Interactions among plant species and their soil biota drive plant–soil feedbacks (PSFs) that play a major role in the dynamics and diversity of plant communities. Among the different components of the soil community, pathogens are considered to be the main drivers of negative PSFs. Despite this, the number of studies that have experimentally quantified the contribution of soil pathogens to PSFs remains considerably low. Here we conducted a greenhouse experiment with oomycete-specific fungicide to quantify the contribution of soil pathogens, and particularly oomycete pathogens, to individual and pairwise PSFs in forest communities. We used as a case study Mediterranean mixed forests dominated by Quercus suber and invaded by the oomycete pathogen Phytophthora cinnamomi. The fungicide treatment was crossed with a competition treatment to explore how conspecific neighbors might modify pathogen effects. To place the results of the experiment in a wider context, we also conducted a systematic review of published papers that explicitly used fungicide to explore the role of pathogens in PSF experiments. Our experimental results showed that oomycete pathogens were the main drivers of individual PSFs in the study forests. Oomycete effects varied among tree species according to their susceptibility to P. cinnamomi, driving negative PSFs in the highly susceptible Q. suber but not in the coexistent Olea europaea. Oomycete-driven PSFs were not modified by intraspecific competition. Oomycete pathogens were also major contributors to negative pairwise PSFs assumed to promote species coexistence. Results from the systematic review supported the novelty of our experimental results, since only three studies had previously used oomycete-specific fungicide in a PSF context and none in systems invaded by exotic oomycetes. Overall, our results provide novel evidence of oomycete pathogens (including the exotic P. cinnamomi) as fundamental drivers of negative individual and pairwise PSFs with implications for species coexistence in invaded communities. Although in the short-term invasive pathogens might contribute to species coexistence by causing self-limitation in dominant species, strong inter-specific variation in self-limitation might undermine coexistence in the long-term. Because of the increasing number of exotic oomycetes worldwide, further attention should be given to oomycetes as drivers of PSFs in plant communities.

中文翻译：

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解开卵菌土壤病原体作为植物-土壤反馈驱动因素的作用

植物物种及其土壤生物群之间的相互作用驱动植物-土壤反馈（PSF），在植物群落的动态和多样性中发挥重要作用。在土壤群落的不同组成部分中，病原体被认为是负 PSF 的主要驱动因素。尽管如此，通过实验量化土壤病原体对 PSF 的贡献的研究数量仍然相当低。在这里，我们使用卵菌特异性杀菌剂进行了温室实验，以量化土壤病原体，特别是卵菌病原体对森林群落中个体和成对 PSF 的贡献。我们以栎木为主并被卵菌病原体肉桂疫霉入侵的地中海混交林作为案例研究. 将杀菌剂处理与竞争处理相结合，以探索同种邻居如何改变病原体的影响。为了将实验结果放在更广泛的背景下，我们还对已发表的论文进行了系统回顾，这些论文明确使用杀菌剂来探索病原体在 PSF 实验中的作用。我们的实验结果表明，卵菌病原体是研究森林中个体 PSF 的主要驱动因素。不同树种的卵菌效应因对肉桂的易感性而异，导致高度易感的Q. suber中的 PSF 为负，但在共存的油橄榄中则没有。. 卵菌驱动的 PSF 没有受到种内竞争的影响。卵菌病原体也是假定促进物种共存的负成对 PSF 的主要贡献者。系统评价的结果支持了我们实验结果的新颖性，因为之前只有三项研究在 PSF 环境中使用了卵菌特异性杀菌剂，而在外来卵菌入侵的系统中则没有。总体而言，我们的研究结果为卵菌病原体（包括外来的P. cinnamomi）作为负面个体和成对PSF的基本驱动因素，对入侵社区中的物种共存有影响。尽管在短期内，入侵病原体可能通过导致优势物种的自限性来促进物种共存，但自限性的强烈种间差异可能会破坏长期的共存。由于世界范围内外来卵菌数量的增加，应进一步关注卵菌作为植物群落中 PSF 的驱动因素。