Multiscale heterogeneity of topsoil properties in southern European old-growth forests

Assessing the contribution of forest soils to carbon (C) and nutrient dynamics is extremely complex due to the high spatial variability of soil properties, at fine to very broad scales. Improving our understanding of soil variability is necessary to scale up sample-based inventory data for the estimation of regional contribution of forest soils to C dynamics. We analyzed the topsoil spatial variability of organic carbon (OC) and matter (OM), nitrogen (N), C/N ratio, texture, and pH in 11 southern European old-growth beech stands. Our aim was to assess the within- and among-stand topsoil variability, and to determine the drivers underlying this variation. For each stand, we sampled the topsoil in 25 quadrats in a 1-ha regular grid where we recorded overstorey structural and compositional attributes, deadwood quantity and quality, microtopography and site conditions. Soil parameters varied highly at both scales: When considering all the topsoil properties together through multivariate analysis, most of the variability occurred at the within-stand scale (mostly due to shifts in organic matter content, C/N, and litter depth); the opposite was true when considering soil properties singularly, with pH, soil texture, and N concentration varying greatly among stands. Most of the among-stand variability depended on climate and on the direct and indirect effects of parent material. Fine-scale variation, instead, depended mainly on overstorey composition and microtopographical variation. Surprisingly, we found no direct influence of overstorey structure on topsoil parameters, likely because soil parameters respond to changes in stand structural features only after a substantial time lag.