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In order to plan for global changing climate experiments are being conducted in many countries, but few have monitored the effects of the climate change treatments (warming, elevated CO2) on the experimental plot microclimate. During three years of an eight year study with year-round feedback-controlled infra-red heater warming (1.5/3.0°C day/night) and growing season free-air CO2 enrichment (600 ppm) in the mixed-grass prairie of Wyoming, USA, we monitored soil, leaf, canopy-air, above-canopy-air temperatures and relative humidity of control and treated experimental plots and evaluated ecologically important temperature differentials. Leaves were warmed somewhat less than the target settings (1.1 & 1.5°C day/night) but soil was warmed more creating an average that matched the target settings extremely well both during the day and night plus the summer and winter. The site typically has about 50% bare or litter covered soil, therefore soil heat transfer is more critical than in dense canopy ecosystems. The Wyoming site commonly has strong winds (5 ms(-1) average) and significant daily and seasonal temperature fluctuations (as much as 30°C daily) but the warming system was nearly always able to maintain the set temperatures regardless of abiotic variation. The within canopy-air was only slightly warmed and above canopy-air was not warmed by the system, therefore convective warming was minor. Elevated CO2 had no direct effect nor interaction with the warming treatment on microclimate. Relative humidity within the plant canopy was only slightly reduced by warming. Soil water content was reduced by warming but increased by elevated CO2. This study demonstrates the importance of monitoring the microclimate in manipulative field global change experiments so that critical physiological and ecological conclusions can be determined. Highly variable energy demand fluctuations showed that passive IR heater warming systems will not maintain desired warming for much of the time.

Future ecosystem properties of grasslands will be driven largely by belowground biomass responses to climate change, which are challenging to understand due to experimental and technical constraints. We used a multi-faceted approach to explore single and combined impacts of elevated CO2 and warming on root carbon (C) and nitrogen (N) dynamics in a temperate, semiarid, native grassland at the Prairie Heating and CO2 Enrichment experiment. To investigate the indirect, moisture mediated effects of elevated CO2, we included an irrigation treatment. We assessed root standing mass, morphology, residence time and seasonal appearance/disappearance of community-aggregated roots, as well as mass and N losses during decomposition of two dominant grass species (a C3 and a C4). In contrast to what is common in mesic grasslands, greater root standing mass under elevated CO2 resulted from increased production, unmatched by disappearance. Elevated CO2 plus warming produced roots that were longer, thinner and had greater surface area, which, together with greater standing biomass, could potentially alter root function and dynamics. Decomposition increased under environmental conditions generated by elevated CO2, but not those generated by warming, likely due to soil desiccation with warming. Elevated CO2, particularly under warming, slowed N release from C4-but not C3-roots, and consequently could indirectly affect N availability through treatment effects on species composition. Elevated CO2 and warming effects on root morphology and decomposition could offset increased C inputs from greater root biomass, thereby limiting future net C accrual in this semiarid grassland.

Atmospheric concentrations of methane (CH4 ) and nitrous oxide (N2 O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2 O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2 O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2 O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2 O emission and uptake occurred at our site with some years showing cumulative N2 O emission and other years showing cumulative N2 O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2 O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2 O expressed in CO2 -equivalents., (© 2013 Blackwell Publishing Ltd.)

Climate change is predicted to increase both drought frequency and duration, and when coupled with substantial warming, will establish a new hydroclimatological model for many regions. Large-scale, warm droughts have recently occurred in North America, Africa, Europe, Amazonia and Australia, resulting in major effects on terrestrial ecosystems, carbon balance and food security. Here we compare the functional response of above-ground net primary production to contrasting hydroclimatic periods in the late twentieth century (1975-1998), and drier, warmer conditions in the early twenty-first century (2000-2009) in the Northern and Southern Hemispheres. We find a common ecosystem water-use efficiency (WUE(e): above-ground net primary production/evapotranspiration) across biomes ranging from grassland to forest that indicates an intrinsic system sensitivity to water availability across rainfall regimes, regardless of hydroclimatic conditions. We found higher WUE(e) in drier years that increased significantly with drought to a maximum WUE(e) across all biomes; and a minimum native state in wetter years that was common across hydroclimatic periods. This indicates biome-scale resilience to the interannual variability associated with the early twenty-first century drought--that is, the capacity to tolerate low, annual precipitation and to respond to subsequent periods of favourable water balance. These findings provide a conceptual model of ecosystem properties at the decadal scale applicable to the widespread altered hydroclimatic conditions that are predicted for later this century. Understanding the hydroclimatic threshold that will break down ecosystem resilience and alter maximum WUE(e) may allow us to predict land-surface consequences as large regions become more arid, starting with water-limited, low-productivity grasslands.

In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO₂ might ameliorate these effects by improving plant water-use efficiency. We examined the effects of CO₂ and water availability on seedlings of two native (Populus deltoides spp. monilifera, Salix exigua) and three exotic (Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO₂-controlled glasshouse, using 1-m-deep pots with different water-table decline rates. Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO₂ increased biomass by 15%, with similar effects on natives and exotics. Elevated CO₂ increased intrinsic water-use efficiency (Δ¹³C(leaf) ), but did not increase biomass more in drier treatments than wetter treatments. The moderate positive effects of elevated CO₂ on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO₂, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species., (No claim to original US government works. New Phytologist © 2012 New Phytologist Trust.)

Nitrogen (N) and phosphorus (P) are essential nutrients for primary producers and decomposers in terrestrial ecosystems. Although climate change affects terrestrial N cycling with important feedbacks to plant productivity and carbon sequestration, the impacts of climate change on the relative availability of N with respect to P remain highly uncertain. In a semiarid grassland in Wyoming, USA, we studied the effects of atmospheric CO(2) enrichment (to 600 ppmv) and warming (1.5/3.0°C above ambient temperature during the day/night) on plant, microbial and available soil pools of N and P. Elevated CO(2) increased P availability to plants and microbes relative to that of N, whereas warming reduced P availability relative to N. Across years and treatments, plant N : P ratios varied between 5 and 18 and were inversely related to soil moisture. Our results indicate that soil moisture is important in controlling P supply from inorganic sources, causing reduced P relative to N availability during dry periods. Both wetter soil conditions under elevated CO(2) and drier conditions with warming can further alter N : P. Although warming may alleviate N constraints under elevated CO(2) , warming and drought can exacerbate P constraints on plant growth and microbial activity in this semiarid grassland., (© 2012 The Authors. New Phytologist © 2012 New Phytologist Trust.)

Climate change, disturbance, and plant invasion threaten grassland ecosystems, but their combined and interactive effects are poorly understood. Here, we examine how the combination of disturbance and plant invasion influences the sensitivity of mixed‐grass prairie to elevated carbon dioxide (eCO2) and warming. We established subplots of intact prairie and disturbed/invaded prairie within a free‐air CO2 enrichment (to 600 ppmv) by infrared warming (+1.5°C day, 3°C night) experiment and followed plant and soil responses for 5 years. Elevated CO2 initially led to moderate increases in biomass and plant diversity in both intact and disturbed/invaded prairie, but these effects shifted due to strong eCO2 responses of the invasive forb Centaurea diffusa. In the final 3 years, biomass responses to eCO2 in disturbed/invaded prairie were 10 times as large as those in intact prairie (+186% vs. +18%), resulting in reduced rather than increased plant diversity (−17% vs. +10%). At the same time, warming interacted with disturbance/invasion and year, reducing the rate of topsoil carbon recovery following disturbance. The strength of these interactions demonstrates the need to incorporate disturbance into predictions of climate change effects. In contrast to expectations from studies in intact ecosystems, eCO2 may threaten plant diversity in ecosystems subject to soil disturbance and invasion. [ABSTRACT FROM AUTHOR]

The structure and function of grassland ecosystems can be altered by a changing climate, including higher temperature and elevated atmospheric CO2 concentration. Previous studies suggest that there is no consistent trend in seed germination and seedling recruitment as affected by these conditions. We collected seeds of two native and two invasive species over 6 years from a field study with elevated CO2 (600 p.p.m.) and temperature (1.5/3.0°C day/night) on the mixed-grass prairie of Wyoming, USA. Seed fill, viability and mass were evaluated and germination tests were conducted under alternating temperatures in growth chambers. Thermal time requirements to reach 50% germination (θ50) and base temperatures (T b) for germination were determined using thermal time models. Climate change conditions had limited effects on seed fill, viability and mass. The combination of CO2 enrichment and warming increased germination of Bouteloua gracilis. Centaurea diffusa and Linaria dalmatica , two invasive species in this study, had the lowest θ50 and T b required for germination among all the species studied. Although final germination percentages of these invasive species were not affected by treatments, previous studies reported increased seed production under future climate conditions, indicating that they could be more invasive at the regeneration stage in the future. We conclude that projected future temperature increases will have little effect on seed reproductive traits of native species. In addition, the distribution and abundance of B. gracilis and invasive species may be favoured by global climate change due to enhanced germination or seed production traits caused by elevated parental CO2 and temperature conditions. [ABSTRACT FROM AUTHOR]

Global changes that alter soil water availability may have profound effects on semiarid ecosystems. Although both elevated CO2 (eCO2) and warming can alter water availability, often in opposite ways, few studies have measured their combined influence on the amount, timing, and temporal variability of soil water. Here, we ask how free air CO2 enrichment (to 600 ppmv) and infrared warming (+ 1.5 °C day, + 3 °C night) effects on soil water vary within years and across wet-dry periods in North American mixed-grass prairie. We found that eCO2 and warming interacted to influence soil water and that those interactions varied by season. In the spring, negative effects of warming on soil water largely offset positive effects of eCO2. As the growing season progressed, however, warming reduced soil water primarily (summer) or only (autumn) in plots treated with eCO2. These interactions constrained the combined effect of eCO2 and warming on soil water, which ranged from neutral in spring to positive in autumn. Within seasons, eCO2 increased soil water under drier conditions, and warming decreased soil water under wetter conditions. By increasing soil water under dry conditions, eCO2 also reduced temporal variability in soil water. These temporal patterns explain previously observed plant responses, including reduced leaf area with warming in summer, and delayed senescence with eCO2 plus warming in autumn. They also suggest that eCO2 and warming may favor plant species that grow in autumn, including winter annuals and C3 graminoids, and species able to remain active under the dry conditions moderated by eCO2. [ABSTRACT FROM AUTHOR]

Abstract: Temporal variation in soil nitrogen (N) availability affects growth of grassland communities that differ in their use and reuse of N. In a 7‐year‐long climate change experiment in a semi‐arid grassland, the temporal stability of plant biomass production varied with plant N turnover (reliance on externally acquired N relative to internally recycled N). Species with high N turnover were less stable in time compared to species with low N turnover. In contrast, N turnover at the community level was positively associated with asynchrony in biomass production, which in turn increased community temporal stability. Elevated CO2 and summer irrigation, but not warming, enhanced community N turnover and stability, possibly because treatments promoted greater abundance of species with high N turnover. Our study highlights the importance of plant N turnover for determining the temporal stability of individual species and plant communities affected by climate change. [ABSTRACT FROM AUTHOR]

Vegetation greenness, detected using digital photography, is useful for monitoring phenology of plant growth, carbon uptake and water loss at the ecosystem level. Assessing ecosystem phenology by greenness is especially useful in spatially extensive, water-limited ecosystems such as the grasslands of the western United States, where productivity is moisture dependent and may become increasingly vulnerable to future climate change., We used repeat photography and a novel means of quantifying greenness in digital photographs to assess how the individual and combined effects of warming and elevated CO2 impact ecosystem phenology (greenness and plant cover) in a semi-arid grassland over an 8-year period., Climate variability within and among years was the proximate driver of ecosystem phenology. Individual and combined effects of warming and elevated CO2 were significant at times, but mediated by variation in both intra- and interannual precipitation. Specifically, warming generally enhanced plant cover and greenness early in the growing season but often had a negative effect during the middle of the summer, offsetting the early season positive effects. The individual effects of elevated CO2 on plant cover and greenness were generally neutral., Opposing seasonal variations in the effects of warming and less so elevated CO2 cancelled each other out over an entire growing season, leading to no net effect of treatments on annual accumulation of greenness. The main effect of elevated CO2 dampened quickly, but warming continued to affect plant cover and plot greenness throughout the experiment. The combination of warming and elevated CO2 had a generally positive effect on greenness, especially early in the growing season and in later years of the experiment, enhanced annual greenness accumulation. However, interannual precipitation variation had larger effect on greenness, with two to three times greater greenness in wet years than in dry years., Synthesis. Seasonal variation in timing and amount of precipitation governs grassland phenology, greenness and the potential for carbon uptake. Our results indicate that concurrent changes in precipitation regimes mediate vegetation responses to warming and elevated atmospheric CO2 in semi-arid grasslands. Even small changes in vegetation phenology and greenness in response to warming and rising atmospheric CO2 concentrations, such as those we report here, can have large consequences for the future of grasslands. [ABSTRACT FROM AUTHOR]

In order to plan for global changing climate experiments are being conducted in many countries, but few have monitored the effects of the climate change treatments (warming, elevated CO2) on the experimental plot microclimate. During three years of an eight year study with year-round feedback-controlled infra-red heater warming (1.5/3.0°C day/night) and growing season free-air CO2 enrichment (600 ppm) in the mixed-grass prairie of Wyoming, USA, we monitored soil, leaf, canopy-air, above-canopy-air temperatures and relative humidity of control and treated experimental plots and evaluated ecologically important temperature differentials. Leaves were warmed somewhat less than the target settings (1.1 & 1.5°C day/night) but soil was warmed more creating an average that matched the target settings extremely well both during the day and night plus the summer and winter. The site typically has about 50% bare or litter covered soil, therefore soil heat transfer is more critical than in dense canopy ecosystems. The Wyoming site commonly has strong winds (5 ms-1 average) and significant daily and seasonal temperature fluctuations (as much as 30°C daily) but the warming system was nearly always able to maintain the set temperatures regardless of abiotic variation. The within canopy-air was only slightly warmed and above canopy-air was not warmed by the system, therefore convective warming was minor. Elevated CO2 had no direct effect nor interaction with the warming treatment on microclimate. Relative humidity within the plant canopy was only slightly reduced by warming. Soil water content was reduced by warming but increased by elevated CO2. This study demonstrates the importance of monitoring the microclimate in manipulative field global change experiments so that critical physiological and ecological conclusions can be determined. Highly variable energy demand fluctuations showed that passive IR heater warming systems will not maintain desired warming for much of the time. [ABSTRACT FROM AUTHOR]

Climate controls vegetation distribution across the globe, and some vegetation types are more vulnerable to climate change, whereas others are more resistant. Because resistance and resilience can influence ecosystem stability and determine how communities and ecosystems respond to climate change, we need to evaluate the potential for resistance as we predict future ecosystem function. In a mixed-grass prairie in the northern Great Plains, we used a large field experiment to test the effects of elevated CO2, warming, and summer irrigation on plant community structure and productivity, linking changes in both to stability in plant community composition and biomass production. We show that the independent effects of CO2 and warming on community composition and productivity depend on interannual variation in precipitation and that the effects of elevated CO2 are not limited to water saving because they differ from those of irrigation. We also show that production in this mixed-grass prairie ecosystem is not only relatively resistant to interannual variation in precipitation, but also rendered more stable under elevated CO2 conditions. This increase in production stability is the result of altered community dominance patterns: Community evenness increases as dominant species decrease in biomass under elevated CO2. In many grasslands that serve as rangelands, the economic value of the ecosystem is largely dependent on plant community composition and the relative abundance of key forage species. Thus, our results have implications for how we manage native grasslands in the face of changing climate. [ABSTRACT FROM AUTHOR]

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. 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. A conceptual model to explain this apparent contradiction, and an analysis of the effect of elevated CO2-which can delay annual life cycle events-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 CO2 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 CO2 concentrations that extend the active period of plant annual life cycles. [ABSTRACT FROM AUTHOR]

As global changes reorganize plant communities, invasive plants may benefit. We hypothesized that elevated CO2 and warming would strongly influence invasive species success in a semi-arid grassland, as a result of both direct and water-mediated indirect effects., To test this hypothesis, we transplanted the invasive forb Linaria dalmatica into mixed-grass prairie treated with free-air CO2 enrichment and infrared warming, and followed survival, growth, and reproduction over 4 yr. We also measured leaf gas exchange and carbon isotopic composition in L. dalmatica and the dominant native C3 grass Pascopyrum smithii., CO2 enrichment increased L. dalmatica biomass 13-fold, seed production 32-fold, and clonal expansion seven-fold, while warming had little effect on L. dalmatica biomass or reproduction. Elevated CO2 decreased stomatal conductance in P. smithii, contributing to higher soil water, but not in L. dalmatica. Elevated CO2 also strongly increased L. dalmatica photosynthesis (87% versus 23% in P. smithii), as a result of both enhanced carbon supply and increased soil water., More broadly, rapid growth and less conservative water use may allow invasive species to take advantage of both carbon fertilization and water savings under elevated CO2. Water-limited ecosystems may therefore be particularly vulnerable to invasion as CO2 increases. [ABSTRACT FROM AUTHOR]

The flux of carbon dioxide (CO2) between terrestrial ecosystems and the atmosphere may ameliorate or exacerbate climate change, depending on the relative responses of ecosystem photosynthesis and respiration to warming temperatures, rising atmospheric CO2, and altered precipitation. The combined effect of these global change factors is especially uncertain because of their potential for interactions and indirectly mediated conditions such as soil moisture. Here, we present observations of CO2 fluxes from a multi-factor experiment in semi-arid grassland that suggests a potentially strong climate – carbon cycle feedback under combined elevated [CO2] and warming. Elevated [CO2] alone, and in combination with warming, enhanced ecosystem respiration to a greater extent than photosynthesis, resulting in net C loss over four years. The effect of warming was to reduce respiration especially during years of below-average precipitation, by partially offsetting the effect of elevated [CO2] on soil moisture and C cycling. Carbon losses were explained partly by stimulated decomposition of soil organic matter with elevated [CO2]. The climate – carbon cycle feedback observed in this semiarid grassland was mediated by soil water content, which was reduced by warming and increased by elevated [CO2]. Ecosystem models should incorporate direct and indirect effects of climate change on soil water content in order to accurately predict terrestrial feedbacks and long-term storage of C in soil. [ABSTRACT FROM AUTHOR]

Atmospheric concentrations of methane ( CH4) and nitrous oxide ( N2O) have increased over the last 150 years because of human activity. Soils are important sources and sinks of both potent greenhouse gases where their production and consumption are largely regulated by biological processes. Climate change could alter these processes thereby affecting both rate and direction of their exchange with the atmosphere. We examined how a rise in atmospheric CO2 and temperature affected CH4 and N2O fluxes in a well-drained upland soil (volumetric water content ranging between 6% and 23%) in a semiarid grassland during five growing seasons. We hypothesized that responses of CH4 and N2O fluxes to elevated CO2 and warming would be driven primarily by treatment effects on soil moisture. Previously we showed that elevated CO2 increased and warming decreased soil moisture in this grassland. We therefore expected that elevated CO2 and warming would have opposing effects on CH4 and N2O fluxes. Methane was taken up throughout the growing season in all 5 years. A bell-shaped relationship was observed with soil moisture with highest CH4 uptake at intermediate soil moisture. Both N2O emission and uptake occurred at our site with some years showing cumulative N2O emission and other years showing cumulative N2O uptake. Nitrous oxide exchange switched from net uptake to net emission with increasing soil moisture. In contrast to our hypothesis, both elevated CO2 and warming reduced the sink of CH4 and N2O expressed in CO2 equivalents (across 5 years by 7% and 11% for elevated CO2 and warming respectively) suggesting that soil moisture changes were not solely responsible for this reduction. We conclude that in a future climate this semiarid grassland may become a smaller sink for atmospheric CH4 and N2O expressed in CO2-equivalents. [ABSTRACT FROM AUTHOR]

In semiarid western North American riparian ecosystems, increased drought and lower streamflows under climate change may reduce plant growth and recruitment, and favor drought-tolerant exotic species over mesic native species. We tested whether elevated atmospheric CO2 might ameliorate these effects by improving plant water-use efficiency., We examined the effects of CO2 and water availability on seedlings of two native ( Populus deltoides spp. monilifera, Salix exigua) and three exotic ( Elaeagnus angustifolia, Tamarix spp., Ulmus pumila) western North American riparian species in a CO2-controlled glasshouse, using 1-m-deep pots with different water-table decline rates., Low water availability reduced seedling biomass by 70-97%, and hindered the native species more than the exotics. Elevated CO2 increased biomass by 15%, with similar effects on natives and exotics. Elevated CO2 increased intrinsic water-use efficiency (Δ13Cleaf), but did not increase biomass more in drier treatments than wetter treatments., The moderate positive effects of elevated CO2 on riparian seedlings are unlikely to counteract the large negative effects of increased aridity projected under climate change. Our results suggest that increased aridity will reduce riparian seedling growth despite elevated CO2, and will reduce growth more for native Salix and Populus than for drought-tolerant exotic species. [ABSTRACT FROM AUTHOR]

Warming and elevated atmospheric CO (eCO) can elicit contrasting responses on different SOM pools, thus to understand the effects of combined factors it is necessary to evaluate individual pools. Over two years, we assessed responses to eCO and warming of SOM pools, their susceptibility to decomposition, and whether these responses were mediated by plant inputs in a semi-arid grassland at the PHACE ( Prairie Heating and CO Enrichment) experiment. We used long-term soil incubations and assessed relationships between plant inputs and the responses of the labile and resistant pools. We found strong and contrasting effects of eCO and warming on the labile C pool. In 2008 labile C was increased by eCO and was positively related to plant biomass. In contrast, in 2007 eCO and warming had interactive effects on the labile C, and the pool size was not related to plant biomass. Effects of warming and eCO in this year were consistent withtreatment effects on soil moisture and temperature and their effects on labile C decomposition. The decomposition rate of the resistant C was positively related to indicators of plant C inputs. Our approach demonstrated that SOM pools in this grassland can have early and contrasting responses to climate change factors. The labile C pool in the mixed-grass prairie was highly responsive to eCO and warming but the factors behind such responses were highly dynamic across years. Results suggest that in this grassland the resistant C pool could be negatively affected by increases in plant-production driven available soil C. [ABSTRACT FROM AUTHOR]

•Simulation models indicate that the nitrogen (N) cycle plays a key role in how other ecosystem processes such as plant productivity and carbon (C) sequestration respond to elevated CO2 and warming. However, combined effects of elevated CO2 and warming on N cycling have rarely been tested in the field. •Here, we studied N cycling under ambient and elevated CO2 concentrations (600 μmol mol−1), and ambient and elevated temperature (1.5 : 3.0°C warmer day:night) in a full factorial semiarid grassland field experiment in Wyoming, USA. We measured soil inorganic N, plant and microbial N pool sizes and NO3− uptake (using a 15N tracer). •Soil inorganic N significantly decreased under elevated CO2, probably because of increased microbial N immobilization, while soil inorganic N and plant N pool sizes significantly increased with warming, probably because of increased N supply. We observed no CO2 × warming interaction effects on soil inorganic N, N pool sizes or NO3− uptake in plants and microbes. •Our results indicate a more closed N cycle under elevated CO2 and a more open N cycle with warming, which could affect long-term N retention, plant productivity, and C sequestration in this semiarid grassland. [ABSTRACT FROM AUTHOR]

There is a need for methodology to warm open-field plots in order to study the likely effects of global warming on ecosystems in the future. Herein, we describe the development of arrays of more powerful and efficient infrared heaters with ceramic heating elements. By tilting the heaters at 45° from horizontal and combining six of them in a hexagonal array, good uniformity of warming was achieved across 3-m-diameter plots. Moreover, there do not appear to be obstacles (other than financial) to scaling to larger plots. The efficiency [ ηh (%); thermal radiation out per electrical energy in] of these heaters was higher than that of the heaters used in most previous infrared heater experiments and can be described by: ηh= 10 + 25exp(− 0.17 u), where u is wind speed at 2 m height (m s− 1). Graphs are presented to estimate operating costs from degrees of warming, two types of plant canopy, and site windiness. Four such arrays were deployed over plots of grass at Haibei, Qinghai, China and another at Cheyenne, Wyoming, USA, along with corresponding reference plots with dummy heaters. Proportional integral derivative systems with infrared thermometers to sense canopy temperatures of the heated and reference plots were used to control the heater outputs. Over month-long periods at both sites, about 75% of canopy temperature observations were within 0.5 °C of the set-point temperature differences between heated and reference plots. Electrical power consumption per 3-m-diameter plot averaged 58 and 80 kW h day− 1 for Haibei and Cheyenne, respectively. However, the desired temperature differences were set lower at Haibei (1.2 °C daytime, 1.7 °C night) than Cheyenne (1.5 °C daytime, 3.0 °C night), and Cheyenne is a windier site. Thus, we conclude that these hexagonal arrays of ceramic infrared heaters can be a successful temperature free-air-controlled enhancement (T-FACE) system for warming ecosystem field plots. [ABSTRACT FROM AUTHOR]

Abstract: Using open-top chambers (OTC) on the shortgrass steppe in northern Colorado, changes of microbial community composition were followed over the latter 3 years of a 5-year study of elevated atmospheric CO2 as well as during 12 months after CO2 amendment ended. The experiment was composed of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360±20μmolmol−1 (ambient treatment), three chambered plots maintained at 720±20μmolmol−1 CO2 (elevated treatment) and three unchambered plots. The abundance of fungal phospholipid fatty acids (PLFAs) shifted in the shortgrass steppe under the influence of elevation of CO2 over the period of 3 years. Whereas the content of the fungal signature molecule (18:2ω6) was similar in soils of the ambient and elevated treatments in the third year of the experiment, CO2 treatment increased the content of 18:2ω6 by around 60% during the two subsequent years. The shift of microbial community composition towards a more fungal dominated community was likely due to slowly changing substrate quality; plant community forage quality declined under elevated CO2 because of a decline of N in all tested species as well as shift in species composition towards greater abundance of the low forage quality species (Stipa comata). In the year after which CO2 enrichment had ceased, abundances of fungal and bacterial PLFAs in the post-CO2 treatment plots shifted slowly back towards the control plots. Therefore, quantity and quality of available substrates had not changed sufficiently to shift the microbial community permanently to a fungal dominated community. We conclude from PLFA composition of soil microorganisms during the CO2 elevation experiment and during the subsequent year after cessation of CO2 treatment that a shift towards a fungal dominated system under higher CO2 concentrations may slow down C cycling in soils and therefore enhance C sequestration in the shortgrass steppe in future CO2-enriched atmospheres. [Copyright &y& Elsevier]

Abstract: Although elevation of CO2 has been reported to impact soil microbial functions, little information is available on the spatial and temporal variation of this effect. The objective of this study was to determine the microbial response in a northern Colorado shortgrass steppe to a 5-year elevation of atmospheric CO2 as well as the reversibility of the microbial response during a period of several months after shutting off the CO2 amendment. The experiment was comprised of nine experimental plots: three chambered plots maintained at ambient CO2 levels of 360μmolmol−1 (ambient treatment), three chambered plots maintained at 720μmolmol−1 CO2 (elevated treatment) and three unchambered plots of equal ground area used as controls to monitor the chamber effect. Elevated CO2 induced mainly an increase of enzyme activities (protease, xylanase, invertase, alkaline phosphatase, arylsulfatase) in the upper 5cm of the soil and did not change microbial biomass in the soil profile. Since rhizodeposition and newly formed roots enlarged the pool of easily available substrates mainly in the upper soil layers, enzyme regulation (production and activity) rather than shifts in microbial abundance was the driving factor for higher enzyme activities in the upper soil. Repeated soil sampling during the third to fifth year of the experiment revealed an enhancement of enzyme activities which varied in the range of 20–80%. Discriminant analysis including all microbiological properties revealed that the enzyme pattern in 1999 and 2000 was dominated by the CO2 and chamber effect, while in 2001 the influence of elevated CO2 increased and the chamber effect decreased. Although microbial biomass did not show any response to elevated CO2 during the main experiment, a significant increase of soil microbial N was detected as a post-treatment effect probably due to lower nutrient (nitrogen) competition between microorganisms and plants in this N-limited ecosystem. Whereas most enzyme activities showed a significant post-CO2 effect in spring 2002 (following the conclusion of CO2 enrichment the previous autumn, 2001), selective depletion of substrates is speculated to be the cause for non-significant treatment effects of most enzyme activities later in summer and autumn, 2002. Therefore, additional belowground carbon input mainly entered the fast cycling carbon pool and contributed little to long-term carbon storage in the semi-arid grassland. [Copyright &y& Elsevier]

Elevated atmospheric CO 2 will change the requirements plants have for minerals, mainly nitrogen, altering the relationship between nutrient demand and plant growth. Here, we evaluated the interacting effects between two concentrations of CO 2 (390 or 750 μL L −1 ) and two nitrogen (N) levels (3 mM or 8 mM) on gas exchange, N use efficiency (NUE), and dry mass responses of papaya seedlings. The study was conducted in a climate-controlled greenhouse using 3.5 L pots and six replicates for 62 days. No significant effects from N or CO 2 were noticed for leaf conductance to water vapor and transpiration. However, CO 2 elevation to 750 μL L −1 increased assimilation rate for both N levels. Elevated CO 2 resulted in an increase of 52% and 16% of instantaneous water-use efficiency under high and low N, respectively, as compared to ambient CO 2 (390 μL L −1 ). Lower N concentrations were observed for all the organs for plants grown under higher concentrations of CO 2 , regardless of the N level used. However, plants grown under elevated CO 2 reached the highest NUEs. There was an N × CO 2 interactive effect on the leaf, stem plus petiole, and root dry mass. However, no significant CO 2 effect on the root:shoot ratio was observed. Our results revealed that elevated CO 2 stimulated total dry mass accumulation of papaya plants, with the results being greatest for plants grown under lower levels of N. [ABSTRACT FROM AUTHOR]