Abstract. The microbial community composition in subsoils remains understudied and it is largely unknown whether subsoil microorganisms show a similar response to global warming as do microorganisms at the soil surface. Since microorganisms are key drivers of soil organic carbon decomposition, this knowledge gap causes uncertainty in predictions of future carbon cycling in the subsoil carbon pool (>50 % of the soil organic carbon stocks are below 30 cm soil depth). In the Blodgett forest field warming experiment (California, USA) we investigated how +4 °C warming the whole soil profile to 100 cm soil depth for 4.5 years has affected the abundance and community structure of microorganisms. We used proxies for bulk microbial biomass carbon (MBC) and functional microbial groups based on lipid biomarkers, such as phospholipid fatty acids (PLFAs) and branched glycerol dialkyl glycerol tetraethers (brGDGTs). Microbial biomass decreased and community composition changed with depth. Our results show that the concentration of PLFAs decreased with warming in the subsoil (below 30 cm) by 28 % but was not affected in the topsoil. Phospholipid fatty acid concentrations changed in concert with soil organic carbon. The microbial community response to warming was depth dependent. The relative abundance of actinobacteria increased in subsoil, and gram+ bacteria in subsoils adapted their cell-membrane structure to warming induced stress as indicated by the ratio of anteiso to iso PLFAs. Our results show for the first time that subsoil microorganisms can be more affected by warming as compared to topsoil microorganisms. These microbial responses could be explained by the observed decrease in subsoil organic carbon concentration in the warmed plots. A decrease in microbial abundance in warmed subsoils might reduce the magnitude of the respiration response over time. The shift in the subsoil microbial community towards more actinobacteria might disproportionately enhance degradation of previously stable subsoil carbon, as this group is able to metabolize complex carbon sources.

中文翻译：

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整个土壤变暖会降低土壤丰度并改变地下土壤而非表层土壤中微生物的群落结构

摘要。对土壤中微生物群落组成的研究仍在研究中，很大程度上未知土壤微生物是否对全球变暖表现出与土壤表面微生物相似的反应。由于微生物是土壤有机碳分解的主要驱动力，因此这种知识鸿沟导致对未来在地下土壤碳库中碳循环的预测存在不确定性（> 50％的土壤有机碳储量在30厘米土壤深度以下）。在布洛杰特森林田间增温实验（美国加利福尼亚）中，我们研究了+4°C在4.5年内将整个土壤剖面升温至100 cm土壤深度如何影响微生物的丰度和群落结构。我们使用了基于脂质生物标志物的微生物总生物量碳（MBC）和功能微生物组的代理，例如磷脂脂肪酸（PLFA）和支链甘油二烷基甘油四醚（brGDGTs）。微生物生物量减少，群落组成随深度变化。我们的研究结果表明，随着土壤温度（30 cm以下）的升高，PLFAs的浓度降低了28％，但在表层土壤中并未受到影响。磷脂脂肪酸浓度随土壤有机碳的变化而变化。微生物群落对变暖的反应取决于深度。土壤中放线菌的相对丰度增加，而土壤中的革兰氏+细菌使它们的细胞膜结构适应由升温引起的胁迫，如 我们的研究结果表明，随着土壤温度（30 cm以下）的升高，PLFAs的浓度降低了28％，但在表层土壤中并未受到影响。磷脂脂肪酸浓度随土壤有机碳的变化而变化。微生物群落对变暖的反应取决于深度。土壤中放线菌的相对丰度增加，而土壤中的革兰氏+细菌使它们的细胞膜结构适应由升温引起的胁迫，如 我们的研究结果表明，随着土壤温度（30 cm以下）的升高，PLFAs的浓度降低了28％，但在表层土壤中并未受到影响。磷脂脂肪酸浓度随土壤有机碳的变化而变化。微生物群落对变暖的反应取决于深度。土壤中放线菌的相对丰度增加，而土壤中的革兰氏+细菌使它们的细胞膜结构适应由升温引起的胁迫，如反异构到ISO磷脂脂肪酸。我们的结果首次表明，与表土微生物相比，地下部微生物受升温的影响更大。这些微生物反应可以通过在升温的土壤中观察到的土壤中有机碳浓度的下降来解释。随着时间的推移，温暖的土壤中微生物丰度的降低可能会降低呼吸反应的强度。由于土壤微生物群落能够代谢复杂的碳源，因此土壤微生物群落向更多的放线菌的转移可能会不成比例地加剧以前稳定的土壤碳的降解。