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Rising atmospheric CO2 concentration directly stimulates plant productivity and affects nutrient dynamics in the soil. However, the influence of CO2 enrichment on soil bacterial communities remains elusive, likely due to their complex interactions with a wide range of plant and soil properties. Here, we investigated the bacterial community response to a decade long preindustrial‐to‐future CO2 gradient (250–500 ppm) among three contrasting soil types using 16S rRNA gene amplicon sequencing. In addition, we examined the effect of seasonal variation and plant species composition on bacterial communities. We found that Shannon index (H') and Faith's phylogenetic diversity (PD) did not change in response to the CO2 gradient (R2 = 0.01, p > 0.05). CO2 gradient and season also had a negligible effect on overall community structure, although silty clay soil communities were better structured on a CO2 gradient (p < 0.001) among three soils. Similarly, CO2 gradient had no significant effect on the relative abundance of different phyla. However, we observed soil‐specific variation of CO2 effects in a few individual families. For example, the abundance of Pirellulaceae family decreased linearly with CO2 gradient, but only in sandy loam soils. Conversely, the abundance of Micromonosporaceae and Gaillaceae families increased with CO2 gradient in clay soils. Soil water content (SWC) and nutrient properties were the key environmental constraints shaping bacterial community structure, one manifestation of which was a decline in bacterial diversity with increasing SWC. Furthermore, the impact of plant species composition on community structure was secondary to the strong influence of soil properties. Taken together, our findings indicate that bacterial communities may be largely unresponsive to indirect effects of CO2 enrichment through plants. Instead, bacterial communities are strongly regulated by edaphic conditions, presumably because soil differences create distinct environmental niches for bacteria. We investigated the bacterial community response to a decade‐long preindustrial‐to‐future CO2 gradient (250–500 ppm) among three contrasting soil types using 16S rRNA gene amplicon sequencing. We found that bacterial communities may be largely unresponsive to indirect effects of CO2 enrichment through plants. Instead, bacterial communities are strongly regulated by edaphic conditions, presumably because soil differences create distinct environmental niches for bacteria. [ABSTRACT FROM AUTHOR]

Aim Plant root traits regulate belowground C inputs, soil nutrient and water uptake, and play critical roles in determining sustainable plant production and consequences for ecosystem C storage. However, the effects of elevated CO2 on root morphology and function have not been well quantified. We reveal general patterns of root trait responses to elevated CO2 from field manipulative experiments. Location North America, Europe, Oceania, Asia. Methods The meta-analysis approach was used to examine the effects of CO2 elevation on 17 variables associated with root morphology, biomass size and distribution, C and N concentrations and pools, turnover and fungal colonization from 110 published studies. Results Elevated CO2 increased root length (+26.0%) and diameter (+8.4%). Elevated CO2 also stimulated total root (+28.8%), fine root (+27.7%) and coarse root biomass (+25.3%), demonstrating strong responses of root morphology and biomass. Elevated CO2 increased the root:shoot ratio (+8.5%) and decreased the proportion of roots in the topsoil (-8.4%), suggesting that plants expand rooting systems. In addition, elevated CO2 decreased N concentration (-7.1%), but did not affect C concentration, and thus increased the C: N ratio (+7.8%). Root C (+29.3%) increased disproportionately relative to root N pools (+9.4%) under elevated CO2. Functional traits were also strongly affected by elevated CO2, which increased respiration (+58.9%), rhizodeposition (+37.9%) and fungal colonization (+3.3%). Main conclusions These results suggest that elevated CO2 promoted root morphological development, root system expansion and C input to soils, implying that the sensitive responses of root morphology and function to elevated CO2 would increase long-term belowground C sequestration. [ABSTRACT FROM AUTHOR]

Soil microbial communities may be able to rapidly respond to changing environments in ways that change community structure and functioning, which could affect climate-carbon feedbacks. However, detecting microbial feedbacks to elevated CO2 (eCO2) or warming is hampered by concurrent changes in substrate availability and plant responses. Whether microbial communities can persistently feed back to climate change is still unknown. We overcame this problem by collecting microbial inocula at subfreezing conditions under eCO2 and warming treatments in a semi-arid grassland field experiment. The inoculant was incubated in a sterilised soil medium at constant conditions for 30 days. Microbes from eCO2 exhibited an increased ability to decompose soil organic matter (SOM) compared with those from ambient CO2 plots, and microbes from warmed plots exhibited increased thermal sensitivity for respiration. Microbes from the combined eCO2 and warming plots had consistently enhanced microbial decomposition activity and thermal sensitivity. These persistent positive feedbacks of soil microbial communities to eCO2 and warming may therefore stimulate soil C loss. [ABSTRACT FROM AUTHOR]

Rising atmospheric CO 2 concentration directly stimulates plant productivity and affects nutrient dynamics in the soil. However, the influence of CO 2 enrichment on soil bacterial communities remains elusive, likely due to their complex interactions with a wide range of plant and soil properties. Here, we investigated the bacterial community response to a decade long preindustrial-to-future CO 2 gradient (250-500 ppm) among three contrasting soil types using 16S rRNA gene amplicon sequencing. In addition, we examined the effect of seasonal variation and plant species composition on bacterial communities. We found that Shannon index (H') and Faith's phylogenetic diversity (PD) did not change in response to the CO 2 gradient (R 2  = 0.01, p > 0.05). CO 2 gradient and season also had a negligible effect on overall community structure, although silty clay soil communities were better structured on a CO 2 gradient (p < 0.001) among three soils. Similarly, CO 2 gradient had no significant effect on the relative abundance of different phyla. However, we observed soil-specific variation of CO 2 effects in a few individual families. For example, the abundance of Pirellulaceae family decreased linearly with CO 2 gradient, but only in sandy loam soils. Conversely, the abundance of Micromonosporaceae and Gaillaceae families increased with CO 2 gradient in clay soils. Soil water content (SWC) and nutrient properties were the key environmental constraints shaping bacterial community structure, one manifestation of which was a decline in bacterial diversity with increasing SWC. Furthermore, the impact of plant species composition on community structure was secondary to the strong influence of soil properties. Taken together, our findings indicate that bacterial communities may be largely unresponsive to indirect effects of CO 2 enrichment through plants. Instead, bacterial communities are strongly regulated by edaphic conditions, presumably because soil differences create distinct environmental niches for bacteria., (© 2018 John Wiley & Sons Ltd.)