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Airborne bacteria are an influential component of the Earth’s microbiomes, but their community structure and biogeographic distribution patterns have yet to be understood. We analyzed the bacterial communities of 370 air particulate samples collected from 63 sites around the world and constructed an airborne bacterial reference catalog with more than 27 million nonredundant 16S ribosomal RNA (rRNA) gene sequences. We present their biogeographic pattern and decipher the interlacing of the microbiome co-occurrence network with surface environments of the Earth. While the total abundance of global airborne bacteria in the troposphere (1.72 × 10 24 cells) is 1 to 3 orders of magnitude lower than that of other habitats, the number of bacterial taxa (i.e., richness) in the atmosphere (4.71 × 10 8 to 3.08 × 10 9 ) is comparable to that in the hydrosphere, and its maximum occurs in midlatitude regions, as is also observed in other ecosystems. The airborne bacterial community harbors a unique set of dominant taxa (24 species); however, its structure appears to be more easily perturbed, due to the more prominent role of stochastic processes in shaping community assembly. This is corroborated by the major contribution of surface microbiomes to airborne bacteria (averaging 46.3%), while atmospheric conditions such as meteorological factors and air quality also play a role. Particularly in urban areas, human impacts weaken the relative importance of plant sources of airborne bacteria and elevate the occurrence of potential pathogens from anthropogenic sources. These findings serve as a key reference for predicting planetary microbiome responses and the health impacts of inhalable microbiomes with future changes in the environment.

The contents and stability of soil dissolved organic matter (DOM) can affect key processes of soil carbon and nitrogen cycle. The responses of DOM content and its spectral structure pro-perties in forest soils to climate change remain unclear. We collected soil samples from two temperate forests, i.e., the broadleaf and Korean pine mixed forest (BKPF) and adjacent secondary white birch forest (WBF), in Changbai Mountains, northeastern China. Using a combination of three-dimensional fluorescence spectrum and parallel factor analysis, a simulated freeze-thaw experiment was conducted in the laboratory. We examined the effects of freeze-thaw intensity, freeze-thaw cycle and their interaction on the content, components and spectral properties of DOM leached from the two forest surface soils with different moisture levels. The results showed that DOM content and components of soil leachates varied with forest types, soil moisture, freeze-thaw intensity and freeze-thaw cycle. The DOM content in the leachates was lowest at medium moisture level and was significantly affected by the high freeze-thaw intensity. In addition, the DOM content increased first and then decreased with the increases of freeze-thaw cycles. Three fluorescence components of DOM in the forest soil leachates were identified as humic acid-like DOM, fulvic acid-like DOM and protein-like DOM. The DOM components of BKPF soil leachates were mainly consisted of fulvic acid-like substances with a high humification index. However, the DOM from WBF soil leachates was dominated by humic acid-like substances with low stability, and the three fluorescence components were significantly affected by the freeze-thaw intensity. Results from the redundancy analysis showed that under the experimental conditions, forest type played a leading role in changing DOM properties. The DOM content and its three fluorescence intensities of WBF soil leachates were higher than those of BKPF. Soil moisture significantly affected the aromaticity of DOM in the forest soil leachates, and the DOM aromaticity of soil leachates from the two forest stands ranked as medium moisturehigh moisturelow moisture. With the increases of freeze-thaw intensity, the DOM aromaticity of BKPF soil leachates significantly decreased. Furthermore, the increases of freeze-thaw cycles significantly increased the humification degree of DOM in the forest soil leachates. Therefore, upon different freeze-thaw disturbance, the DOM content and bioavailability of soil leachates with low moisture tended to increase, particularly in the WBF soil leachates, which may result in an increased lea-ching of DOM in temperate forest soils during spring freeze-thaw periods. The results provide a refe-rence for further investigating DOM turnover in temperate forest soils during spring freeze-thaw periods.土壤溶解性有机质(DOM)含量及其稳定性影响土壤碳氮循环关键过程,目前气候变化下森林土壤DOM含量及其光谱结构特征仍不明确.本研究利用长白山阔叶红松混交林和次生白桦林表层土壤进行室内冻融模拟试验,结合三维荧光光谱-平行因子分析方法,研究冻融强度和冻融循环次数及其交互作用对不同湿度温带森林土壤渗漏液DOM含量、组分和光谱结构特征的影响.结果表明: 森林土壤渗漏液DOM含量及其组分因林分类型、土壤湿度、冻融强度、冻融循环次数不同而存在差异.2种林分土壤渗漏液DOM含量均在中湿度下最低,并受高强度冻融影响显著,且随冻融循环次数增加呈现先增加后降低的趋势.可鉴别DOM的3个荧光组分:胡敏酸类DOM、富里酸类DOM和蛋白类DOM;阔叶红松混交林土壤渗漏液DOM组分以富里酸类物质为主,腐殖化程度较高;而次生白桦林土壤渗漏液DOM组分以胡敏酸类物质为主,3组分受冻融强度显著影响,稳定性较低.经冗余分析(RDA)发现,林分在很大程度上决定森林土壤DOM属性变化,次生白桦林土壤渗漏液DOM含量及其3组分荧光强度大于阔叶红松混交林;土壤湿度显著影响DOM芳香性,2种林分土壤渗漏液DOM芳香性均呈中湿度高湿度低湿度的趋势;随冻融强度增加,阔叶红松混交林土壤渗漏液DOM芳香性显著降低;多次冻融循环显著提高2种林分土壤渗漏液DOM腐殖化程度.因此,不同冻融作用下,低湿度温带森林土壤渗漏液DOM含量及其生物有效性呈现增加的趋势,尤其是次生白桦林土壤,可能会增加春季冻融期温带森林土壤溶解性有机质淋溶损失.这些结果可为深入研究野外冻融期温带森林土壤溶解性有机质周转机制提供参考.

Vertical profiles of low molecular weight dicarboxylic acids, related organic compounds and SOA tracer compounds in particle phase have not yet been simultaneously explored in East Asia, although there is growing evidence that aqueous phase oxidation of volatile organic compounds may be responsible for the elevated organic aerosols (OA) in the free troposphere. Here, we found consistently good correlation of oxalic acid, the most abundant organics globally, with its precursors as well as biogenic-derived secondary OA (SOA) compounds in Chinese tropospheric aerosols by aircraft measurements. Anthropogenically derived dicarboxylic acids (i.e., C5 and C6 diacids) at high altitudes were 4–20 times higher than those from surface measurements and even occasionally dominant over oxalic acid at altitude higher than 2 km, which is in contrast to the predominance of oxalic acid previously reported globally including the tropospheric and surface aerosols. This indicates an enhancement of tropospheric SOA formation from anthropogenic precursors. Furthermore, oxalic acid-tosulfate ratio maximized at altitude of ~2 km, explaining aqueous-phase SOA production that was supported by good correlations with predicted liquid water content, organic carbon and biogenic SOA tracers. These results demonstrate that elevated oxalic acid and related SOA compounds from both the anthropogenic and biogenic sources may substantially contribute to tropospheric OA burden over polluted regions of China, implying aerosol-associated climate effects and intercontinental transport.