黄土-古土壤原核生物群落对古气候变化的响应

【目的】黄土-古土壤序列是记录第四纪气候环境变化的良好载体，其内部的土壤微生物特征是蕴含土壤环境变化的重要信息。由于黄土与古土壤成壤环境的气候差异，微生物群落结构特征可能会有不同的响应，但针对该问题的研究还十分有限。【方法】选择任家坡(R)和九州台(J)两地黄土(RL和JL)-古土壤(RS和JS)序列，运用高通量测序技术和线性判别分析效应大小(linear discriminant analysis effect size, LEfSe)识别土壤原核生物群落结构和类群差异，基于原核生物分类单元功能注释(functional annotation of prokaryotic taxa, FAPROTAX)数据库进行群落功能预测，以及利用Mantel test探讨影响土壤原核生物群落稳定的环境因子。【结果】土壤中碳氮营养物质与气候变化的代用指标磁化率、Rb/Sr变化趋势一致，含量整体表现为古土壤(RS和JS)高，对应的黄土(RL和JL)低，这一特征在任家坡古土壤(RS)中尤为显著；在同一气候时期，九州台较任家坡更为干冷，并且九州台古土壤沉积阶段也受到较强冬季风的影响，使其气候冷干与暖湿转变呈渐变型。原核生物群落结构中酸杆菌门(Acidobacteria)、泉古菌门(Crenarchaeota)、绿弯菌门(Chloroflexi)等具有嗜热嗜温性质的细菌和古菌在任家坡黄土-古土壤(RL和RS)中丰度较高，芽单胞菌门(Gemmatimonadetes)、放线菌门(Actinobacteria)、厚壁菌门(Firmicutes)、广古菌门(Euryarchaeota)、异常球菌-栖热菌门(Deinococcus-Thermus)等耐旱、适宜极端环境中生存的细菌和古菌在九州台黄土-古土壤中(JL和JS)丰度较高。同时，生命产能、氮、锰、铁、氯元素循环相关功能基因在任家坡古土壤(RS)中表达量最高，而碳、氢、硫元素循环相关功能基因在任家坡黄土(RL)中表达量最高。与任家坡相比，九州台原核生物群落具有物种多样性高、功能种类少的特点。Mantel test分析进一步表明，有机碳(soil organic carbon, SOC)、含水率(soil water content, SWC)、总氮(total nitrogen, TN)和硝态氮(nitrate nitrogen, NO₃^--N)是影响任家坡原核生物群落和功能稳定的关键环境因子，而TN、SOC、pH值和铵态氮(NH₄⁺-N)是影响九州台原核生物群落和功能稳定的关键环境因子。【结论】在暖湿期，微生物群落分化出更多的功能种类，具有更旺盛的生命活动；在冷干期，微生物群落通过提高物种多样性来完成主要的生命活动功能，通过协同共生维持群落生存和稳定来适应环境胁迫。研究成果对认识气候变化对土壤微生物多样性和功能的影响具有重要意义。

Response of prokaryotic community in loess-paleosol to paleoclimate change

[Objective] Loess-paleosol sequence (LPS) is a good carrier recording the changes of Quaternary climate and environment, and the characteristics of soil microorganisms in it indicates important information about the changes of soil environment. Due to the climate difference between loess and paleosoil, the soil microbial community may have different responses in the structural characteristics. The research on this problem, however, is limited. [Methods] In this paper, the loess (RL and JL)-paleosol (RS and JS) sequences in Renjiapo (R) and Jiuzhoutai (J) were selected, and high-throughput sequencing and linear discriminant analysis effect size (LEfSe) were employed to gain insights into the community structure and group differences of soil prokaryotes. Furthermore, functional annotation of prokaryotic taxa (FAPROTAX) was used to predict the community function, and the Mantel test was carried out to identify the environmental factors affecting the community stability of soil prokaryotes. [Results] The carbon and nitrogen in soil showed changes consistent with the magnetic susceptibility and Rb/Sr ratio, the alternative indicators of climate change. The content of carbon and nitrogen was high in the paleosol (RS and JS, especially in RS) and low in the corresponding loess (RL and JL). In the same climate era, Jiuzhoutai was drier and colder than Renjiapo. The paleosol deposition stage in Jiuzhoutai was affected by strong winter monsoon, which ultimately led to the gradual change from the dry-cold to wet-warm climate. In the prokaryotic community, thermophilic or mesophilic bacteria and archaea, such as Acidobacteria, Crenarchaeota, and Chloroflexi, were abundant in RL and RS, while those with tolerance to drought and extreme environments, such as Gemmatimonadetes, Actinobacteria, Firmicutes, Euryarchaeota, and Deinococcus-Thermus, had high abundance in JL and JS. The functional genes related to energy source and nitrogen, manganese, iron, and chlorine cycling had the highest expression levels in RS, while those involved in carbon, hydrogen, and sulfur cycling showed the highest expression levels in RL. The prokaryotic community in Jiuzhoutai had higher species diversity and fewer functional species than that in Renjiapo. Mantel test results indicated that soil organic carbon (SOC), soil water content (SWC), total nitrogen (TN), and nitrate nitrogen (NO₃^--N) were the key environmental factors influencing the stability and functions of the prokaryotic community in Renjiapo, while the influencing factors in Jiuzhoutai were TN, SOC, pH, and ammonium nitrogen (NH₄⁺-N). [Conclusion] During the warm-humid period, the microbial community differentiated into more functional categories and exhibited more vigorous life activities. When the climate was dry and cold, the microbial community completed the main life activities by improving species diversity and jointly maintaining the community survival and stability to adapt to environmental stress. The findings are of great significance for understanding the impacts of climate change on the diversity and functions of soil microorganisms.