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The productivity of rainforests growing on highly weathered tropical soils is expected to be limited by phosphorus availability 1 . Yet, controlled fertilization experiments have been unable to demonstrate a dominant role for phosphorus in controlling tropical forest net primary productivity. Recent syntheses have demonstrated that responses to nitrogen addition are as large as to phosphorus 2 , and adaptations to low phosphorus availability appear to enable net primary productivity to be maintained across major soil phosphorus gradients 3 . Thus, the extent to which phosphorus availability limits tropical forest productivity is highly uncertain. The majority of the Amazonia, however, is characterized by soils that are more depleted in phosphorus than those in which most tropical fertilization experiments have taken place 2 . Thus, we established a phosphorus, nitrogen and base cation addition experiment in an old growth Amazon rainforest, with a low soil phosphorus content that is representative of approximately 60% of the Amazon basin. Here we show that net primary productivity increased exclusively with phosphorus addition. After 2 years, strong responses were observed in fine root (+29%) and canopy productivity (+19%), but not stem growth. The direct evidence of phosphorus limitation of net primary productivity suggests that phosphorus availability may restrict Amazon forest responses to CO 2 fertilization 4 , with major implications for future carbon sequestration and forest resilience to climate change. Nutrient manipulation of low-phosphorus soil in an old growth Amazon rainforest shows that phosphorus availability drives forest productivity and is likely to limit the response to increasing atmospheric CO 2 concentrations.

Summary •  The consequences of increasing atmospheric carbon dioxide for long‐term adaptation of forest ecosystems remain uncertain, with virtually no studies undertaken at the genetic level. A global analysis using cDNA microarrays was conducted following 6 yr exposure of Populus × euramericana (clone I‐214) to elevated [CO2] in a FACE (free‐air CO2 enrichment) experiment. •  Gene expression was sensitive to elevated [CO2] but the response depended on the developmental age of the leaves, and < 50 transcripts differed significantly between different CO2 environments. For young leaves most differentially expressed genes were upregulated in elevated [CO2], while in semimature leaves most were downregulated in elevated [CO2]. •  For transcripts related only to the small subunit of Rubisco, upregulation in LPI 3 and downregulation in LPI 6 leaves in elevated CO2 was confirmed by anova. Similar patterns of gene expression for young leaves were also confirmed independently across year 3 and year 6 microarray data, and using real‐time RT–PCR. •  This study provides the first clues to the long‐term genetic expression changes that may occur during long‐term plant response to elevated CO2.

Dietary deficiencies of zinc and iron are a major global public health problem. An estimated two billion people suffer these deficiencies causing a loss of 63 million life years annually. Most of these people depend upon grains and legumes as their primary dietary source of zinc and iron. This manuscript reports findings from the analysis of 540 pairs of crop samples grown at contemporary and elevated [CO2] from six different FACE experiments involving six food crops. We tested the nutrient concentrations of the edible portions of rice (Oryza sativa, 18 cultivars), wheat (Triticum aestivum, 8 cultivars), maize (Zea mays, 2 cultivars), soybeans (Glycine max, 7 cultivars), field peas (Pisum sativum, 4 cultivars) and sorghum (Sorghum bicolor, 1 cultivar). In all six experiments, the elevated [CO2] was in the range of 550-584 ppm. Each crop sample grown at elevated [CO2] was paired with an identical cultivar grown under the same conditions but at contemporary [CO2]. Our main outcomes were fractional changes in concentrations of the nutrients between samples grown at elevated and contemporary [CO2] levels, estimated using a linear mixed effects statistical model. We found that elevated [CO2] was associated with significant decreases in the concentrations of zinc and iron in all C3 grasses and legumes. For example, in wheat grains grown at elevated [CO2] compared with contemporary [CO2], zinc decreased 9.6% and iron decreased 5.2%. We also found that elevated [CO2] was associated with lower protein in C3 grasses with a 6.5% decrease in wheat grains and a 7.9% (95% CI: -8.9, -6.9) decrease in rice grains. Elevated [CO2] showed no significant effect on protein in C3 legumes or C4 crops. Response differences between cultivars suggest breeding crops for reduced sensitivity to elevations in atmospheric [CO2]. Such breeding efforts may partly address the new challenges to global health that these findings highlight.

The rhizosphere priming effect (RPE) is a mechanism by which plants interact with soil functions. The large impact of the RPE on soil organic matter decomposition rates (from 50% reduction to 380% increase) warrants similar attention to that being paid to climatic controls on ecosystem functions. Furthermore, global increases in atmospheric CO2 concentration and surface temperature can significantly alter the RPE. Our analysis using a game theoretic model suggests that the RPE may have resulted from an evolutionarily stable mutualistic association between plants and rhizosphere microbes. Through model simulations based on microbial physiology, we demonstrate that a shift in microbial metabolic response to different substrate inputs from plants is a plausible mechanism leading to positive or negative RPEs. In a case study of the Duke Free-Air CO2 Enrichment experiment, performance of the PhotoCent model was significantly improved by including an RPE-induced 40% increase in soil organic matter decomposition rate for the elevated CO2 treatment – demonstrating the value of incorporating the RPE into future ecosystem models. Overall, the RPE is emerging as a crucial mechanism in terrestrial ecosystems, which awaits substantial research and model development.

Stimulation of terrestrial plant production by rising CO₂ concentration is projected to reduce the airborne fraction of anthropogenic CO₂ emissions. Coupled climate–carbon cycle models are sensitive to this negative feedback on atmospheric CO₂, but model projections are uncertain because of the expectation that feedbacks through the nitrogen (N) cycle will reduce this so-called CO₂ fertilization effect. We assessed whether N limitation caused a reduced stimulation of net primary productivity (NPP) by elevated atmospheric CO₂ concentration over 11 y in a free-air CO₂ enrichment (FACE) experiment in a deciduous Liquidambar styraciflua (sweetgum) forest stand in Tennessee. During the first 6 y of the experiment, NPP was significantly enhanced in forest plots exposed to 550 ppm CO₂ compared with NPP in plots in current ambient CO₂, and this was a consistent and sustained response. However, the enhancement of NPP under elevated CO₂ declined from 24% in 2001–2003 to 9% in 2008. Global analyses that assume a sustained CO₂ fertilization effect are no longer supported by this FACE experiment. N budget analysis supports the premise that N availability was limiting to tree growth and declining over time—an expected consequence of stand development, which was exacerbated by elevated CO₂. Leaf- and stand-level observations provide mechanistic evidence that declining N availability constrained the tree response to elevated CO₂; these observations are consistent with stand-level model projections. This FACE experiment provides strong rationale and process understanding for incorporating N limitation and N feedback effects in ecosystem and global models used in climate change assessments.

Global climate change accompanied by continuous increases in atmospheric carbon dioxide (CO2) concentration and temperature affects the growth and yield of important crops. The present study investigated the effect of elevated temperature and CO2 concentrations on the growth, yield, and photosynthesis of potato (Solanum tuberosum L. cv. Superior) crops using Korean Soil-Plant-Atmosphere-Research chambers that allow the regulation of temperature and CO2 concentration under daylight conditions. Based on the average temperature from 1991 to 2010 in the Jeonju area, South Korea, potato plants were exposed to four different conditions: ambient weather (400 μmol mol-1, aCaT), elevated temperature (+4°C, aCeT), elevated CO2 concentration (800 μmol mol-1, eCaT), and concurrently elevated CO2 concentration and temperature (eCeT). Under aCeT conditions, the temperature exceeded the optimal growth temperature range towards the late growth phase that decreased stomatal conductance and canopy net photosynthetic rate and subsequently reduced biomass and tuber yield. Stomatal conductance and chlorophyll concentration were lower under eCaT conditions than under aCaT conditions, whereas late-growth phase biomass and tuber yield were greater. Compared to other conditions, eCeT yielded a distinct increase in growth and development and canopy net photosynthetic rate during tuber initiation and bulking. Consequently, biomass and canopy net photosynthesis increased, and tuber yield increased by 20.3%, which could be attributed to the increased tuber size, rather than increased tuber number. Elevated CO2 reduced chlorophyll, magnesium, and phosphorus concentrations; reducing nitrogen concentration (by approximately 39.7%) increased the C:N ratio. The data indicate that future climate conditions will likely change nutrient concentration and quality of crops. The present study shows that while elevated temperature may negatively influence the growth and yield of potato crops, especially towards the late-growth phase, the concurrent and appropriate elevation of CO2 and temperature could promote balanced development of source and sink organs and positively effect potato productivity and quality.

The plant enzyme Rubisco is the main enzyme converting atmospheric carbon dioxide into biological compounds, however, this enzymatic process is inefficient in vascular plants; this study demonstrates that tobacco plants can be engineered to fix carbon with a faster cyanobacterial Rubisco, thus potentially improving plant photosynthesis. Introducing algal Rubisco into a crop plant Rubisco — a major enzyme assimilating atmospheric CO 2 into the biosphere — is an important target for efforts to improve the photosynthetic efficiency of plants. These authors successfully engineered tobacco plants containing a functioning Rubisco from a cyanobacterium. The cyanobacterial (photosynthetic blue–green algae) enzyme has a greater catalytic rate than any 'C3' plant. The lines generated here pave the way for future addition of the remaining components of the cyanobacterial CO 2 -concentrating mechanism, an important step towards enhancing photosynthetic efficiency and improving crop yields. In photosynthetic organisms, d -ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) is the major enzyme assimilating atmospheric CO 2 into the biosphere 1 . Owing to the wasteful oxygenase activity and slow turnover of Rubisco, the enzyme is among the most important targets for improving the photosynthetic efficiency of vascular plants 2 , 3 . It has been anticipated that introducing the CO 2 -concentrating mechanism (CCM) from cyanobacteria into plants could enhance crop yield 4 , 5 , 6 . However, the complex nature of Rubisco’s assembly has made manipulation of the enzyme extremely challenging, and attempts to replace it in plants with the enzymes from cyanobacteria and red algae have not been successful 7 , 8 . Here we report two transplastomic tobacco lines with functional Rubisco from the cyanobacterium Synechococcus elongatus PCC7942 (Se7942). We knocked out the native tobacco gene encoding the large subunit of Rubisco by inserting the large and small subunit genes of the Se7942 enzyme, in combination with either the corresponding Se7942 assembly chaperone, RbcX, or an internal carboxysomal protein, CcmM35, which incorporates three small subunit-like domains 9 , 10 . Se7942 Rubisco and CcmM35 formed macromolecular complexes within the chloroplast stroma, mirroring an early step in the biogenesis of cyanobacterial β-carboxysomes 11 , 12 . Both transformed lines were photosynthetically competent, supporting autotrophic growth, and their respective forms of Rubisco had higher rates of CO 2 fixation per unit of enzyme than the tobacco control. These transplastomic tobacco lines represent an important step towards improved photosynthesis in plants and will be valuable hosts for future addition of the remaining components of the cyanobacterial CCM, such as inorganic carbon transporters and the β-carboxysome shell proteins 4 , 5 , 6 .

A grassland warming and CO 2 enrichment experiment shows that temperature increase brings forward the growing season of early leafing species, but does not affect or delays senescence in late species, the latter enhanced by elevated CO 2 . Growing seasons lengthened by high CO 2 Lengthening plant growing seasons in temperate and polar regions in recent years have been attributed to rising temperatures, but the effect on individual species can be to bring the growing season forward without actually lengthening it. In a series of warming and CO 2 enrichment experiments in temperate grasslands in Wyoming, these authors show that temperature increase brings forward the growing season of early leafing species, whereas late-season species extend their life cycle, leading to a longer growing season. The latter effect is enhanced by elevated CO 2 , particularly when water availability is limited. Observations of a longer growing season through earlier plant growth in temperate to polar regions have been thought to be a response to climate warming 1 , 2 , 3 , 4 , 5 . However, data from experimental warming studies indicate that many species that initiate leaf growth and flowering earlier also reach seed maturation and senesce earlier, shortening their active and reproductive periods 6 , 7 , 8 , 9 , 10 . A conceptual model to explain this apparent contradiction 11 , and an analysis of the effect of elevated CO 2 —which can delay annual life cycle events 12 , 13 , 14 —on changing season length, have not been tested. Here we show that experimental warming in a temperate grassland led to a longer growing season through earlier leaf emergence by the first species to leaf, often a grass, and constant or delayed senescence by other species that were the last to senesce, supporting the conceptual model. Elevated CO 2 further extended growing, but not reproductive, season length in the warmed grassland by conserving water, which enabled most species to remain active longer. Our results suggest that a longer growing season, especially in years or biomes where water is a limiting factor, is not due to warming alone, but also to higher atmospheric CO 2 concentrations that extend the active period of plant annual life cycles.

The first generation of forest free‐air CO₂ enrichment (FACE) experiments has successfully provided deeper understanding about how forests respond to an increasing CO₂ concentration in the atmosphere. Located in aggrading stands in the temperate zone, they have provided a strong foundation for testing critical assumptions in terrestrial biosphere models that are being used to project future interactions between forest productivity and the atmosphere, despite the limited inference space of these experiments with regards to the range of global ecosystems. Now, a new generation of FACE experiments in mature forests in different biomes and over a wide range of climate space and biodiversity will significantly expand the inference space. These new experiments are: EucFACE in a mature Eucalyptus stand on highly weathered soil in subtropical Australia; AmazonFACE in a highly diverse, primary rainforest in Brazil; BIFoR‐FACE in a 150‐yr‐old deciduous woodland stand in central England; and SwedFACE proposed in a hemiboreal, Pinus sylvestris stand in Sweden. We now have a unique opportunity to initiate a model–data interaction as an integral part of experimental design and to address a set of cross‐site science questions on topics including responses of mature forests; interactions with temperature, water stress, and phosphorus limitation; and the influence of biodiversity.

Recent metaanalyses suggest biodiversity loss affects the functioning of ecosystems to a similar extent as other global environmental change agents. However, the abundance and functioning of soil organisms have been hypothesized to be much less responsive to such changes, particularly in plant diversity, than aboveground variables, although tests of this hypothesis are extremely rare. We examined the responses of soil food webs (soil microorganisms, nematodes, microarthropods) to 13-y manipulation of multiple environmental factors that are changing at global scales—specifically plant species richness, atmospheric CO₂, and N deposition—in a grassland experiment in Minnesota. Plant diversity was a strong driver of the structure and functioning of soil food webs through several bottom-up (resource control) effects, whereas CO₂ and N only had modest effects. We found few interactions between plant diversity and CO₂ and N, likely because of weak interactive effects of those factors on resource availability (e.g., root biomass). Plant diversity effects likely were large because high plant diversity promoted the accumulation of soil organic matter in the site's sandy, organic matter-poor soils. Plant diversity effects were not explained by the presence of certain plant functional groups. Our results underline the prime importance of plant diversity loss cascading to soil food webs (density and diversity of soil organisms) and functions. Because the present results suggest prevailing plant diversity effects and few interactions with other global change drivers, protecting plant diversity may be of high priority to maintain the biodiversity and functioning of soils in a changing world.