Bacterial communities in the organic leaf litter layer and bulk (mineral and organic) soil are sensitive to environmental change. However, despite close interactions between these communities, the leaf litter layer has historically been studied in isolation from the bulk soil. Whether bacterial response to environmental change is uniform throughout the surface soil remains unclear. Here, we simultaneously characterized how bacterial community composition in three surface soil layers (the leaf litter layer, 0–2 cm of bulk soil, and 0–10 cm of bulk soil) responded to a wildfire burning through a 13-year drought simulation in two adjacent ecosystems, a grassland and coastal sage scrubland. We found that bacterial communities in all three surface soil layers were distinct in composition and varied with drought, ecosystem type, and temporal variation. Moreover, the impact of these environmental changes on bacterial community composition decreased with depth in the surface soil. Bacterial response to drought was three-fold higher in the leaf litter layer than in the top 10 cm of bulk soil, with the drought treatment explaining 4.8% and 1.6% of the compositional variation, respectively. Wildfire altered bacterial composition in the leaf litter layer but not within the top 10 cm of bulk soil. Further, previous exposure of the bacterial communities in the leaf litter layer to drought did not influence its response to the wildfire. Thus, considering soil depth when assessing the impact of environmental conditions on the surface soil microbiome may improve predictions about the degree to which microbial communities, and therefore soil carbon, will respond to future environmental change.

有机落叶层和大块（矿物和有机）土壤中的细菌群落对环境变化很敏感。然而，尽管这些群落之间存在密切的相互作用，但在历史上，落叶层的研究一直与大块土壤隔离开来。整个表层土壤中细菌对环境变化的反应是否一致仍不清楚。在这里，我们通过 13 年的干旱模拟，同时描述了三个表层土壤层（落叶层、0-2 厘米大块土壤和 0-10 厘米大块土壤）中的细菌群落组成如何响应野火燃烧。两个相邻的生态系统，一个草原和沿海鼠尾草灌丛。我们发现所有三个表层土壤层中的细菌群落组成不同，并随着干旱、生态系统类型和时间变化而变化。此外，这些环境变化对细菌群落组成的影响随着表层土壤的深度而降低。落叶层中的细菌对干旱的反应是土壤表层 10 cm 的三倍，干旱处理分别解释了 4.8% 和 1.6% 的成分变化。野火改变了落叶层中的细菌组成，但没有改变土壤顶部 10 厘米的细菌组成。此外，先前将落叶层中的细菌群落暴露于干旱并不影响其对野火的反应。因此，在评估环境条件对地表土壤微生物组的影响时考虑土壤深度可能会改善对微生物群落以及土壤碳对未来环境变化的反应程度的预测。这些环境变化对细菌群落组成的影响随着表层土壤的深度而降低。落叶层中的细菌对干旱的反应是土壤表层 10 cm 的三倍，干旱处理分别解释了 4.8% 和 1.6% 的成分变化。野火改变了落叶层中的细菌组成，但没有改变土壤顶部 10 厘米的细菌组成。此外，先前将落叶层中的细菌群落暴露于干旱并不影响其对野火的反应。因此，在评估环境条件对地表土壤微生物组的影响时考虑土壤深度可能会改善对微生物群落以及土壤碳对未来环境变化的反应程度的预测。这些环境变化对细菌群落组成的影响随着表层土壤的深度而降低。落叶层中的细菌对干旱的反应是土壤表层 10 cm 的三倍，干旱处理分别解释了 4.8% 和 1.6% 的成分变化。野火改变了落叶层中的细菌组成，但没有改变土壤顶部 10 厘米的细菌组成。此外，先前将落叶层中的细菌群落暴露于干旱并不影响其对野火的反应。因此，在评估环境条件对地表土壤微生物组的影响时考虑土壤深度可能会改善对微生物群落以及土壤碳对未来环境变化的反应程度的预测。