1. Plant–fungal associations strongly influence forest carbon and nitrogen cycling. The prevailing framework for understanding these relationships is through the relative abundance of arbuscular (AM) versus ectomycorrhizal (EcM) trees. Ericoid mycorrhizal (ErM) shrubs are also common in forests and interactions between co-occurring ErM shrubs and AM and EcM trees could shift soil biogeochemical responses. Here we test hypotheses that the effects of ErM shrubs on soil carbon and nitrogen either extend or are redundant with those of EcM trees.
2. Using regional vegetation inventory data (>3,500 plot observations) we evaluated the frequency, richness and relative abundance of ErM plants in temperate forests in the eastern United States and examined their relationship with EcM plant cover. We then used surface soil (7 cm) data from 414 plots within a single forest to analyse relationships between ErM plant cover, relative EcM tree basal area and soil carbon and nitrogen concentrations while accounting for other biogeochemical controls, such as soil moisture.
3. At both scales, we found a positive relationship between ErM and EcM plants, and the majority of ErM plants were in the shrub layer. Within the forest site, ErM plants strongly modulated tree mycorrhizal dominance effects. We found negative relationships between EcM relative basal area and soil carbon and nitrogen concentrations, but these relationships were weak to negligible in the absence of ErM plants. Both EcM relative basal area and ErM plant cover were positively associated with the soil carbon-to-nitrogen ratio. However, this relationship was driven by relatively lower nitrogen for EcM trees and higher carbon for ErM plants. As such, the functional effects of ErM plants on soil biogeochemistry neither extended nor were redundant with those of EcM trees.
4. Synthesis. We found that ErM shrubs strongly influenced the relationship between tree mycorrhizal associations and soil biogeochemistry, and the effects of ErM shrubs and EcM trees on carbon and nitrogen were functionally distinct. Our findings suggest that ErM shrubs could confound interpretation of AM versus EcM tree effects in ecosystems where they co-occur but also bolster growing calls to consider mycorrhizal functional types as variables that strongly influence forest biogeochemistry.

中文翻译：

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Ericoid菌根灌木改变乔木菌根优势与土壤碳氮的关系

1. 植物-真菌关联强烈影响森林碳和氮循环。理解这些关系的主要框架是通过丛枝 (AM) 与外生菌根 (EcM) 树的相对丰度。Ericoid 菌根 (ErM) 灌木在森林中也很常见，共生 ErM 灌木与 AM 和 EcM 树之间的相互作用可能会改变土壤生物地球化学反应。在这里，我们测试了假设，即 ErM 灌木对土壤碳和氮的影响与 EcM 树木的影响扩展或冗余。
2. 我们使用区域植被清单数据（> 3,500 个地块观察）评估了美国东部温带森林中 ErM 植物的频率、丰富度和相对丰度，并检查了它们与 EcM 植物覆盖的关系。然后，我们使用来自单个森林内 414 个地块的表层土壤 (7 cm) 数据来分析 ErM 植物覆盖率、相对 EcM 树基面积和土壤碳和氮浓度之间的关系，同时考虑其他生物地球化学控制，如土壤水分。
3. 在两个尺度上，我们发现 ErM 和 EcM 植物之间存在正相关关系，并且大多数 ErM 植物位于灌木层。在森林场地内，ErM 植物强烈调节树木菌根优势效应。我们发现 EcM 相对基础面积与土壤碳和氮浓度之间存在负相关关系，但在没有 ErM 植物的情况下，这些关系很弱甚至可以忽略不计。EcM 相对基面积和 ErM 植物覆盖与土壤碳氮比呈正相关。然而，这种关系是由 EcM 树的相对较低的氮和 ErM 植物的较高碳驱动的。因此，ErM 植物对土壤生物地球化学的功能影响与 EcM 树的功能影响既不扩展也不重复。
4. 合成。我们发现 ErM 灌木强烈影响树木菌根关联和土壤生物地球化学之间的关系，并且 ErM 灌木和 EcM 树木对碳和氮的影响在功能上是不同的。我们的研究结果表明，ErM 灌木可能会混淆它们共同发生的生态系统中 AM 与 EcM 树效应的解释，但也支持日益增长的将菌根功能类型视为强烈影响森林生物地球化学的变量的呼声。