Table_2_Chronic Physical Disturbance Substantially Alters the Response of Biological Soil Crusts to a Wetting Pulse, as Characterized by Metatranscriptomic Sequencing.xls

Biological soil crusts (biocrusts) are microbial communities that are a feature of arid surface soils worldwide. In drylands where precipitation is pulsed and ephemeral, the ability of biocrust microbiota to rapidly initiate metabolic activity is critical to their survival. Community gene expression was compared after a short duration (1 h) wetting pulse in both intact and soils disturbed by chronic foot trampling. Across the metatranscriptomes the majority of transcripts were cyanobacterial in origin, suggesting that cyanobacteria accounted for the bulk of the transcriptionally active cells. Chronic trampling substantially altered the functional profile of the metatranscriptomes, specifically resulting in a significant decrease in transcripts for nitrogen fixation. Soil depth (biocrust and below crust) was a relatively small factor in differentiating the metatranscriptomes, suggesting that the metabolically active bacteria were similar between shallow soil horizons. The dry samples were consistently enriched for hydrogenase genes, indicating that molecular hydrogen may serve as an energy source for the desiccated soil communities. The water pulse was associated with a restructuring of the metatranscriptome, particularly for the biocrusts. Biocrusts increased transcripts for photosynthesis and carbon fixation, suggesting a rapid resuscitation upon wetting. In contrast, the trampled surface soils showed a much smaller response to wetting, indicating that trampling altered the metabolic response of the community. Finally, several biogeochemical cycling genes in carbon and nitrogen cycling were assessed for their change in abundance due to wetting in the biocrusts. Different transcripts encoding the same gene product did not show a consensus response, with some more abundant in dry or wet biocrusts, highlighting the challenges in relating transcript abundance to biogeochemical cycling rates. These observations demonstrate that metatranscriptome sequencing was able to distinguish alterations in the function of arid soil microbial communities at two varying temporal scales, a long-term ecosystems disturbance through foot trampling, and a short term wetting pulse. Thus, community metatranscriptomes have the potential to inform studies on the response and resilience of biocrusts to various environmental perturbations.

生物土壤结皮（biological soil crusts，以下简称biocrusts）是广泛分布于全球干旱地表土壤中的微生物群落。在降水呈脉冲式且短暂的干旱地区，生物土壤结皮微生物群快速启动代谢活动的能力对其存活至关重要。本研究对比了未受干扰的完整土壤样地与长期受人类慢性踩踏干扰的土壤，在经历短时（1小时）湿润脉冲后的群落基因表达情况。在所有宏转录组（metatranscriptome）样本中，绝大多数转录本均为蓝细菌来源，表明蓝细菌构成了转录活性细胞的主体。长期踩踏显著改变了宏转录组的功能谱，具体表现为固氮相关转录本的丰度显著下降。土壤分层（结皮层及其下层土壤）是区分宏转录组的相对次要因素，表明浅层土壤剖面中代谢活跃的细菌类群具有相似性。干燥样本始终富集氢化酶基因，提示分子氢可能作为干燥土壤群落的能量来源。湿润脉冲与宏转录组的结构重构密切相关，这一现象在生物土壤结皮样本中尤为显著。结皮样本中光合作用与固碳相关的转录本丰度上升，表明湿润后群落可快速复苏。与之相反，受踩踏的地表土壤对湿润的响应幅度要小得多，这表明踩踏改变了群落的代谢响应模式。最后，本研究针对生物土壤结皮中参与碳、氮生物地球化学循环的若干基因，分析了湿润处理后其丰度的变化情况。编码同一基因产物的不同转录本并未表现出一致的响应模式，部分转录本在干燥或湿润结皮样本中丰度更高，这凸显了将转录本丰度与生物地球化学循环速率相关联的难度。上述结果表明，宏转录组测序能够区分干旱土壤微生物群落在两种不同时间尺度下的功能变化：一是长期的人类踩踏生态干扰，二是短期的湿润脉冲事件。因此，群落宏转录组学技术有望为研究生物土壤结皮对各类环境扰动的响应与恢复力提供理论参考。