An increasing number of studies demonstrate that tree species biodiversity can affect primary productivity and nutrient cycling in forests due to several factors, such as complementarity, facilitation or selection effects. For instance, resource partitioning in soils has been found to allow a more optimized nutrient uptake in mixed species plots compared with monocultures. However, how these effects will be modified by climate change – such as water availability – is not as well understood, especially in deep soil layers. Therefore, we specifically asked how water availability may influence the effect of tree mixtures on soil microorganism activity by measuring extracellular enzyme activities (EEAs) and available nutrients along the soil profile (down to 90 cm) in a 10-year-old plantation in southwestern France, which manipulates tree species composition (Pinus pinaster and Betula pendula, in monocultures and in mixed plots) and irrigation. Our results showed that EEAs directly depend on tree species composition and water conditions in interaction with soil depth; we found a positive effect of mixing birch and pine on carbon and nitrogen-related EEAs at an intermediate soil depth (15–30 cm soil layer), while the effect of increasing water availability increased phosphorus-related EEAs mostly in the upper soil layers (0–30 cm). However, we found no significant interactive effect between tree diversity and water availability on EEAs, underlying that the negative effect of lower water availability cannot be offset by the positive effect of mixing tree species. Differences in EEAs and available nutrients along the soil profile highlight the importance to look at different soil depths to better understand how nutrient cycling may be affected by increasing tree diversity and changes in water availability.

越来越多的研究表明，由于多种因素，例如互补性，促进作用或选择效应，树木生物多样性可以影响森林的初级生产力和养分循环。例如，发现土壤中的资源分配与单种养殖相比，可以在混合物种地块中更好地吸收养分。但是，人们对如何通过气候变化来改善这些影响（例如水的可利用性）的了解还不够广，尤其是在深层土壤中。因此，我们特别询问水的可利用性如何通过测量西南部一个已有10年历史的人工林中的细胞外酶活性（EEA）和沿土壤剖面（低至90 cm）的有效养分来影响树木混合物对土壤微生物活性的影响。法国，樟子松和桦，单一文化和混合地块）和灌溉。我们的结果表明，EEA与土壤深度的相互作用直接取决于树种的组成和水分条件。我们发现，在中等土壤深度（15–30 cm土层），桦木和松树混合对碳和氮相关的EEA具有积极作用，而增加水分利用的效果则主要在上层土壤中增加了磷相关的EEA（ 0–30厘米）。然而，我们发现树木多样性与水利用效率对EEA之间没有显着的交互作用，这说明较低的水利用效率的负面影响不能被混合树种的正面影响所抵消。