Drought increases microbial allocation to stress tolerance but with few tradeoffs among community-level traits

Climate change will increase soil drying, altering microbial communities via increasing water stress and decreasing resource availability. The responses of these microbial communities to changing environments is likely governed by physiological tradeoffs between high yield, resource acquisition, and stress tolerance (Y-A-S framework). We leveraged a unique field experiment that manipulates both drought and carbon availability across two years and three land uses, and we used both metagenomic and bioassay indicators of the three microbial community traits to test the following hypotheses: 1. Drought increases microbial allocation to stress tolerance functions, at the expense of growth and resource acquisition. 2. Because microbes are resource-limited under drought, increased carbon will enable greater expression of stress tolerance. 3. All three key life history traits described in the YAS framework will trade off, especially when resources are limited. Drought did increase microbial physiological investment in stress tolerance (measured via trehalose production), but we saw few other changes in microbial communities under drought. Carbon addition increased resource acquisition (measured via enzyme activity and resource acquisition gene abundance) and stress tolerance (trehalose assay), but did so in both drought and average rainfall environments. We found no evidence of trait tradeoffs, as we found no significant negative correlations between traits (measured via bioassay and metagenomics). In summary, we found C addition, and to a lesser extent, drought, both altered microbial community function and functional genes. However, resources did not alter drought response in a way that was consistent with theory of life history tradeoffs.气候变化将加剧土壤干旱化，通过增加水分胁迫与降低资源可获得性，改变微生物群落结构。这类微生物群落对环境变化的响应，大概率受高产量、资源获取与胁迫耐受三者间的生理权衡（Y-A-S framework）所调控。 本研究依托一项独特的野外控制实验，该实验在两年周期内、覆盖三种土地利用类型，同时调控干旱程度与碳源可获得性；我们同时采用宏基因组（metagenomic）与生物测定（bioassay）两类指标，针对三种微生物群落功能性状开展以下假说检验：1. 干旱会使微生物将资源分配至胁迫耐受功能，以牺牲生长与资源获取能力为代价。2. 由于干旱环境下微生物受资源限制，增加碳源供给可提升胁迫耐受性状的表达水平。3. Y-A-S框架中提及的三类关键生活史性状均会发生权衡，尤其在资源受限的情境下。 研究结果显示，干旱确实提升了微生物在胁迫耐受方面的生理投入（通过海藻糖（trehalose）合成量衡量），但干旱条件下微生物群落其余特征几乎未发生变化。 碳源添加提升了资源获取能力（通过酶活性与资源获取基因丰度衡量）与胁迫耐受能力（海藻糖生物测定（trehalose assay）），且该效应在干旱与正常降雨环境中均有体现。 本研究未发现性状权衡的相关证据：通过生物测定（bioassay）与宏基因组学（metagenomics）手段测得的性状间，均未出现显著负相关关系。 综上，本研究发现碳源添加（以及程度较轻的干旱胁迫）均可改变微生物群落功能与功能基因表达模式。但资源供给并未按照生活史权衡理论所预测的方式，改变微生物对干旱胁迫的响应模式。