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The carbon stored in soil exceeds that of plant biomass and atmospheric carbon and its stability can impact global climate. Growth of decomposer microorganisms mediates both the accrual and loss of soil carbon. Growth is sensitive to temperature and given the vast biological diversity of soil microorganisms, the response of decomposer growth rates to warming may be strongly idiosyncratic, varying among taxa, making ecosystem predictions difficult. Here, we show that 15 years of warming by transplanting plant–soil mesocosms down in elevation, strongly reduced the growth rates of soil microorganisms, measured in the field using undisturbed soil. The magnitude of the response to warming varied among microbial taxa. However, the direction of the response—reduced growth—was universal and warming explained twofold more variation than did the sum of taxonomic identity and its interaction with warming. For this ecosystem, most of the growth responses to warming could be explained without taxon‐specific information, suggesting that in some cases microbial responses measured in aggregate may be adequate for climate modeling. Long‐term experimental warming also reduced soil carbon content, likely a consequence of a warming‐induced increase in decomposition, as warming‐induced changes in plant productivity were negligible. The loss of soil carbon and decreased microbial biomass with warming may explain the reduced growth of the microbial community, more than the direct effects of temperature on growth. These findings show that direct and indirect effects of long‐term warming can reduce growth rates of soil microbes, which may have important feedbacks to global warming. [ABSTRACT FROM AUTHOR]

This study reports the aboveground biomass response of a fire-regenerated Florida scrub-oak ecosystem exposed to elevated CO2 (1996–2007), from emergence after fire through canopy closure. Eleven years exposure to elevated CO2 caused a 67% increase in aboveground shoot biomass. Growth stimulation was sustained throughout the experiment; although there was significant variability between years. The absolute stimulation of aboveground biomass generally declined over time, reflecting increasing environmental limitations to long-term growth response. Extensive defoliation caused by hurricanes in September 2004 was followed by a strong increase in shoot density in 2005 that may have resulted from reopening the canopy and relocating nitrogen from leaves to the nutrient-poor soil. Biomass response to elevated CO2 was driven primarily by stimulation of growth of the dominant species, Quercus myrtifolia, while Quercus geminata, the other co-dominant oak, displayed no significant CO2 response. Aboveground growth also displayed interannual variation, which was correlated with total annual rainfall. The rainfall × CO2 interaction was partially masked at the community level by species-specific responses: elevated CO2 had an ameliorating effect on Q. myrtifolia growth under water stress. The results of this long-term study not only show that atmospheric CO2 concentration had a consistent stimulating effect on aboveground biomass production, but also showed that available water is the primary driver of interannual variation in shoot growth and that the long-term response to elevated CO2 may have been caused by other factors such as nutrient limitation and disturbance. [ABSTRACT FROM AUTHOR]

AbstractThe effect of elevated atmospheric CO2 concentration (C a ) on the aboveground biomass of three oak species, Quercus myrtifolia , Q. geminata , and Q. chapmanii , was estimated nondestructively using allometric relationships between stem diameter and aboveground biomass after four years of experimental treatment in a naturally fire-regenerated scrub-oak ecosystem. After burning a stand of scrub-oak vegetation, re-growing plants were exposed to either current ambient (379 µL L-1 CO2 ) or elevated (704 µL L-1 CO2 ) C a in 16 open-top chambers over a four-year period, and measurements of stem diameter were carried out annually on all oak shoots within each chamber. Elevated C a significantly increased aboveground biomass, expressed either per unit ground area or per shoot; elevated C a had no effect on shoot density. The relative effect of elevated C a on aboveground biomass increased each year of the study from 44% (May 96–Jan 97), to 55% (Jan 97–Jan 98), 66% (Jan 98–Jan 99), and 75% (Jan 99–Jan 00). The effect of elevated C a was species specific: elevated C a significantly increased aboveground biomass of the dominant species, Q. myrtifolia , and tended to increase aboveground biomass of Q. chapmanii , but had no effect on aboveground biomass of the subdominant, Q. geminata . These results show that rising atmospheric CO2 has the potential to stimulate aboveground biomass production in ecosystems dominated by woody species, and that species-specific growth responses could, in the long term, alter the composition of the scrub-oak community. [ABSTRACT FROM AUTHOR]

SummaryThe effect of elevated atmospheric CO2 concentration (Ca) on soil carbon and nitrogen accumulation and soil microbial biomass and activity in a native Florida scrub oak community was studied. The plant community, dominated by Quercus myrtifolia Willd. and Q. geminata Small, was exposed for 2 years to elevated Ca in open-top chambers. Buried subsoil bags were retrieved after 1 year of exposure to elevated Ca. In addition, soil cores were taken twice from the chambers within two weeks in July 1998 (the first after a long dry spell and the second after 25 mm of rainfall) and divided into rhizosphere and bulk soil. Soil organic matter accumulation (excluding roots) into the buried subsoil bags was lower in elevated than in ambient Ca. Concentrations of soluble carbon and ninhydrin-reactive nitrogen (Nninh) in the rhizosphere soil were reduced by elevated Ca for the first sampling date and unaffected for the second sampling date. Microbial activity, measured as fluorescein diacetate (FDA) hydrolysis, decreased in elevated Ca for the first sampling date. Microbial biomass carbon and nitrogen in the bulk soil were unaffected by elevated Ca. There was no effect of elevated Ca on bacterial numbers in the rhizosphere. [ABSTRACT FROM AUTHOR]

SummaryWe report changes in nitrogen cycling in Florida scrub oak in response to elevated atmospheric CO2 during the first 14 months of experimental treatment. Elevated CO2 stimulated above-ground growth, nitrogen mass, and root nodule production of the nitrogen-fixing vine, Galactia elliottii Nuttall. During this period, elevated CO2 reduced rates of gross nitrogen mineralization in soil, and resulted in lower recovery of nitrate on resin lysimeters. Elevated CO2 did not alter nitrogen in the soil microbial biomass, but increased the specific rate of ammonium immobilization (NH4+ immobilized per unit microbial N) measured over a 24-h period. Increased carbon input to soil through greater root growth combined with a decrease in the quality of that carbon in elevated CO2 best explains these changes. These results demonstrate that atmospheric CO2 concentration influences both the internal cycling of nitrogen (mineralization, immobilization, and nitrification) as well as the processes that regulate total ecosystem nitrogen mass (nitrogen fixation and nitrate leaching) in Florida coastal scrub oak. If these changes in nitrogen cycling are sustained, they could cause long-term feedbacks to the growth responses of plants to elevated CO2. Greater nitrogen fixation and reduced leaching could stimulate nitrogen-limited plant growth by increasing the mass of labile nitrogen in the ecosystem. By contrast, reduced nitrogen mineralization and increased immobilization will restrict the supply rate of plant-available nitrogen, potentially reducing plant growth. Thus, the net feedback to plant growth will depend on the balance of these effects through time. [ABSTRACT FROM AUTHOR]

SummaryWinter wheat was grown at ambient and elevated (ambient plus 350 μL L–1) CO2 concentrations in open top chambers and in field-tracking sun-lit climatized enclosures (elevated is 718 μL L–1). There was no significant effect of CO2 concentration on sheath, leaf and root biomass and leaf area in the early spring (January to April). 24-h canopy CO2 exchange rate (CCER) was not significantly affected either. However, elevated CO2 concentration increased CCER at midday, decreased evapotranspiration rate and increased instantaneous water-use-efficiency during early spring. Leaf, sheath and root nitrogen concentration per unit dry weight decreased and nonstructural carbohydrate concentration increased under elevated CO2, and N-uptake per unit ground area decreased significantly (– 22%) towards the end of this period. These results contrast with results from the final harvest, when grain yield and biomass were increased by 19% under elevated CO2. N concentration per dry weight was reduced by 5%, but N-uptake per unit ground area was significantly higher (+ 11%) for the elevated CO2 treatment. 24-h and midday-CCER increased significantly more in late spring (period of 21 April to 30 May) (respectively by + 40% and 53%) than in the early spring (respectively 5% and 19%) in response to elevated CO2. Midday evapotranspiration rate was reduced less by elevated CO2 in the late spring (– 13%) than in early spring (– 21%). The CO2 response of midday and 24-h CCER decreased again (+ 27% and + 23% resp.) towards the end of the growing season. We conclude that the low response to CO2 concentration during the early spring was associated with a growth-restriction, caused by low temperature and irradiance levels. The reduction of nitrogen concentration, the increase of nonstructural carbohydrate, and the lower evapotranspiration indicated that CO2 did have an effect towards the end of early spring, but not on biomass accumulation. Regression analysis showed that both irradiance and temperature affected the response to CO2. [ABSTRACT FROM AUTHOR]

The effect of elevated [CO2] on the productivity of spring wheat, winter wheat and faba bean was studied in experiments in climatized crop enclosures in the Wageningen Rhizolab in 1991-93. Simulation models for crop growth were used to explore possible causes for the observed differences in the CO2 response. Measurements of the canopy gas exchange (CO2 and water vapour) were made continuously from emergence until harvest. At an external [CO2] of 700 µmol mol-1 Maximum Canopy CO2 Exchange Rate (CCERmax) at canopy closure was stimulated by 51% for spring wheat and by 71% for faba bean. At the end of the growing season, above ground biomass increase at 700 µmol mol-1 was 58% (faba bean), 35% (spring wheat) and 19% (winter wheat) and the harvest index did not change. For model exploration, weather data sets for the period 1975-88 and 1991-93 were used, assuming adequate water supply and [CO2] at 350 and 700 µmol mol-1. For spring wheat the simulated responses (35-50%) were at the upper end of the experimental results. In agreement with experiments, simulations showed smaller responses for winter wheat and larger responses for faba bean. Further model explorations showed that this differential effect in the CO2 response may not be primarily due to fundamental physiological differences between the crops, but may be at least partly due to differences in the daily air temperatures during comparable stages of growth of these crops. Simulations also showed that variations between years in CO2 response can be largely explained by differences in weather conditions (especially temperature) between growing seasons. [ABSTRACT FROM AUTHOR]