by single-site analysis and meta-analysis. At each study site, which comprised adjacent stands of the investigated tree species with similar age, soil samples were taken down to the solid bedrock or 80cm depth at six representative locations of each stand. Replacement of spruce or pine by Douglas fir or beech resulted in a considerable, in most cases significant decrease of the forest floor C/N ratio in the order spruce, pine>mixture of spruce or pine with Douglas fir>Douglas fir>beech. Additionally we noticed a significant decrease of forest floor OC (on average: -38% to -45%) and N stocks (on average: -24% to -29%) in the same order. After replacement of spruce or pine by Douglas fir or beech, we observed insignificant changes of mineral soil OC stocks, but significantly increased (on average: +5% to +8%) mineral soil N stocks. For the total soil, including the forest floor and the mineral soil down to 50cm depth, replacement of spruce with Douglas fir or beech resulted in significantly decreased soil OC stocks (average of all sites: -7% and -11%, respectively). Only two sites with original Scots pine stands located at poorer sites were included in the study: here, the changes of soil C and N stocks as induced by transformation into Douglas fir stands were in the same direction and order of magnitude as for the spruce stands. Replacement of pure spruce by 50%/50%-mixtures of spruce with Douglas fir resulted in statistically insignificant, slight increases of the soil OC stock (average +4%), and significantly increased soil N stocks (average: +7%). The magnitude of replacement effects was dependent on stand age and site properties, such as site N or moisture status. The circumstance that mixtures of Douglas fir with Norway spruce seem to be particularly effective in sequestrating C and N in the soil emphasizes the benefit of mixed stands not only with regard to ecosystem stability, but also with regard to mitigation of atmospheric CO ₂ enrichment, and minimization of N exports into groundwater aquifers. Our study demonstrates the potential of meta-analysis to (i) identify and quantify small, but significant changes of soil variables with considerable spatial variation, and (ii) reveal site factors which affect these changes.