BACKGROUND AND AIMS Growing evidence suggests that the net effect of soil microbes on plants depends on both abiotic and biotic conditions, but the context-dependency of soil feedback effects remains poorly understood. Here we test for interactions between the presence of conspecific soil microbes, plant competition and light availability on tree seedling performance. METHODS Seedlings of two congeneric tropical tree species, Bauhinia brachycarpa and Bauhinia variegata, were grown in either sterilized soil or soil conditioned by conspecific soil microorganisms in a two-phase, greenhouse feedback experiment. We examined the interactive effects of soil treatment (live, sterilized), light availability (low, high) and plant competition (no competition, intraspecific and interspecific competition) on tree seedling biomass. We also investigated the linkages between the outcomes of soil feedback effects and soil microbial community structure. KEY RESULTS The outcomes of soil feedback effects on seedling biomass varied depending on both competition treatment and light availability. Under low light conditions, soil feedback effects were neutral irrespective of competition treatment and plant species. Soil feedback effects were negative in high light for seedlings with interspecific competition, but positive for seedlings growing alone or with intraspecific competition. Soil feedback effects for seedlings were driven by variation in the Gram positive-Gram negative bacteria ratio. Light and conspecific soil microbes had interactive effects on the competitive environment experienced by tree species; in low light, the presence of conspecific soil microbes decreased plant competition intensity, whereas in high light, both the intensity and the importance of competition increased for seedlings in the presence of soil microbes, irrespective of plant species. CONCLUSIONS Our findings underline the importance of light and plant competition for the outcomes of soil feedback effects on young tree seedlings, and suggest that reduced light availability may reduce the influence of conspecific soil microbes on plant-plant interactions.

中文翻译：

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土壤微生物在植物竞争和对比光照条件下改变幼苗性能和生物相互作用

背景和目标 越来越多的证据表明，土壤微生物对植物的净效应取决于非生物和生物条件，但土壤反馈效应的环境依赖性仍然知之甚少。在这里，我们测试了同种土壤微生物的存在、植物竞争和光照对树苗性能之间的相互作用。方法 在两阶段温室反馈实验中，两种同属热带树种 Bauhinia brachycarpa 和 Bauhinia variegata 的幼苗在无菌土壤或同种土壤微生物条件下生长。我们研究了土壤处理（活的、消毒的）、光照（低、高）和植物竞争（无竞争、种内和种间竞争）对树苗生物量的交互影响。我们还研究了土壤反馈效应的结果与土壤微生物群落结构之间的联系。主要结果 土壤反馈对幼苗生物量的影响结果因竞争处理和光照可用性而异。在弱光条件下，无论竞争处理和植物种类如何，土壤反馈效应都是中性的。高光条件下，种间竞争幼苗的土壤反馈效应为负，而单独生长或种内竞争幼苗的土壤反馈效应为正。对幼苗的土壤反馈效应是由革兰氏阳性菌与革兰氏阴性菌比率的变化驱动的。光和同种土壤微生物对树种所经历的竞争环境具有交互作用；在光线不足的情况下，同种土壤微生物的存在降低了植物的竞争强度，而在高光下，无论植物种类如何，在土壤微生物存在的情况下，幼苗的竞争强度和重要性都会增加。结论我们的研究结果强调了光和植物竞争对幼树幼苗的土壤反馈效应结果的重要性，并表明减少光照可能会减少同种土壤微生物对植物-植物相互作用的影响。