The objective of this study was to examine the interactive influence of tree diversity and water availability on microbial functioning in surface soil (0–10 cm). The study was conducted at two field experiments (ages 4.2 and 9.0 years, respectively) of the TreeDivNet platform that established plantations with a gradient of tree species richness exposed to high and low water availability treatments. Soil microbial biomass, activity, functional diversity and functional identity were estimated using the MicroResp™ method. Soil organic carbon and several other variables potentially associated with soil processes (soil moisture, soil and air temperature, soil pH, soil C:N ratio, tree biomass) were also measured. Our results indicate that tree species richness significantly increased soil microbial biomass and, to a lesser extent, soil microbial activity in the high water availability treatment, at both trial sites. However, tree species richness effects on microbial processes were not expressed in the low water availability treatment, contrary to the stress-gradient hypothesis. Tree species richness and water treatment interaction impacted microbial biomass and activity through their effects on microbial functional diversity and identity at one site and air temperature at the other site. In conclusion, the mechanisms through which tree diversity creates bottom-up effects on biological soil processes depend on environmental conditions and time following the establishment of tree communities. Future research relating diversity and water availability effects on soil microbial processes should consider functional traits of tree species and a larger range of trophic levels including herbivores and soil fauna to better understand biodiversity-soil ecosystem function relationships in forest ecosystems.

这项研究的目的是研究树木多样性和水分供应对表层土壤（0–10 cm）微生物功能的交互影响。这项研究是在TreeDivNet平台的两个野外试验（分别为4.2和9.0岁）上进行的，该试验建立了种植园，这些种植园的树木种类丰富程度受到高和低水分利用处理的影响。使用MicroResp™方法估算了土壤微生物的生物量，活性，功能多样性和功能特性。还测量了土壤有机碳和其他可能与土壤过程相关的变量（土壤湿度，土壤和空气温度，土壤pH，土壤碳氮比，树木生物量）。我们的结果表明，树木物种丰富度显着提高了土壤微生物的生物量，在较小程度上，在两个试验地点均进行了高水分利用处理中的土壤微生物活动。然而，与水分梯度梯度假说相反，低水分利用度处理并未显示树木物种丰富度对微生物过程的影响。树种的丰富性和水处理相互作用通过影响一个地点的微生物功能多样性和同一性以及另一地点的气温，影响了微生物的生物量和活性。总之，树木多样性对生物土壤过程产生自下而上影响的机制取决于建立树木群落后的环境条件和时间。