Andrew McLeod

Free-air Carbon Dioxide Enrichment (FACE) in Global Change Research: A Review

Summary There have been many experimental studies to evaluate the response of vegetation to the effect of increases in the partial pressure of carbon dioxide in the atmosphere (p CO2) that are expected to occur during the next century. This knowledge is important for the future protection of food supplies, for understanding changes in natural ecosystems and for quantifying the role of terrestrial plants in regulating the rate of change of p CO2 and resulting changes in the global climate. Most of our knowledge about these effects has derived from experimental studies that have used open-top or closed chambers. These methods are subject to “chamber effects” caused by differences in energy balance and water relations that may significantly modify the response of vegetation to elevated p CO2. The small plot sizes imposed by these techniques add other limitations both to interpretation of results and scope of investigations. Free-air carbon dioxide enrichment (FACE) provides an experimental technique for studying the effects of elevated p CO2 on vegetation and other ecosystem components in large unenclosed plots (>20 m diameter). FACE avoids many modifications to the microclimate imposed by chamber methods and therefore provides some of the most reliable estimates of plant response to elevated p CO2. Control of p CO2 in large-scale FACE experiments has now been developed to an extent where performance is similar to that achieved with sophisticated closed-chamber facilities. Experience has shown that, when FACE facilities are fully utilised, the cost per unit of usable ground area enriched with CO2, is significantly lower than alternative methods. The large scale of FACE plots can support a range of integrated studies on the same material, thereby achieving a more complete analysis than has been possible with other methods of elevating p CO2. This review considers the technical aspects of FACE methodology, outlines the major FACE experiments and summarises the advances in understanding of p CO2 effects on ecosystems that it has allowed. Published data on large-scale FACE experiments with adequate plot replication are limited to experiments on four crop/vegetation types at three locations. FACE has been used for experiments on two crops, cotton and wheat, at Maricopa, Arizona, and on grassland species, principally ryegrass and clover, at Eschikon, Switzerland. The method has also been adapted for the first study of mature forest trees, loblolly pine at Duke Forest, North Carolina. A number of other large-scale FACE experiments are in progress and the method has been adapted for use in much smaller experimental plots. The results of the major FACE experiments represent important advances over understanding obtained from previous p CO2 treatment methods. Most significant in terms of the global climate and atmosphere system are the clear observations with cotton and wheat crops that elevated p CO2 increases the ratio of sensible: latent heat transfer and causes daytime warming of the surface vegetation. This results from decreased water use and loss, and has been evident at a range of scales. The scale of FACE plots has allowed quantitative and detailed studies of the dynamics of below-ground production and C accumulation in a range of systems, and all have shown surprisingly large increases. Of particular note are the increases observed in grassland grown with low N, where there was no response of the above-ground biomass, but an increased rate of turnover of leaves and input of surface litter. FACE has allowed cultivation of crops at a scale appropriate to agronomic trials and shown statistically significant increases in the yields of wheat, cotton and pasture crops, although some of these increases are less than suggested by chamber experiments. Against expectations, the FACE experiments at Maricopa have shown a greater relative increase in yield in crops grown under water shortage than in water-sufficient crops. Acclimatory loss of photosynthetic capacity has been widely anticipated to offset the increase in photosynthesis that follows initial transfer of vegetation to elevated p CO2. None of the FACE experiments provides any evidence of such a loss; however, changes which will allow a re-optimisation of N distribution within plants have been reported. FACE methods have now been demonstrated to be feasible and effective within a range of crops and vegetation types. The information from past experiments has greatly improved our understanding of the impacts of global atmospheric change on terrestrial ecosystems.

Ultraviolet radiation drives methane emissions from terrestrial plant pectins

Recent studies demonstrating an in situ formation of methane (CH(4)) within foliage and separate observations that soil-derived CH(4) can be released from the stems of trees have continued the debate about the role of vegetation in CH(4) emissions to the atmosphere. Here, a study of the role of ultraviolet (UV) radiation in the formation of CH(4) and other trace gases from plant pectins in vitro and from leaves of tobacco (Nicotiana tabacum) in planta is reported. Plant pectins were investigated for CH(4 )production under UV irradiation before and after de-methylesterification and with and without the singlet oxygen scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO). Leaves of tobacco were also investigated under UV irradiation and following leaf infiltration with the singlet oxygen generator rose bengal or the bacterial pathogen Pseudomonas syringae. Results demonstrated production of CH(4), ethane and ethylene from pectins and from tobacco leaves following all treatments, that methyl-ester groups of pectin are a source of CH(4), and that reactive oxygen species (ROS) arising from environmental stresses have a potential role in mechanisms of CH(4) formation. Rates of CH(4 )production were lower than those previously reported for intact plants in sunlight but the results clearly show that foliage can emit CH(4) under aerobic conditions.

OPEN-AIR FUMIGATION OF FIELD CROPS - CRITERIA AND DESIGN FOR A NEW EXPERIMENTAL SYSTEM

Abstract The criteria for designing open-air systems to expose field crops to gaseous air pollutants were examined. A computer simulation of gas dispersion was then used to investigate the distribution of gas concentration produced by different source patterns and to develop a new design which permitted a spatially uniform exposure of an experimental area. A system constructed to the new design has been operated to fumigate winter cereals with sulphur dioxide throughout two complete growing seasons. Rates of gas release were controlled by a small computer to follow predetermined patterns of concentration, and spatial uniformity of exposure was achieved. Effective control of concentration was demonstrated with examples of how frequency distributions of concentration and diurnal patterns can be reproduced experimentally.

The role of ultraviolet radiation, photosensitizers, reactive oxygen species and ester groups in mechanisms of methane formation from pectin.

Ultraviolet (UV) radiation has recently been demonstrated to drive an aerobic production of methane (CH(4)) from plant tissues and pectins, as do agents that generate reactive oxygen species (ROS) in vivo independently of UV. As the major building-blocks of pectin do not absorb solar UV found at the earths surface (i.e. >280 nm), we explored the hypothesis that UV radiation affects pectin indirectly via generation of ROS which themselves release CH(4) from pectin. Decreasing the UV absorbance of commercial pectin by ethanol washing diminished UV-dependent CH(4) production, and this was restored by the addition of the UV photosensitizer tryptophan. Certain ROS scavengers [mannitol, a hydroxyl radical ((*)OH) scavenger; 1,4-diazabicyclo[2.2.2] octane; and iodide] strongly inhibited UV-induced CH(4) production from dry pectin. Furthermore, pectin solutions emitted CH(4) in darkness upon the addition of (*)OH, but not superoxide or H(2)O(2). Model carbohydrates reacted similarly if they possessed -CH(3) groups [e.g. methyl esters or (more weakly) acetyl esters but not rhamnose]. We conclude that UV evokes CH(4) production from pectic methyl groups by interacting with UV photosensitizers to generate (*)OH. We suggest that diverse processes generating (*)OH could contribute to CH(4) emissions independently of UV irradiation, and that environmental stresses and constitutive physiological processes generating ROS require careful evaluation in studies of CH(4) formation from foliage.

Methane formation in aerobic environments

Methane (CH_4), the second principal anthropogenic greenhouse gas after CO_2, is the most abundant reduced organic compound in the atmosphere and plays a central role in atmospheric chemistry. Therefore a comprehensive understanding of its sources and sinks and the parameters that control emissions is prerequisite to simulate past, present and future atmospheric conditions. Until recently biological CH_4 formation has been associated exclusively with anoxic environments and methanogenic activity. However, there is growing and convincing evidence of alternative pathways in the aerobic biosphere including terrestrial plants, soils, marine algae and animals. Identifying and describing these sources is essential to complete our understanding of the biogeochemical cycles that control CH_4 in the atmospheric environment and its influence as a greenhouse gas.

Global methane emission estimates from ultraviolet irradiation of terrestrial plant foliage

SUMMARY *Several studies have reported in situ methane (CH(4)) emissions from vegetation foliage, but there remains considerable debate about its significance as a global source. Here, we report a study that evaluates the role of ultraviolet (UV) radiation-driven CH(4) emissions from foliar pectin as a global CH(4) source. *We combine a relationship for spectrally weighted CH(4) production from pectin with a global UV irradiation climatology model, satellite-derived leaf area index (LAI) and air temperature data to estimate the potential global CH(4) emissions from vegetation foliage. *Our results suggest that global foliar CH(4) emissions from UV-irradiated pectin could account for 0.2-1.0 Tg yr(-1), of which 60% is from tropical latitudes, corresponding to < 0.2% of total CH(4) sources. *Our estimate is one to two orders of magnitude lower than previous estimates of global foliar CH(4) emissions. Recent studies have reported that pectin is not the only molecular source of UV-driven CH(4) emissions and that other environmental stresses may also generate CH(4). Consequently, further evaluation of such mechanisms of CH(4) generation is needed to confirm the contribution of foliage to the global CH(4) budget.

Outdoor supplementation systems for studies of the effects of increased UV-B radiation

Studies of the effects of increases in ultraviolet-B (UV-B) radiation on plants and terrestrial ecosystems have been undertaken using a variety of methods including: controlled-environment cabinets, glasshouses, outdoor filtration and outdoor supplementation using fluorescent UV-B lamps. Outdoor supplementation systems provide a method of study which creates only small alterations to the microclimate and the number of such studies has increased during the past 3 years. These supplementation systems differ in their methods of operation, equipment, UV-B exposure regime and experimental design. This essay surveys the systems currently developed, considers problems associated with their use and discusses these in relation to the interpretation of biological effects.

Ultraviolet-radiation-induced methane emissions from meteorites and the Martian atmosphere

Almost a decade after methane was first reported in the atmosphere of Mars there is an intensive discussion about both the reliability of the observations—particularly the suggested seasonal and latitudinal variations—and the sources of methane on Mars. Given that the lifetime of methane in the Martian atmosphere is limited, a process on or below the planet’s surface would need to be continuously producing methane. A biological source would provide support for the potential existence of life on Mars, whereas a chemical origin would imply that there are unexpected geological processes. Methane release from carbonaceous meteorites associated with ablation during atmospheric entry is considered negligible. Here we show that methane is produced in much larger quantities from the Murchison meteorite (a type CM2 carbonaceous chondrite) when exposed to ultraviolet radiation under conditions similar to those expected at the Martian surface. Meteorites containing several per cent of intact organic matter reach the Martian surface at high rates, and our experiments suggest that a significant fraction of the organic matter accessible to ultraviolet radiation is converted to methane. Ultraviolet-radiation-induced methane formation from meteorites could explain a substantial fraction of the most recently estimated atmospheric methane mixing ratios. Stable hydrogen isotope analysis unambiguously confirms that the methane released from Murchison is of extraterrestrial origin. The stable carbon isotope composition, in contrast, is similar to that of terrestrial microbial origin; hence, measurements of this signature in future Mars missions may not enable an unambiguous identification of biogenic methane.

The Liphook Forest Fumigation Project: studies of sulphur dioxide and ozone effects on coniferous trees

Abstract The Liphook Forest Fumigation Project is a long-term field study of the effects of sulphur dioxide (SO 2 ) and ozone (O 3 ) on the coniferous ecosystem including the growth and performance of three species of conifer: Picea sitchensis (Bong.) Carr, Picea abies (L.) Karsten and Pinus sylvestris L. Seven experimental plots (diameter 50 m) were planted with 200 2-year-old seedlings of each species in 1985. Five of the plots receive computer-controlled fumigation treatments with two levels of SO 2 fumigation (annual means 14 and 24 ppb; parts in 10 9 by volume), one level of O 3 fumigation (1.5 times ambient) and the combination of these treatments. SO 2 fumigation commenced in early 1987 and O 3 fumigation in early 1988. Over 35 studies are in progress at the site including work on tree growth, physiology, nutrient status, litter decomposition, soil chemistry, root growth, insects, micro-organisms, throughfall chemistry and lichens. This wide range of studies using a large number of trees under realistic environmental conditions demonstrates the potential of this method for investigating air-pollutant effects on forest ecology.

Energy policy and institutional context: Marine energy innovation systems

A process of UK energy policy review in the early 2000s has seen renewable energy technologies moving from the policy margins to centre-stage. The review process drew on international experiences of renewables innovation, including an innovation systems framework that emphasises ‘social capital’ (collaborative learning between distributed agents). However, the UK energy system reflects a longstanding policy commitment to ‘financial capital’ (market competition and avoiding ‘picking winners’). This paper analyses policy tensions between social and financial capital by focusing on marine energy innovation, especially in the emerging Scottish policy arena. Recent initiatives to promote renewables innovation in the UK, though significant, face continuing challenges. Copyright , Beech Tree Publishing.