Climatic water availability mainly drives context-dependency of tree functional diversity effects on soil organic carbon storage in European forests

The interplay of forest stand and environmental factors shape soil organic C (SOC) storage in forest ecosystems but little is known about their relative impacts in different soil layers. Moreover, how environmental factors modulate the impact of stand factors, particularly species mixing, on SOC storage, is largely unexplored.  In this study conducted in 21 forest triplets (two-species mixed stand and respective monocultures nearby) distributed in Europe, we tested the hypothesis that stand factors (functional identity and diversity) have stronger effects on topsoil (FF+0-10 cm) C storage than environmental factors (climatic water availability, clay+silt content, oxalate-extractable Al - Alox) but that the opposite occurs in the subsoil (10-40 cm). We also tested the hypothesis that functional diversity improves SOC storage under high climatic water availability, clay+silt contents, Alox. We characterized functional identity as the proportion of broadleaved species (beech and/or oak), a..., This study was conducted in 21 forest triplets across Europe. A triplet consisted of two-species mixed stand and their corresponding monocultures at the same site. The three forest stands in each triplet were of similar ages (based on tree cores and forest archives) and had homogenous soil conditions based on texture analyses on soil samples in the 10-20 cm depth. The triplets were of three types: five beech-oak (Fagus sylvatica L. - Quercus petraea (Matt.) Liebl.), eight pine-beech (Pinus sylvestris L. - Fagus sylvatica L.), and eight pine-oak (Pinus sylvestris L. - Quercus robur L. / Quercus petraea (Matt.) Liebl.). These tree species are widely distributed in Europe and are very important for forestry.
We placed ten (10) sampling points in each mixed stand and five (5) points each in the corresponding monocultures. At each sampling point, we sampled the forest floor (organic layer above the mineral soil) with 30 cm x 30 cm metal frame. Subsequently, we dug sampling pits in 10 cm inte..., It is saved in txt, which is accessible by almost all softwares.,