Global mean temperature is predicted to increase by 2-7 1C and precipitation to change across the globe by the end of this century. To quantify climate effects on ecosystem processes, a number of climate change experiments have been established around the world in various ecosystems. Despite these efforts, general responses of terrestrial ecosystems to changes in temperature and precipitation, and especially to their combined effects, remain unclear. We used metaanalysis to synthesize ecosystem-level responses to warming, altered precipitation, and their combination. We focused on plant growth and ecosystem carbon (C) balance, including biomass, net primary production (NPP), respiration, net ecosystem exchange (NEE), and ecosystem photosynthesis, synthesizing results from 85 studies. We found that experimental warming and increased precipitation generally stimulated plant growth and ecosystem C fluxes, whereas decreased precipitation had the opposite effects. For example, warming significantly stimulated total NPP, increased ecosystem photosynthesis, and ecosystem respiration. Experimentally reduced precipitation suppressed aboveground NPP (ANPP) and NEE, whereas supplemental precipitation enhanced ANPP and NEE. Plant productivity and ecosystem C fluxes generally showed higher sensitivities to increased precipitation than to decreased precipitation. Interactive effects of warming and altered precipitation tended to be smaller than expected from additive, single-factor effects, though low statistical power limits the strength of these conclusions. New experiments with combined temperature and precipitation manipulations are needed to conclusively determine the importance of temperature-precipitation interactions on the C balance of terrestrial ecosystems under future climate conditions.

We determined the proximate chemical composition as well as the construction costs of leaves of 27 species, grown at ambient and at a twice-ambient partial pressure of atmospberic CO2. Tbese species comprised wild and agricultural berbaceous plants as well as tree seedlings. Botb average responses across species and tbe range in response were considered. Expressed on a total dry weigbt basis, tbe main cbange in cbemical composition due to CO2 was tbe accumulation of total non-structural carbobydrates (TNC). To a lesser extent, decreases were found for organic N compounds and minerals. Hardly any cbange was observed for total structural carbobydrates (cellulose plus bemicellulose), lignin and lipids. Wben expressed on a TNC-free basis, decreases in organic N compounds and minerals were still present. On tbis basis, tbere was also an increase in tbe concentration of soluble pbenolics.In terms of glucose required for biosynthesis, tbe increase in costs for one cbemical compound -TNCwas balanced by a decrease in tbe costs for organic N compounds. Tberefore, tbe construction costs, tbe total amount of glucose required to produce 1 g of leaf, were ratber similar for tbe two CO2 treatments; on average a small decrease of 3% was found. Tbis decrease was attributable to a decrease of up to 30% in tbe growtb respiration coefficient, tbe total CO2 respired [mainly for N AD(P)H and ATP] in tbe process of constructing 1 g of biomass. Tbe main reasons for tbis reduction were tbe decrease in organic N compounds and tbe increase in TNC.

The Rehai and Ruidian geothermal fields, located in Tengchong County, Yunnan Province, China, host a variety of geochemically distinct hot springs. In this study, we report a comprehensive, cultivation-independent census of microbial communities in 37 samples collected from these geothermal fields, encompassing sites ranging in temperature from 55.1 to 93.6°C, in pH from 2.5 to 9.4, and in mineralogy from silicates in Rehai to carbonates in Ruidian. Richness was low in all samples, with 21–123 species-level OTUs detected. The bacterial phylum Aquificae or archaeal phylum Crenarchaeota were dominant in Rehai samples, yet the dominant taxa within those phyla depended on temperature, pH, and geochemistry. Rehai springs with low pH (2.5–2.6), high temperature (85.1–89.1°C), and high sulfur contents favored the crenarchaeal order Sulfolobales, whereas those with low pH (2.6–4.8) and cooler temperature (55.1–64.5°C) favored the Aquificae genus Hydrogenobaculum. Rehai springs with neutral-alkaline pH (7.2–9.4) and high temperature (>80°C) with high concentrations of silica and salt ions (Na, K, and Cl) favored the Aquificae genus Hydrogenobacter and crenarchaeal orders Desulfurococcales and Thermoproteales. Desulfurococcales and Thermoproteales became predominant in springs with pH much higher than the optimum and even the maximum pH known for these orders. Ruidian water samples harbored a single Aquificae genus Hydrogenobacter, whereas microbial communities in Ruidian sediment samples were more diverse at the phylum level and distinctly different from those in Rehai and Ruidian water samples, with a higher abundance of uncultivated lineages, close relatives of the ammonia-oxidizing archaeon “Candidatus Nitrosocaldus yellowstonii”, and candidate division O1aA90 and OP1. These differences between Ruidian sediments and Rehai samples were likely caused by temperature, pH, and sediment mineralogy. The results of this study significantly expand the current understanding of the microbiology in Tengchong hot springs and provide a basis for comparison with other geothermal systems around the world.

Bacteria grow and transform elements at different rates, and as yet, quantifying this variation in the environment is difficult. Determining isotope enrichment with fine taxonomic resolution after exposure to isotope tracers could help, but there are few suitable techniques. We propose a modification to stable isotope probing (SIP) that enables the isotopic composition of DNA from individual bacterial taxa after exposure to isotope tracers to be determined. In our modification, after isopycnic centrifugation, DNA is collected in multiple density fractions, and each fraction is sequenced separately. Taxon-specific density curves are produced for labeled and nonlabeled treatments, from which the shift in density for each individual taxon in response to isotope labeling is calculated. Expressing each taxon's density shift relative to that taxon's density measured without isotope enrichment accounts for the influence of nucleic acid composition on density and isolates the influence of isotope tracer assimilation. The shift in density translates quantitatively to isotopic enrichment. Because this revision to SIP allows quantitative measurements of isotope enrichment, we propose to call it quantitative stable isotope probing (qSIP). We demonstrated qSIP using soil incubations, in which soil bacteria exhibited strong taxonomic variations in 18

Rising temperatures are expected to reduce global soil carbon (C) stocks, driving a positive feedback to climate change 1-3. However, the mechanisms underlying this prediction are not well understood, including how temperature affects microbial enzyme kinetics, growth efficiency (MGE), and turnover 4,5. Here, in a laboratory study, we show that microbial turnover accelerates with warming and, along with enzyme kinetics, determines the response of microbial respiration to temperature change. In contrast, MGE, which is generally thought to decline with warming 6-8 , showed no temperature sensitivity. Using a microbial-enzyme model, we show that temperature-sensitive microbial turnover promotes soil C accumulation with warming, in contrast to reduced soil C predicted by traditional biogeochemical models. Furthermore, the effect of increased microbial turnover differs from the effects of reduced MGE, causing larger increases in soil C stocks. Our results demonstrate that the response of soil C to warming is affected by changes in microbial turnover. This control should be included in the next generation of models to improve prediction of soil C feedbacks to warming.

No abstract

There is abundant evidence that a reduction in mitochondrial respiration of plants occurs when atmospheric CO 2 (C a ) is increased. Recent reviews suggest that doubling the present C a will reduce the respiration rate [per unit dry weight (DW)] by 15 to 18%. The effect has two components: an immediate, reversible effect observed in leaves, stems, and roots of plants as well as soil microbes, and an irreversible effect which occurs as a consequence of growth in elevated C a and appears to be specific to C 3 species. The direct effect has been correlated with inhibition of certain respiratory enzymes, namely cytochromec-oxidase and succinate dehydrogenase, and the indirect or acclimation effect may be related to changes in tissue composition. Although no satisfactory mechanisms to explain these effects have been demonstrated, plausible mechanisms have been proposed and await experimental testing. These are carbamylation of proteins and direct inhibition of enzymes of respiration. A reduction of foliar respiration of 15% by doubling present ambient C a would represent 3 Gt of carbon per annum in the global carbon budget.Key-words: acclimation to rising CO 2 ; dark respiration; global carbon cycle; rising CO 2 .Abbreviations: ATP, adenosine triphosphate; K m , MichaelisMenton coefficient; C a , concentration of CO 2 in the air (µmol mol -1 ); NAD, oxidized nicotin adenine dinucleotide; NADH, reduced nicotin adenine dinucleotide; NADP, oxidized nicotin adenine phosphate dinucleotide; NADPH, reduced nicotine adenine phosphate dinucleotide; R, rate of respiration per unit DW [µmol g DW -1 ], Rubisco, ribulose-1,5-bisphosphate carboxylase/oxygenase; V c , max , maximum in vivo rate of carboxylation at Rubisco (µmol m -2 s -1 ).

24Bacteria grow and transform elements at different rates, yet quantifying this variation in the 25 environment is difficult. Determining isotope enrichment with fine taxonomic resolution after 26 exposure to isotope tracers could help, but there are few suitable techniques. We propose a 27 modification to Stable Isotope Probing (SIP) that enables determining the isotopic composition 28 of DNA from individual bacterial taxa after exposure to isotope tracers. In our modification, after 29 isopycnic centrifugation, DNA is collected in multiple density fractions, and each fraction is 30 sequenced separately. Taxon specific density curves are produced for labeled and non-labeled 31 treatments, from which the shift in density for each individual taxon in response to isotope 32 labeling is calculated. Expressing each taxon's density shift relative to that taxon's density 33 measured without isotope enrichment accounts for the influence of nucleic acid composition on 34 density and isolates the influence of isotope tracer assimilation.