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

<b>Abstract</b><br/><p>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 - Al<sub>ox</sub>) 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, Al<sub>ox</sub>. We characterized functional identity as the proportion of broadleaved species (beech and/or oak), and functional diversity as the product of broadleaved and conifer (pine) proportions. The results show that functional identity was the main driver of topsoil C storage while climatic water availability had the largest control on subsoil C storage. Contrary to expectations, functional diversity decreased topsoil C storage under increasing climatic water availability but the opposite was observed in the subsoil. Functional diversity effects on topsoil C increased with increasing clay+silt content, while its effects on subsoil C was negative at increasing Al<sub>ox</sub> content. This suggests that functional diversity effect on SOC storage along environmental gradients depends on the specific environmental factor and the soil depth under consideration.</p>

摘要
林分与环境因子的交互作用塑造森林生态系统的土壤有机碳（SOC）储量，但目前对不同土层中二者的相对影响尚不清楚。此外，环境因子如何调控林分因子（尤其是树种混交）对SOC储量的影响，在很大程度上仍未被探索。本研究在欧洲分布的21组林分三联体（两种树种混交林及其邻近的各自纯林）中开展试验，验证两个假说：其一，林分因子（功能特征组成与功能多样性）对表层土壤（FF+0~10 cm）碳储量的影响强于环境因子（气候水分有效性、黏粒+粉粒含量、草酸浸提态铝（Al_ox）），而亚表层土壤（10~40 cm）则呈现相反规律；其二，在高气候水分有效性、高黏粒+粉粒含量及高Al_ox条件下，功能多样性可提升SOC储量。本研究将功能特征组成定义为阔叶树种（山毛榉和/或橡树）的占比，将功能多样性定义为阔叶树与针叶树（松树）占比的乘积。研究结果表明，功能特征组成是表层土壤碳储量的主要驱动因子，而气候水分有效性对亚表层土壤碳储量的调控作用最强。与预期相反的是，随着气候水分有效性升高，功能多样性会降低表层土壤碳储量，但在亚表层土壤中则呈现相反趋势。功能多样性对表层土壤碳储量的影响随黏粒+粉粒含量升高而增强，而其对亚表层土壤碳的影响则随Al_ox含量升高呈负向变化。这表明，功能多样性对SOC储量沿环境梯度的效应，取决于具体的环境因子与所研究的土壤深度。