Ten years of elevated atmospheric carbon dioxide alters soil nitrogen transformations in a sheep‐grazed pasture

The increasing concentration of atmospheric carbon dioxide (CO₂) is expected to lead to enhanced competition between plants and microorganisms for the available nitrogen (N) in soil. Here, we present novel results from a ¹⁵N tracing study conducted with a sheep‐grazed pasture soil that had been under 10 years of CO₂ enrichment. Our study aimed to investigate changes in process‐specific gross N transformations in a soil previously exposed to an elevated atmospheric CO₂ (eCO₂) concentration and to examine indicators for the occurrence of progressive nitrogen limitation (PNL). Our results show that the mineralization–immobilization turnover (MIT) was enhanced under eCO₂, which was driven by the mineralization of recalcitrant organic N. The retention of N in the grassland was enhanced by increased dissimilatory NO₃^? reduction to NH₄⁺ (DNRA) and decreased NH₄⁺ oxidation. Our results indicate that heterotrophic processes become more important under eCO₂. We conclude that higher MIT of recalcitrant organic N and enhanced N retention are mechanisms that may alleviate PNL in grazed temperate grassland.     

Positive climate feedbacks of soil microbial communities in a semi‐arid grassland

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.     

Distinct responses of soil microbial communities to elevated CO2 and O3 in a soybean agro-ecosystem

The concentrations of atmospheric carbon dioxide (CO₂) and tropospheric ozone (O₃) have been rising due to human activities. However, little is known about how such increases influence soil microbial communities. We hypothesized that elevated CO₂ (eCO₂) and elevated O₃ (eO₃) would significantly affect the functional composition, structure and metabolic potential of soil microbial communities, and that various functional groups would respond to such atmospheric changes differentially. To test these hypotheses, we analyzed 96 soil samples from a soybean free-air CO₂ enrichment (SoyFACE) experimental site using a comprehensive functional gene microarray (GeoChip 3.0). The results showed the overall functional composition and structure of soil microbial communities shifted under eCO₂, eO₃ or eCO₂+eO₃. Key functional genes involved in carbon fixation and degradation, nitrogen fixation, denitrification and methane metabolism were stimulated under eCO₂, whereas those involved in N fixation, denitrification and N mineralization were suppressed under eO₃, resulting in the fact that the abundance of some eO₃-supressed genes was promoted to ambient, or eCO₂-induced levels by the interaction of eCO₂+eO₃. Such effects appeared distinct for each treatment and significantly correlated with soil properties and soybean yield. Overall, our analysis suggests possible mechanisms of microbial responses to global atmospheric change factors through the stimulation of C and N cycling by eCO₂, the inhibition of N functional processes by eO₃ and the interaction by eCO₂ and eO₃. This study provides new insights into our understanding of microbial functional processes in response to global atmospheric change in soybean agro-ecosystems.     

Elevated carbon dioxide increases soil nitrogen and phosphorus availability in a phosphorus‐limited Eucalyptus woodland

Free‐air CO₂ enrichment (FACE) experiments have demonstrated increased plant productivity in response to elevated (e)CO₂, with the magnitude of responses related to soil nutrient status. Whilst understanding nutrient constraints on productivity responses to eCO₂ is crucial for predicting carbon uptake and storage, very little is known about how eCO₂ affects nutrient cycling in phosphorus (P)‐limited ecosystems. Our study investigates eCO₂ effects on soil N and P dynamics at the EucFACE experiment in Western Sydney over an 18‐month period. Three ambient and three eCO₂ (+150 ppm) FACE rings were installed in a P‐limited, mature Cumberland Plain Eucalyptus woodland. Levels of plant accessible nutrients, evaluated using ion exchange resins, were increased under eCO₂, compared to ambient, for nitrate (+93%), ammonium (+12%) and phosphate (+54%). There was a strong seasonality to responses, particularly for phosphate, resulting in a relatively greater stimulation in available P, compared to N, under eCO₂ in spring and summer. eCO₂ was also associated with faster nutrient turnover rates in the first six months of the experiment, with higher N (+175%) and P (+211%) mineralization rates compared to ambient rings, although this difference did not persist. Seasonally dependant effects of eCO₂ were seen for concentrations of dissolved organic carbon in soil solution (+31%), and there was also a reduction in bulk soil pH (‐0.18 units) observed under eCO₂. These results demonstrate that CO₂ fertilization increases nutrient availability – particularly for phosphate – in P‐limited soils, likely via increased plant belowground investment in labile carbon and associated enhancement of microbial turnover of organic matter and mobilization of chemically bound P. Early evidence suggests that there is the potential for the observed increases in P availability to support increased ecosystem C‐accumulation under future predicted CO₂ concentrations.     

Water availability affects seasonal CO2‐induced photosynthetic enhancement in herbaceous species in a periodically dry woodland

Elevated atmospheric CO₂ (eCO₂) is expected to reduce the impacts of drought and increase photosynthetic rates via two key mechanisms: first, through decreased stomatal conductance (g_s) and increased soil water content (V_SWC) and second, through increased leaf internal CO₂ (C_i) and decreased stomatal limitations (S_lim). It is unclear if such findings from temperate grassland studies similarly pertain to warmer ecosystems with periodic water deficits. We tested these mechanisms in three important C₃ herbaceous species in a periodically dry Eucalyptus woodland and investigated how eCO₂‐induced photosynthetic enhancement varied with seasonal water availability, over a 3 year period. Leaf photosynthesis increased by 10%–50% with a 150 μmol mol^?1 increase in atmospheric CO₂ across seasons. This eCO₂‐induced increase in photosynthesis was a function of seasonal water availability, given by recent precipitation and mean daily V_SWC. The highest photosynthetic enhancement by eCO₂ (>30%) was observed during the most water‐limited period, for example, with V_SWC <0.07 in this sandy surface soil. Under eCO₂ there was neither a significant decrease in g_s in the three herbaceous species, nor increases in V_SWC, indicating no “water‐savings effect” of eCO₂. Periods of low V_SWC showed lower g_s (less than ≈ 0.12 mol m^?2 s^?1), higher relative S_lim (>30%) and decreased C_i under the ambient CO₂ concentration (aCO₂), with leaf photosynthesis strongly carboxylation‐limited. The alleviation of S_lim by eCO₂ was facilitated by increasing C_i, thus yielding a larger photosynthetic enhancement during dry periods. We demonstrated that water availability, but not eCO₂, controls g_s and hence the magnitude of photosynthetic enhancement in the understory herbaceous plants. Thus, eCO₂ has the potential to alter vegetation functioning in a periodically dry woodland understory through changes in stomatal limitation to photosynthesis, not by the “water‐savings effect” usually invoked in grasslands.     

Impacts of long‐term elevated atmospheric CO2 concentrations on communities of arbuscular mycorrhizal fungi

The ecological impacts of long‐term elevated atmospheric CO₂ (eCO₂) levels on soil microbiota remain largely unknown. This is particularly true for the arbuscular mycorrhizal (AM) fungi, which form mutualistic associations with over two‐thirds of terrestrial plant species and are entirely dependent on their plant hosts for carbon. Here, we use high‐resolution amplicon sequencing (Illumina, HiSeq) to quantify the response of AM fungal communities to the longest running (>15 years) free‐air carbon dioxide enrichment (FACE) experiment in the Northern Hemisphere (GiFACE); providing the first evaluation of these responses from old‐growth (>100 years) semi‐natural grasslands subjected to a 20% increase in atmospheric CO₂. eCO₂ significantly increased AM fungal richness but had a less‐pronounced impact on the composition of their communities. However, while broader changes in community composition were not observed, more subtle responses of specific AM fungal taxa were with populations both increasing and decreasing in abundance in response to eCO₂. Most population‐level responses to eCO₂ were not consistent through time, with a significant interaction between sampling time and eCO₂ treatment being observed. This suggests that the temporal dynamics of AM fungal populations may be disturbed by anthropogenic stressors. As AM fungi are functionally differentiated, with different taxa providing different benefits to host plants, changes in population densities in response to eCO₂ may significantly impact terrestrial plant communities and their productivity. Thus, predictions regarding future terrestrial ecosystems must consider changes both aboveground and belowground, but avoid relying on broad‐scale community‐level responses of soil microbes observed on single occasions.     

Microbial community dynamics in soil aggregates shape biogeochemical gas fluxes from soil profiles – upscaling an aggregate biophysical model

Microbial communities inhabiting soil aggregates dynamically adjust their activity and composition in response to variations in hydration and other external conditions. These rapid dynamics shape signatures of biogeochemical activity and gas fluxes emitted from soil profiles. Recent mechanistic models of microbial processes in unsaturated aggregate‐like pore networks revealed a highly dynamic interplay between oxic and anoxic microsites jointly shaped by hydration conditions and by aerobic and anaerobic microbial community abundance and self‐organization. The spatial extent of anoxic niches (hotspots) flicker in time (hot moments) and support substantial anaerobic microbial activity even in aerated soil profiles. We employed an individual‐based model for microbial community life in soil aggregate assemblies represented by 3D angular pore networks. Model aggregates of different sizes were subjected to variable water, carbon and oxygen contents that varied with soil depth as boundary conditions. The study integrates microbial activity within aggregates of different sizes and soil depth to obtain estimates of biogeochemical fluxes from the soil profile. The results quantify impacts of dynamic shifts in microbial community composition on CO₂ and N₂O production rates in soil profiles in good agreement with experimental data. Aggregate size distribution and the shape of resource profiles in a soil determine how hydration dynamics shape denitrification and carbon utilization rates. Results from the mechanistic model for microbial activity in aggregates of different sizes were used to derive parameters for analytical representation of soil biogeochemical processes across large scales of practical interest for hydrological and climate models.     

The phylogenetic composition and structure of soil microbial communities shifts in response to elevated carbon dioxide

One of the major factors associated with global change is the ever-increasing concentration of atmospheric CO₂. Although the stimulating effects of elevated CO₂ (eCO₂) on plant growth and primary productivity have been established, its impacts on the diversity and function of soil microbial communities are poorly understood. In this study, phylogenetic microarrays (PhyloChip) were used to comprehensively survey the richness, composition and structure of soil microbial communities in a grassland experiment subjected to two CO₂ conditions (ambient, 368 p.p.m., versus elevated, 560 p.p.m.) for 10 years. The richness based on the detected number of operational taxonomic units (OTUs) significantly decreased under eCO₂. PhyloChip detected 2269 OTUs derived from 45 phyla (including two from Archaea), 55 classes, 99 orders, 164 families and 190 subfamilies. Also, the signal intensity of five phyla (Crenarchaeota, Chloroflexi, OP10, OP9/JS1, Verrucomicrobia) significantly decreased at eCO₂, and such significant effects of eCO₂ on microbial composition were also observed at the class or lower taxonomic levels for most abundant phyla, such as Proteobacteria, Firmicutes, Actinobacteria, Bacteroidetes and Acidobacteria, suggesting a shift in microbial community composition at eCO₂. Additionally, statistical analyses showed that the overall taxonomic structure of soil microbial communities was altered at eCO₂. Mantel tests indicated that such changes in species richness, composition and structure of soil microbial communities were closely correlated with soil and plant properties. This study provides insights into our understanding of shifts in the richness, composition and structure of soil microbial communities under eCO₂ and environmental factors shaping the microbial community structure.     

Lower‐than‐expected CH4 emissions from rice paddies with rising CO2 concentrations

Elevated atmospheric CO₂ (eCO₂) generally increases carbon input in rice paddy soils and stimulates the growth of methane‐producing microorganisms. Therefore, eCO₂ is widely expected to increase methane (CH₄) emissions from rice agriculture, a major source of anthropogenic CH₄. Agricultural practices strongly affect CH₄ emissions from rice paddies as well, but whether these practices modulate effects of eCO₂ is unclear. Here we show, by combining a series of experiments and meta‐analyses, that whereas eCO₂ strongly increased CH₄ emissions from paddies without straw incorporation, it tended to reduce CH₄ emissions from paddy soils with straw incorporation. Our experiments also identified the microbial processes underlying these results: eCO₂ increased methane‐consuming microorganisms more strongly in soils with straw incorporation than in soils without straw, with the opposite pattern for methane‐producing microorganisms. Accounting for the interaction between CO₂ and straw management, we estimate that eCO₂ increases global CH₄ emissions from rice paddies by 3.7%, an order of magnitude lower than previous estimates. Our results suggest that the effect of eCO₂ on CH₄ emissions from rice paddies is smaller than previously thought and underline the need for judicious agricultural management to curb future CH₄ emissions.     

Soil aggregates as biogeochemical reactors and implications for soil–atmosphere exchange of greenhouse gases—A concept

Soil–atmosphere exchange significantly influences the global atmospheric abundances of carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O). These greenhouse gases (GHGs) have been extensively studied at the soil profile level and extrapolated to coarser scales (regional and global). However, finer scale studies of soil aggregation have not received much attention, even though elucidating the GHG activities at the full spectrum of scales rather than just coarse levels is essential for reducing the large uncertainties in the current atmospheric budgets of these gases. Through synthesizing relevant studies, we propose that aggregates, as relatively separate micro‐environments embedded in a complex soil matrix, can be viewed as biogeochemical reactors of GHGs. Aggregate reactivity is determined by both aggregate size (which determines the reactor size) and the bulk soil environment including both biotic and abiotic factors (which further influence the reaction conditions). With a systematic, dynamic view of the soil system, implications of aggregate reactors for soil–atmosphere GHG exchange are determined by both an individual reactor's reactivity and dynamics in aggregate size distributions. Emerging evidence supports the contention that aggregate reactors significantly influence soil–atmosphere GHG exchange and may have global implications for carbon and nitrogen cycling. In the context of increasingly frequent and severe disturbances, we advocate more analyses of GHG activities at the aggregate scale. To complement data on aggregate reactors, we suggest developing bottom‐up aggregate‐based models (ABMs) that apply a trait‐based approach and incorporate soil system heterogeneity.     

Elevated atmospheric carbon dioxide impairs the performance of root‐feeding vine weevils by modifying root growth and secondary metabolites

Predicting how insect crop pests will respond to global climate change is an important part of increasing crop production for future food security, and will increasingly rely on empirically based evidence. The effects of atmospheric composition, especially elevated carbon dioxide (eCO₂), on insect herbivores have been well studied, but this research has focussed almost exclusively on aboveground insects. However, responses of root‐feeding insects to eCO₂ are unlikely to mirror these trends because of fundamental differences between aboveground and belowground habitats. Moreover, changes in secondary metabolites and defensive responses to insect attack under eCO₂ conditions are largely unexplored for root herbivore interactions. This study investigated how eCO₂ (700 μmol mol^?1) affected a root‐feeding herbivore via changes to plant growth and concentrations of carbon (C), nitrogen (N) and phenolics. This study used the root‐feeding vine weevil, Otiorhynchus sulcatus and the perennial crop, Ribes nigrum. Weevil populations decreased by 33% and body mass decreased by 23% (from 7.2 to 5.4 mg) in eCO₂. Root biomass decreased by 16% in eCO₂, which was strongly correlated with weevil performance. While root N concentrations fell by 8%, there were no significant effects of eCO₂ on root C and N concentrations. Weevils caused a sink in plants, resulting in 8–12% decreases in leaf C concentration following herbivory. There was an interactive effect of CO₂ and root herbivory on root phenolic concentrations, whereby weevils induced an increase at ambient CO₂, suggestive of defensive response, but caused a decrease under eCO₂. Contrary to predictions, there was a positive relationship between root phenolics and weevil performance. We conclude that impaired root‐growth underpinned the negative effects of eCO₂ on vine weevils and speculate that the plant's failure to mount a defensive response at eCO₂ may have intensified these negative effects.     

Response of maize biomass and soil water fluxes on elevated CO2 and drought—From field experiments to process‐based simulations

The rising concentration of atmospheric carbon dioxide (CO₂) is known to increase the total aboveground biomass of several C3 crops, whereas C4 crops are reported to be hardly affected when water supply is sufficient. However, a free‐air carbon enrichment (FACE) experiment in Braunschweig, Germany, in 2007 and 2008 resulted in a 25% increased biomass of the C4 crop maize under restricted water conditions and elevated CO₂ (550 ppm). To project future yields of maize under climate change, an accurate representation of the effects of eCO₂ and drought on biomass and soil water conditions is essential. Current crop growth models reveal limitations in simulations of maize biomass under eCO₂ and limited water supply. We use the coupled process‐based hydrological‐plant growth model Catchment Modeling Framework‐Plant growth Modeling Framework to overcome this limitation. We apply the coupled model to the maize‐based FACE experiment in Braunschweig that provides robust data for the investigation of combined CO₂ and drought effects. We approve hypothesis I that CO₂ enrichment has a small direct‐fertilizing effect with regard to the total aboveground biomass of maize and hypothesis II that CO₂ enrichment decreases water stress and leads to higher yields of maize under restricted water conditions. Hypothesis III could partly be approved showing that CO₂ enrichment decreases the transpiration of maize, but does not raise soil moisture, while increasing evaporation. We emphasize the importance of plant‐specific CO₂ response factors derived by use of comprehensive FACE data. By now, only one FACE experiment on maize is accomplished applying different water levels. For the rigorous testing of plant growth models and their applicability in climate change studies, we call for datasets that go beyond single criteria (only yield response) and single effects (only elevated CO₂).     

Constraints to nitrogen acquisition of terrestrial plants under elevated CO2

A key part of the uncertainty in terrestrial feedbacks on climate change is related to how and to what extent nitrogen (N) availability constrains the stimulation of terrestrial productivity by elevated CO₂ (eCO₂), and whether or not this constraint will become stronger over time. We explored the ecosystem‐scale relationship between responses of plant productivity and N acquisition to eCO₂ in free‐air CO₂ enrichment (FACE) experiments in grassland, cropland and forest ecosystems and found that: (i) in all three ecosystem types, this relationship was positive, linear and strong (r² = 0.68), but exhibited a negative intercept such that plant N acquisition was decreased by 10% when eCO₂ caused neutral or modest changes in productivity. As the ecosystems were markedly N limited, plants with minimal productivity responses to eCO₂ likely acquired less N than ambient CO₂‐grown counterparts because access was decreased, and not because demand was lower. (ii) Plant N concentration was lower under eCO₂, and this decrease was independent of the presence or magnitude of eCO₂‐induced productivity enhancement, refuting the long‐held hypothesis that this effect results from growth dilution. (iii) Effects of eCO₂ on productivity and N acquisition did not diminish over time, while the typical eCO₂‐induced decrease in plant N concentration did. Our results suggest that, at the decennial timescale covered by FACE studies, N limitation of eCO₂‐induced terrestrial productivity enhancement is associated with negative effects of eCO₂ on plant N acquisition rather than with growth dilution of plant N or processes leading to progressive N limitation.     

Short‐term carbon cycling responses of a mature eucalypt woodland to gradual stepwise enrichment of atmospheric CO2 concentration

Projections of future climate are highly sensitive to uncertainties regarding carbon (C) uptake and storage by terrestrial ecosystems. The Eucalyptus Free‐Air CO₂ Enrichment (EucFACE) experiment was established to study the effects of elevated atmospheric CO₂ concentrations (eCO₂) on a native mature eucalypt woodland with low fertility soils in southeast Australia. In contrast to other FACE experiments, the concentration of CO₂ at EucFACE was increased gradually in steps above ambient (+0, 30, 60, 90, 120, and 150 ppm CO₂ above ambient of ~400 ppm), with each step lasting approximately 5 weeks. This provided a unique opportunity to study the short‐term (weeks to months) response of C cycle flux components to eCO₂ across a range of CO₂ concentrations in an intact ecosystem. Soil CO₂ efflux (i.e., soil respiration or R_soil) increased in response to initial enrichment (e.g., +30 and +60 ppm CO₂) but did not continue to increase as the CO₂ enrichment was stepped up to higher concentrations. Light‐saturated photosynthesis of canopy leaves (A_sat) also showed similar stimulation by elevated CO₂ at +60 ppm as at +150 ppm CO₂. The lack of significant effects of eCO₂ on soil moisture, microbial biomass, or activity suggests that the increase in R_soil likely reflected increased root and rhizosphere respiration rather than increased microbial decomposition of soil organic matter. This rapid increase in R_soil suggests that under eCO_2, additional photosynthate was produced, transported belowground, and respired. The consequences of this increased belowground activity and whether it is sustained through time in mature ecosystems under eCO₂ are a priority for future research.     

Soil [N] modulates soil C cycling in CO2‐fumigated tree stands: a meta‐analysis

Under elevated atmospheric CO₂ concentrations, soil carbon (C) inputs are typically enhanced, suggesting larger soil C sequestration potential. However, soil C losses also increase and progressive nitrogen (N) limitation to plant growth may reduce the CO₂ effect on soil C inputs with time. We compiled a data set from 131 manipulation experiments, and used meta‐analysis to test the hypotheses that: (1) elevated atmospheric CO₂ stimulates soil C inputs more than C losses, resulting in increasing soil C stocks; and (2) that these responses are modulated by N. Our results confirm that elevated CO₂ induces a C allocation shift towards below‐ground biomass compartments. However, the increased soil C inputs were offset by increased heterotrophic respiration (Rh), such that soil C content was not affected by elevated CO₂. Soil N concentration strongly interacted with CO₂ fumigation: the effect of elevated CO₂ on fine root biomass and –production and on microbial activity increased with increasing soil N concentration, while the effect on soil C content decreased with increasing soil N concentration. These results suggest that both plant growth and microbial activity responses to elevated CO₂ are modulated by N availability, and that it is essential to account for soil N concentration in C cycling analyses.     

Global change belowground: impacts of elevated CO2, nitrogen,and summer drought on soil food webs and biodiversity

The world's ecosystems are subjected to various anthropogenic global change agents, such as enrichment of atmospheric CO₂ concentrations, nitrogen (N) deposition, and changes in precipitation regimes. Despite the increasing appreciation that the consequences of impending global change can be better understood if varying agents are studied in concert, there is a paucity of multi‐factor long‐term studies, particularly on belowground processes. Herein, we address this gap by examining the responses of soil food webs and biodiversity to enrichment of CO₂, elevated N, and summer drought in a long‐term grassland study at Cedar Creek, Minnesota, USA (BioCON experiment). We use structural equation modeling (SEM), various abiotic and biotic explanatory variables, and data on soil microorganisms, protozoa, nematodes, and soil microarthropods to identify the impacts of multiple global change effects on drivers belowground. We found that long‐term (13‐year) changes in CO₂ and N availability resulted in modest alterations of soil biotic food webs and biodiversity via several mechanisms, encompassing soil water availability, plant productivity, and – most importantly – changes in rhizodeposition. Four years of manipulation of summer drought exerted surprisingly minor effects, only detrimentally affecting belowground herbivores and ciliate protists at elevated N. Elevated CO₂ increased microbial biomass and the density of ciliates, microarthropod detritivores, and gamasid mites, most likely by fueling soil food webs with labile C. Moreover, beneficial bottom‐up effects of elevated CO₂ compensated for detrimental elevated N effects on soil microarthropod taxa richness. In contrast, nematode taxa richness was lowest at elevated CO₂ and elevated N. Thus, enrichment of atmospheric CO₂ concentrations and N deposition may result in taxonomically and functionally altered, potentially simplified, soil communities. Detrimental effects of N deposition on soil biodiversity underscore recent reports on plant community simplification. This is of particular concern, as soils house a considerable fraction of global biodiversity and ecosystem functions.     

Elevated carbon dioxide accelerates the spatial turnover of soil microbial communities

Although elevated CO₂ (eCO₂) significantly affects the α‐diversity, composition, function, interaction and dynamics of soil microbial communities at the local scale, little is known about eCO₂ impacts on the geographic distribution of micro‐organisms regionally or globally. Here, we examined the β‐diversity of 110 soil microbial communities across six free air CO₂ enrichment (FACE) experimental sites using a high‐throughput functional gene array. The β‐diversity of soil microbial communities was significantly (P < 0.05) correlated with geographic distance under both CO₂ conditions, but declined significantly (P < 0.05) faster at eCO₂ with a slope of ?0.0250 than at ambient CO₂ (aCO₂) with a slope of ?0.0231 although it varied within each individual site, indicating that the spatial turnover rate of soil microbial communities was accelerated under eCO₂ at a larger geographic scale (e.g. regionally). Both distance and soil properties significantly (P < 0.05) contributed to the observed microbial β‐diversity. This study provides new hypotheses for further understanding their assembly mechanisms that may be especially important as global CO₂ continues to increase.     

Strong photosynthetic acclimation and enhanced water‐use efficiency in grassland functional groups persist over 21 years of CO2 enrichment,independent of nitrogen supply

Uncertainty about long‐term leaf‐level responses to atmospheric CO₂ rise is a major knowledge gap that exists because of limited empirical data. Thus, it remains unclear how responses of leaf gas exchange to elevated CO₂ (eCO₂) vary among plant species and functional groups, or across different levels of nutrient supply, and whether they persist over time for long‐lived perennials. Here, we report the effects of eCO₂ on rates of net photosynthesis and stomatal conductance in 14 perennial grassland species from four functional groups over two decades in a Minnesota Free‐Air CO₂ Enrichment experiment, BioCON. Monocultures of species belonging to C₃ grasses, C₄ grasses, forbs, and legumes were exposed to two levels of CO₂ and nitrogen supply in factorial combinations over 21 years. eCO₂ increased photosynthesis by 12.9% on average in C₃ species, substantially less than model predictions of instantaneous responses based on physiological theory and results of other studies, even those spanning multiple years. Acclimation of photosynthesis to eCO₂ was observed beginning in the first year and did not strengthen through time. Yet, contrary to expectations, the response of photosynthesis to eCO₂ was not enhanced by increased nitrogen supply. Differences in responses among herbaceous plant functional groups were modest, with legumes responding the most and C₄ grasses the least as expected, but did not further diverge over time. Leaf‐level water‐use efficiency increased by 50% under eCO₂ primarily because of reduced stomatal conductance. Our results imply that enhanced nitrogen supply will not necessarily diminish photosynthetic acclimation to eCO₂ in nitrogen‐limited systems, and that significant and consistent declines in stomatal conductance and increases in water‐use efficiency under eCO₂ may allow plants to better withstand drought.     

Low phosphorus supply constrains plant responses to elevated CO2: A meta‐analysis

Phosphorus (P) is an essential macro‐nutrient required for plant metabolism and growth. Low P availability could potentially limit plant responses to elevated carbon dioxide (eCO₂), but consensus has yet to be reached on the extent of this limitation. Here, based on data from experiments that manipulated both CO₂ and P for young individuals of woody and non‐woody species, we present a meta‐analysis of P limitation impacts on plant growth, physiological, and morphological response to eCO₂. We show that low P availability attenuated plant photosynthetic response to eCO₂ by approximately one‐quarter, leading to a reduced, but still positive photosynthetic response to eCO₂ compared to those under high P availability. Furthermore, low P limited plant aboveground, belowground, and total biomass responses to eCO₂, by 14.7%, 14.3%, and 12.4%, respectively, equivalent to an approximate halving of the eCO₂ responses observed under high P availability. In comparison, low P availability did not significantly alter the eCO₂‐induced changes in plant tissue nutrient concentration, suggesting tissue nutrient flexibility is an important mechanism allowing biomass response to eCO₂ under low P availability. Low P significantly reduced the eCO₂‐induced increase in leaf area by 14.3%, mirroring the aboveground biomass response, but low P did not affect the eCO₂‐induced increase in root length. Woody plants exhibited stronger attenuation effect of low P on aboveground biomass response to eCO₂ than non‐woody plants, while plants with different mycorrhizal associations showed similar responses to low P and eCO₂ interaction. This meta‐analysis highlights crucial data gaps in capturing plant responses to eCO₂ and low P availability. Field‐based experiments with longer‐term exposure of both CO₂ and P manipulations are critically needed to provide ecosystem‐scale understanding. Taken together, our results provide a quantitative baseline to constrain model‐based hypotheses of plant responses to eCO₂ under P limitation, thereby improving projections of future global change impacts.