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Aims: Soil microbes are key to myriad processes in terrestrial ecosystems. Elevated CO2 represents a dominant driver of global climate change; however, it remains unclear to what extent elevated CO2 impacts soil microbial communities at ecosystem and global scales. Here, we sought to address the following questions: (a) Do the compositions of microbial communities shift from K‐ to r‐strategists under elevated CO2? (b) What is the extent of the compositional shifts of microbial communities affected by elevated CO2 concentrations, experimental duration, ecosystem types and/or background climates? (c) Are the responses of microbial communities to elevated CO2 associated with changes in soil pH and carbon and nitrogen availabilities? Location: Global. Time period: 1998–2020. Major taxa studied: Soil microbes. Methods: We performed a global meta‐analysis of 965 observations from 122 studies, which tested the effects of elevated CO2 on microbial communities. The data covered broad variations in ecosystems, climate, CO2 concentrations, experimental duration, and soil factors. Results: We revealed that elevated CO2 decreased the K‐ to r‐strategist ratios with decreasing fungi : bacteria, Gram+ : Gram– bacteria, and Acidobacteria : Proteobacteria ratios, and increased bacterial biomass, microbial biomass carbon, Gram– bacteria, and Acidobacteria abundance. Moreover, the shifts from K‐ to r‐strategists were more pronounced under higher CO2 concentrations and longer experimental durations. The responses of microbial attributes to elevated CO2 did not differ significantly among croplands, forests and grasslands. Furthermore, the response of microbial biomass to elevated CO2 was negatively correlated with the response of soil pH, while those of bacterial biomass and fungi : bacteria ratios were positively correlated with those of soil organic carbon and soil carbon : nitrogen ratios, respectively. Main conclusions: Our results suggest that elevated CO2 shifts soil microbial communities from K‐ to r‐strategists, and provide supportive evidence for understanding responses of soil microbial processes to elevated CO2. [ABSTRACT FROM AUTHOR]

Evaluates the variation in trembling aspen productivity in British Columbia. Relationships between site index and environmental factors; Effect of topography on aspen growth; Soils associated with aspen stands.

• Soil nitrogen concentration increase over time since restoration. • Soil phosphorus concentration decreases over time. • Shift from nitrogen to phosphorus limitation over time. • Nutrients and stoichiometry become increasingly akin to those in primary forests. Soil stoichiometry (C:N:P ratio) is an important indicator of soil fertility and plant nutrient status. To understand the impacts of the long-term restoration of degraded subtropical broadleaved mixed forests on soil stoichiometry, we employed a chronosequence to examine the responses of soil nutrient and C-N-P stoichiometry at three soil depths (0–10, 10–20, and 20–30 cm) over time (0 to 35 years) since the cessation of anthropogenic disturbances and used primary forests as a reference in subtropical Southeast China. The soil organic carbon and total nitrogen concentrations increased significantly with longer duration restoration, whereas the soil phosphorus concentration decreased. In the 33–35-year age class, the P concentration in the subtropical degraded broadleaved mixed forest was akin to the primary forest. The soil organic carbon and total nitrogen concentrations decreased with soil depth, while the soil phosphorus concentrations did not differ significantly with soil depth. The soil carbon: phosphorus and nitrogen: phosphorus ratios increased over time, whereas the carbon: nitrogen ratio decreased weakly. Soil stoichiometry ratio in the 33–35-year age class was closer to soil stoichiometry ratio in primary forests. Our results suggest that N-limitation weakens with time since restoration, but P-limitation strengthens with stand development following restoration. [ABSTRACT FROM AUTHOR]

• Forest conversion has a profound impact on soil nanoscale pores. • Vegetation types alter soil nanoscale pores in different diameter ranges. • Soil microbes may be one of the factors that affects soil nanoscale pores. Long term land-use change, such as forest conversion, has been shown to strongly affect soil pore properties. The soil pores mediate soil processes and determine how soil functions. Despite their importance for understanding biological, water, and carbon stability, the properties of soil pores following long term forest conversion remain unclear in subtropical forests. We examined soil nanoscale pores in response to conversion from native forests to plantations and addressed how bacterial and fungal communities influenced soil pore properties. We found significant changes in soil pores 40 years after the conversion from native forests to Chinese fir forests or bamboo forests. The pores in mixed forest soils were similar to those in native forests, while Chinese fir forest soils contained the most pores, followed by bamboo forest soils. Chinese fir forests also had a higher surface area and volume of both total soil pores and macropores, whereas bamboo forests had a higher volume of mesopores and larger specific surface areas of the total, macro-, and mesopores. Both soil bacteria and fungi significantly influenced soil pore properties. The relative abundance of soil bacterial phyla (Planctomycetes and Verrucomicrobia), fungal phylum (Chytridiomycota), and bacterial and fungal diversity negatively affected average pore diameter. In addition, soil fungal phylum (Basidiomycota) and the total sequencing number positively affected soil mesopore volume and the specific surface areas of both total pores and mesopores. Our study demonstrated that forest conversion drove the changes in soil pores at the nanoscale. Changes in soil microbial abundance and diversity may be one of the factors that affect soil nanoscale pores, while soil fungal compositions influence soil mesopores and soil specific surface area. [ABSTRACT FROM AUTHOR]

The vertical distribution of microbial biomass carbon (MBC) is important for terrestrial soil C cycling. However, we still do not understand what controls the vertical distribution of MBC along the soil profile, which limits our ability to estimate C cycling, especially in deep soils. Here, we conducted a global synthesis of 163 observations from 82 published studies to examine the direct and indirect effects of climate, soil physicochemical properties and ecosystem type on the vertical distribution of MBC. We further explored how climate and soil physicochemical properties influence the relationship between the vertical distributions of MBC and SOC. We used decrease rates of MBC and SOC with soil depth (k MBC and k SOC) to describe the vertical distribution of MBC and SOC, with a greater decrease rate indicating a more surface-skewed distribution. k SOC in our analysis was the main predictor of k MBC. Ecosystem type and clay concentration had indirect positive and negative, respectively, effects on k MBC , through k SOC. Although forests had steeper decrease rates of SOC than agriculture and grasslands, k MBC was invariable across ecosystem types, which we attributed to the effects of k SOC on k MBC being pH-dependent. The on average lower pH of forests might have slowed down decrease rates in MBC, notwithstanding the steeper changes in SOC with depth. We show that the vertical distribution of SOC is a robust indicator of the vertical distribution of MBC; however, the vertical distribution of MBC in soils with high pH was, nonetheless, more sensitive to the decrease in SOC with depth, leading to a greater decrease in MBC along the soil profile. Our analysis offers important insights into the spatial patterns of soil microbial communities and their activities and thus improves our understanding of soil C cycling. [ABSTRACT FROM AUTHOR]

• N deposition reduces soil microbial biomass (SMB) in grasslands and forests. • N deposition increases SMB in croplands. • SMB was most negatively affected by N deposition in acid soils. • Soil acidification act as a key mechanism in the responses of SMB to N deposition. • N deposition overall increased SMB by 10.0% across the globe during 2000–2020. Soil microorganisms are key for biodiversity and ecosystem processes. Recent meta -analyses based on nitrogen (N) addition experiments reported an overall negative impact of elevated N on soil microbial biomass on a global scale. However, individual studies have reported divergent effects of N addition, ranging from strongly negative to even positive. Moreover, N deposition varies temporally and spatially worldwide. It remains uncertain how the effects of N deposition on soil microbial biomass vary across global terrestrial ecosystems over time. Through the synthesis of 374 N addition experiments across six continents, we revealed that low quantities of N increased the soil microbial biomass, but high N amounts strongly reduced it. Moreover, the N addition effects were strongly contingent on the ecosystem type, being highly negative in grasslands (−19.3 ± 6.2%, mean and 95% confidence intervals), negative in forests (−8.6 ± 4.2%), and positive in croplands (15.1 ± 12.3%). Further, the soil microbial biomass was most negatively affected by N addition in acidic soils. By combining our meta -analysis results from N addition experiments and global N deposition data, we revealed that the global soil microbial biomass increased by 10.0% in response to cumulative N deposition from 2000–2020. However, regions encompassing the Eastern U.S., Southern Brazil, Europe, and Eastern Asia, with high N deposition rates and large forested areas of acidic soils, were hotspots for microbial biomass loss. Our findings challenge the long-held notion that N deposition has universal negative impacts on soil microbial biomass. Instead, we show that the N deposition impacts on soil microbial biomass are dependent on the amounts of elevated N, ecosystem type, and soil pH, for which N-deposition-induced soil acidification acts as an internal mechanism. [ABSTRACT FROM AUTHOR]

• C:N in plants-soils-microorganisms increased under elevated CO 2. • CO 2 effects increased with CO 2 concentrations and duration. • CO 2 effects were similar across ecosystems and background climates. The stoichiometry of carbon to nitrogen ratio (C:N) plays an important role in biogeochemical cycling in terrestrial ecosystems. As time goes by, the increase in atmospheric CO 2 levels is expected; however, the impact of elevated atmospheric CO 2 on the C:N stoichiometry of soils, plants, and microorganisms remains largely unclear. The results of the meta-analysis that included 174 studies with 1009 observations demonstrated that above- and below-ground C concentrations and C:N ratio increased under elevated CO 2 regimes, whereas N concentrations decreased. Importantly, these responses were more pronounced with rising CO 2 concentrations and longer experimental durations. Moreover, the responses of C, N, and C:N to elevated CO 2 were similar across croplands, forests, and grasslands ecosystems with varying climates. Our results revealed that the C:N stoichiometry of soils, plants, and microorganisms responded consistently to the rising global CO 2 levels, which indicated that terrestrial ecosystems might have the capacity to mitigate increased atmospheric CO 2 concentrations by increasing C sequestration in plants, soils, and microorganisms in future. [ABSTRACT FROM AUTHOR]

Soil microbial extracellular polymeric substances (EPS) are important components of soil organic carbon (SOC), which are thought to promote the formation of soil structures and induce the stabilization of SOC. Earthworms may not only stimulate the production of EPS through the promotion of microbial activity and microbial biomass, but might also accelerate the decomposition of EPS via ingestion, resulting in SOC sequestration or loss. However, the exact impacts of earthworms on EPS in soils have never been evaluated. Here, a pioneering experiment involving two species of endogeic earthworms (Drawida gisti and Metaphire guillelmi) with different body sizes was conducted to investigate their impacts on soil EPS. It was found that the D. gisti and M. guillelmi earthworms reduced the soil EPS-protein concentrations by 12.6 % and 17.6 %, respectively; however, they did not affect the EPS-polysaccharide concentration. Moreover, the presence of these endogeic earthworms increased soil microbial biomass, and elevated microbial respiration, microbial metabolic quotient, dissolved organic carbon, and soil pH. The results of this study provide the first evidence for the decoupling effects of earthworms on extracellular residual components. This suggests that earthworms may alter the molecular composition of SOM, thereby influencing its persistence and soil-atmosphere carbon exchange. [Display omitted] [ABSTRACT FROM AUTHOR]

Microbes play critical roles in the cycling of nutrients in forest ecosystems. Many studies have shown that the application of organic fertilizer can alter bacterial diversity; however, the responses of soil microbial functional genes to organic fertilization (particularly biogas slurry fertilization, or in combination with biochar) have not been elucidated as yet. We examined the responses of the abundance, diversity, and composition of soil microbial functional genes under five-years of fertilization. This fertilization involved low and high application rates of biogas slurry, at 250 and 375 m3 ha−1 yr−1, respectively, or in combination with low and high application rates of biochar, at 80 and 120 t ha−1, respectively, in a coastal poplar plantation of Eastern China. Compared to the control, the normalized signal intensity of genes in the major functional categories increased significantly in response to the quantity of applied biogas slurry, but insignificantly in response to the additional application of biochar. Subsequent to the application of biogas slurry, the functional gene evenness and Shannon index increased significantly; however, the addition of biochar to the biogas slurry had insignificant effects on the α-diversity of functional genes. These functional gene compositions under five different treatments were well distinguished, primarily through the application of biogas slurry. Our results revealed that the abundance of a large number of key functional genes involved in C, N, and P cycling were effectively increased, and these changes were well correlated with most soil properties. This study reveals the molecular level mechanisms that promote plant growth following organic fertilizer application, through the enhancement of soil microbial functional gene diversity. • Biogas application increases soil microbial functional gene abundance. • Biogas application increases soil functional gene diversity. • Additonal biochar has minimal effects on soil functional genes. • Functional gene responses provide insights into fertilization effects on growth. [ABSTRACT FROM AUTHOR]