A comparison of isoprene and monoterpene emission rates from the perennial bioenergy crops short‐rotation coppice willow and Miscanthus and the annual arable crops wheat and oilseed rape

Biogenic volatile organic compounds (BVOC) emissions from bioenergy crops may differ from those of conventional crops. We compared emission rates of isoprene and a number of monoterpenes from the lignocellulosic bioenergy crops short‐rotation coppice (SRC) willow and Miscanthus, with the conventional crops wheat and oilseed rape. BVOC emission rates were measured via dynamic vegetation enclosure and GC‐MS analysis approximately monthly between April 2010 and August 2012 at a location in England and from SRC willow at two locations in Scotland. The largest BVOC emission rates were measured from willow in England and varied between years. Isoprene emission rates varied between μg g^?1 h^?1. Of the monoterpenes detected from willow, α‐pinene emission rates were highest (μg g^?1 h^?1), followed by μg g^?1 h^?1 for δ‐3‐carene, μg g^?1 h^?1 for β‐pinene and μg g^?1 h^?1 for limonene. BVOC emission rates measured in Scotland were much lower. Low emission rates of isoprene and α‐pinene were measured from Miscanthus in 2010 (μg g^?1 h^?1 and μg g^?1 h^?1, respectively) but were not detected in subsequent years. Emission rates from wheat of isoprene were negligible but relatively high for monoterpenes (μg g^?1 h^?1 and μg g^?1 h^?1 for α‐pinene and limonene, respectively). No significant emission rates of BVOCs were measured from oilseed rape. The measured emission rates followed a clear seasonal trend. Crude extrapolations based solely on data gathered here indicate that isoprene emissions from willow could correspond to 0.004–0.03% (UK) and 0.76–5.5% (Europe) of current global isoprene if 50% of all land potentially available for bioenergy crops is planted with willow.     

Seasonal differences in isoprene and light-dependent monoterpene emission by Amazonian tree species

Whereas for extra‐tropical regions model estimates of the emission of volatile organic compounds (VOC) predict strong responses to the strong annual cycles of foliar biomass, light intensity and temperature, the tropical regions stand out as a dominant source year round, with only little variability mainly due to the annual cycle of foliar biomass of drought‐deciduous trees. As part of the Large Scale Biosphere Atmosphere Experiment in Amazônia (LBA‐EUSTACH), a remote secondary tropical forest site was visited in the dry‐to‐wet season transition campaign, and the trace gas exchange of a strong isoprene emitter and a monoterpene emitter are compared to the wet‐to‐dry season transition investigations reported earlier. Strong seasonal differences of the emission capacity were observed. The standard emission factor for isoprene emission of young mature leaves of Hymenaea courbaril was about twofold in the end of the dry season (111.5 μgC g^?1 h^?1 or 41.2 nmol m^?2 s^?1) compared to old mature leaves investigated in the end of the wet season (45.4 μgC g^?1 h^?1 or 24.9 nmol m^?2 s^?1). Standardized monoterpene emission rate of Apeiba tibourbou were 2.1 and 3.6 μgC g^?1 h^?1 (or 0.3 and 0.8 nmol m^?2 s^‐1), respectively. This change in species‐specific VOC emission capacity was mirrored by a concurrent change in the ambient mixing ratios. The growth conditions vary less in tropical areas than in temperate regions of the world, and the seasonal differences in emission strength could not be reconciled solely with meteorological data of instantaneous light intensity and temperature. Hence the inadequacy of using a single standard emission factor to represent an entire seasonal cycle is apparent. Among a host of other potential factors, including the leaf developmental stage, water and nutrient status, and abiotic stresses like the oxidative capacity of the ambient air, predominantly the long‐term growth temperature may be applied to predict the seasonal variability of the isoprene emission capacity. The dry season isoprene emission rates of H. courbaril measured at the canopy top were also compared to isoprene emissions of the shade‐adapted species Sorocea guilleminiana growing in the understory. Despite the difference in VOC emission composition and canopy position, one common algorithm was able to predict the diel emission pattern of all three tree species.     

Seasonal emission of monoterpenes by the Mediterranean tree Quercus ilex in field conditions: Relations with photosynthetic rates, temperature and volatility

The relationships of monoterpene emission with temperature, light, photosynthesis and stomatal conductance (g_s) were studied in Quercus ilex L. trees throughout the four annual seasons under field conditions. The highest monoterpene emission was measured in spring and summer (midday average of 11 μg [g DW]^?1 h^?1), whereas the lowest rates were found in autumn and winter (midday averages of 0.51 and 0.23 μg [g DW]^?1 h^?1, respectively). In spring and summer, limonene was the monoterpene emitted at highest rate (midday averages of 5.27–6.69 μg [g DW]^?1 h^?1), whereas α-pinene was emitted the most in autumn and winter (midday averages of 0.31 μg [g DW]^?1 h^?1). The monoterpenes limonene, α-pinene and β-pinene represented about 75–95% of total detected monoterpenes. The total monoterpene emission rates represented about 0.04% of carbon fixed in autumn, 0.17% in winter, 0.84–2.51% in spring and 1.22–5.13% in summer. Significant correlations of total monoterpene emission with temperature were found when considering either summer emission or the emission over the entire year, whereas significant correlations with net photosynthetic rates were only found when considering summer season. Among individual terpenes, the most volatile, α-pinene and β-pinene, were more correlated with temperature than with net photosynthetic rates whereas the less volatile limonene was more correlated with net photosynthetic rate. Thus, under field conditions it seems that dependency of monoterpene emission on photosynthetic rate or temperature is partly related with volatility of the compounds. Influences of seasonality, temperature, photosynthetic rates and volatility should be considered in inventories and models of emission rates in Mediterranean ecosystems.     

Controls on isoprene emission from trees in a subtropical dry forest

Isoprene emission from vegetation is the single most important source of photochemically active reduced compounds to the atmosphere. We present the first controlled-environment measurements of isoprene emission from leaves of tropical forest trees. Our studies were conducted in the Guanica State Forest in Puerto Rico. We report the effects of temperature and light variations on biogenic isoprene emissions during 1995. Maximum emission rates varied among species from 0 to 268 nmol m^?2 s^?1. Values at the upper end of this range of maximum emission rates are 2–3 times higher than values reported from any temperate taxa. Isoprene emission showed strong sensitivity to light and temperature variations. In contrast to temperate plants, whose emissions tend to saturate at a light intensity of ～1000 μmol m^?2 s^?1, emissions from the tropical species increased with light intensity up to 2500 μmol m^?2 s^?1. The temperature optima for emissions from these plants were similar to those previously reported for temperate plants: ～40 °C. The high maximum emission rates and lack of light saturation indicate that estimates of isoprene emission from tropical forests need to be revised upwards.     

Competition between isoprene emission and pigment synthesis during leaf development in aspen

In growing leaves, lack of isoprene synthase (IspS) is considered responsible for delayed isoprene emission, but competition for dimethylallyl diphosphate (DMADP), the substrate for both isoprene synthesis and prenyltransferase reactions in photosynthetic pigment and phytohormone synthesis, can also play a role. We used a kinetic approach based on post‐illumination isoprene decay and modelling DMADP consumption to estimate in vivo kinetic characteristics of IspS and prenyltransferase reactions, and to determine the share of DMADP use by different processes through leaf development in Populus tremula. Pigment synthesis rate was also estimated from pigment accumulation data and distribution of DMADP use from isoprene emission changes due to alendronate, a selective inhibitor of prenyltransferases. Development of photosynthetic activity and pigment synthesis occurred with the greatest rate in 1‐ to 5‐day‐old leaves when isoprene emission was absent. Isoprene emission commenced on days 5 and 6 and increased simultaneously with slowing down of pigment synthesis. In vivo Michaelis–Menten constant (K_m) values obtained were 265 nmol m^?2 (20 μm ) for DMADP‐consuming prenyltransferase reactions and 2560 nmol m^?2 (190 μm ) for IspS. Thus, despite decelerating pigment synthesis reactions in maturing leaves, isoprene emission in young leaves was limited by both IspS activity and competition for DMADP by prenyltransferase reactions.     

Carbon dioxide,methane, and nitrous oxide exchanges in an age‐sequence of temperate pine forests

We investigated soil carbon dioxide (CO₂), methane (CH₄), and nitrous oxide (N₂O) exchanges in an age‐sequence (4, 17, 32, 67 years old) of eastern white pine (Pinus strobus L.) forests in southern Ontario, Canada, for the period of mid‐April to mid‐December in 2006 and 2007. For both CH₄ and N₂O, we observed uptake and emission ranging from ?160 to 245 μg CH₄ m^?2 h^?1 and ?52 to 21 μg N₂O m^?2 h^?1, respectively (negative values indicate uptake). Mean fluxes from mid‐April to mid‐December across the 4, 17, 32, 67 years old stands were similar for CO₂ fluxes (259, 246, 220, and 250 mg CO₂ m^?2 h^?1, respectively), without pattern for N₂O fluxes (?3.7, 1.5, ?2.2, and ?7.6 μg N₂O m^?2 h^?1, respectively), whereas the uptake rates of CH₄ increased with stand age (6.4, ?7.9, ?10.8, and ?23.3 μg CH₄ m^?2 h^?1, respectively). For the same period, the combined contribution of CH₄ and N₂O exchanges to the global warming potential (GWP) calculated from net ecosystem exchange of CO₂ and aggregated soil exchanges of CH₄ and N₂O was on average 4%, <1%, <1%, and 2% for the 4, 17, 32, 67 years old stand, respectively. Soil CO₂ fluxes correlated positively with soil temperature but had no relationship with soil moisture. We found no control of soil temperature or soil moisture on CH₄ and N₂O fluxes, but CH₄ emission was observed following summer rainfall events. LFH layer removal reduced CO₂ emissions by 43%, increased CH₄ uptake during dry and warm soil conditions by more than twofold, but did not affect N₂O flux. We suggest that significant alternating sink and source potentials for both CH₄ and N₂O may occur in N‐ and soil water‐limited forest ecosystems, which constitute a large portion of forest cover in temperate areas.     

Ecosystem‐scale volatile organic compound fluxes during an extreme drought in a broadleaf temperate forest of the Missouri Ozarks (central USA)

Considerable amounts and varieties of biogenic volatile organic compounds (BVOCs) are exchanged between vegetation and the surrounding air. These BVOCs play key ecological and atmospheric roles that must be adequately represented for accurately modeling the coupled biosphere–atmosphere–climate earth system. One key uncertainty in existing models is the response of BVOC fluxes to an important global change process: drought. We describe the diurnal and seasonal variation in isoprene, monoterpene, and methanol fluxes from a temperate forest ecosystem before, during, and after an extreme 2012 drought event in the Ozark region of the central USA. BVOC fluxes were dominated by isoprene, which attained high emission rates of up to 35.4 mg m^?2 h^?1 at midday. Methanol fluxes were characterized by net deposition in the morning, changing to a net emission flux through the rest of the daylight hours. Net flux of CO₂ reached its seasonal maximum approximately a month earlier than isoprenoid fluxes, which highlights the differential response of photosynthesis and isoprenoid emissions to progressing drought conditions. Nevertheless, both processes were strongly suppressed under extreme drought, although isoprene fluxes remained relatively high compared to reported fluxes from other ecosystems. Methanol exchange was less affected by drought throughout the season, confirming the complex processes driving biogenic methanol fluxes. The fraction of daytime (7–17 h) assimilated carbon released back to the atmosphere combining the three BVOCs measured was 2% of gross primary productivity (GPP) and 4.9% of net ecosystem exchange (NEE) on average for our whole measurement campaign, while exceeding 5% of GPP and 10% of NEE just before the strongest drought phase. The megan v2.1 model correctly predicted diurnal variations in fluxes driven mainly by light and temperature, although further research is needed to address model BVOC fluxes during drought events.     

Photoinhibition of Photosynthesis in Plants Growing in Natural Tropical Forest Gaps. A Chlorophyll Fluorescence Study

Photoinhibition of photosynthesis was monitored by means of chlorophyll a fluorescence in leaves of plants growing in 60–80 m² light gaps in a moist tropical lowland forest located on Barro Colorado Island in central Panama. In these forest gaps, photon flux density was low (less than 100 μmol photons m^?2 s^?1) during most of the day, but increased on clear days to 1.7-1.8 mmol photons m^?2 s^?1 for 1–2 h during midday. Nine species representing different taxa and life-forms were examined. Leaves of all species exhibited substantial photoinhibition in situ during high light exposure, as manifested by a decrease in the ratio of variable to maximum fluorescence emission, F_V/F_M. Recovery (reversion of fluorescence quenching) took place in the shade following high light exposure. The major part of recovery occurred in a fast phase within about 1 h after the high light period. A slow phase of recovery proceeded for another 4–5 h until sunset. After 30–60 min of recovery in the shade, calculated rates of PSII electron transport remained significantly (5–15%) reduced in comparison to rates obtained prior to high light exposure; after about 2 h of recovery, inhibition was negligible. All species responded to the high light periods and following shade periods in a very similar manner. It is concluded that photoinhibition and recovery exhibited by these gap leaves reflect a dynamic regulatory mechanism of thermal energy dissipation that allows plants of different life-forms to cope with periods of high light in tropical forest gaps.     

Chronic photoinhibition in seedlings of tropical trees

Seedlings of five canopy species of tropical trees from Costa Rica and Puerto Rico were grown in full shade (midday range of photosynthetic photon flux density [PPFD], 100–140 μmol m^?2 s^?1), partial shade (midday PPFD, 400–600 μmol m^?2 s^?1) and full sun (midday PPFD, 1 500–1 800 μmol m^?2 s^?1) for 3 months. The species were Ochroma lagopus (Bombacaceae), a pioneer species; Inga edulis (Fabaceae), found in secondary forest; and Dipteryx panamensis (Fabaceae), Hampea appendiculata (Malvaceae), and Manilkara bidentata (Sapotaceae), three species characteristic of primary forest. After the plants were placed in the dark overnight, chlorophyll fluorescence characteristics were measured for recently expanded and mature leaves. The ratio of variable fluorescence to maximum fluorescence (F_v/F_m) was used to estimate the degree of chronic photoinhibition. Only individuals of one species, Dipteryx panamensis, showed significant depression of F_v/F_m after long-term exposure to full sun. The depression was highly correlated with quantum yield of O₂ evolution which also declined after exposure to full sun. The decline may have been related to foliar N concentration. Although all plants were supplied with ample nutrients, foliar N did not increase significantly for Dipteryx seedlings in full sun, whereas it did for Ochroma and Inga. Leaf age affected F_v/F_m only in the cases of Manilkara, where it was slightly lower in recently expanded leaves, and of Dipteryx where it interacted with the effects of light regime. We conclude that chronic photoinhibition is not common in seedlings of canopy trees of tropical rain forests except when availability of mineral nutrients may be limiting.     

Stress-induced changes in carbon sources for isoprene production in Populus deltoides

Isoprene is emitted from leaves of numerous plant species and has important implications for plant metabolism and atmospheric chemistry. The ability to use stored carbon (alternative carbon sources), as opposed to recently assimilated photosynthate, for isoprene production may be important as plants routinely experience photosynthetic depression in response to environmental stress. A CO₂‐labelling study was performed and stable isotopes of carbon were used to examine the role of alternative carbon sources in isoprene production in Populus deltoides during conditions of water stress and high leaf temperature. Isotopic fractionation during isoprene production was higher in heat‐ and water‐stressed leaves (?8.5 and ?9.3‰, respectively) than in unstressed controls (?2.5 to ?3.2‰). In unstressed plants, 84–88% of the carbon in isoprene was derived from recently assimilated photosynthate. A significant shift in the isoprene carbon composition from photosynthate to alternative carbon sources was observed only under severe photosynthetic limitation (stomatal conductance < 0.05 mol m^?2 s^?1). The contribution of photosynthate to isoprene production decreased to 77 and 61% in heat‐ and water‐stressed leaves, respectively. Across water‐ and heat‐stress experiments, allocation of photosynthate was negatively correlated to the ratio of isoprene emission to photosynthesis. In water‐stressed plants, the use of alternative carbon was also related to stomatal conductance. It has been proposed that isoprene emission may be regulated by substrate availability. Thus, understanding carbon partitioning to isoprene production from multiple sources is essential for building predictive models of isoprene emission.     

Isoprene emission by plants is affected by transmissible wound signals

Isoprene (2-methyl 1,3-butadiene) is emitted from many plants, but the signals regulating isoprene emission are unknown. Mounting leaves in a gas exchange chamber or taking small leaf punches for biochemical analysis was found to reduce the rate of isoprene emission (Loreto & Sharkey 1993). This phenomenon was investigated by putting terminal leaflets of velvet bean (Mucuna deeringeniana L.) and kudzu [Pueraria lobaia (Willd) Ohwi.] into a gas exchange chamber and monitoring isoprene emission and photosynthesis. Lateral leaflets or remote leaves were then wounded or mechanically stimulated. The rate of isoprene emission was reduced after 1 min by up to 75% by burning a lateral leaflet with a match. Even a 7 ms^?1 (25km h^?1) wind imposed on a lateral leaflet reduced isoprene emission from the terminal leaflet by 18%. Photosynthesis rates were either unaffected by these treatments or reduced more slowly than isoprene emission rates, indicating that the effect of isoprene emission rates was not a consequence of changes in photosynthetic activity. Isoprene emission from a terminal leaflet was reduced by burning leaves above and below the monitored leaflet when on the same stem. The effect was much reduced if the burned leaf (all three leaflets) was on a different stem from the monitored leaflet. Reduction of the rate of isoprene emission was observed even when the burned leaf was 52 cm distant from the measured leaflet. Increasing the distance between the stressed leaf and the monitored leaf caused the effect to be slower and smaller. It is speculated that a signal is generated by wounding which propagates through the plant at 1.3 mm s^?1. This velocity was consistent throughout the measurements and is similar to the rate of propagation of electrical signals such as action potentials and variation potentials. The effect of the environmental stress, and particularly the wind effect, can be frequent in nature and should be considered when estimating local and regional emission of isoprene for modelling atmospheric chemistry. If leaf samples used for isoprene determination are exposed to the type of stress we investigated, isoprene emission inventories based on leaf level measurements will be underestimated.     

Rate of acclimation of the capacity for isoprene emission in response to light and temperature

Isoprene emission from plants accounts for nearly half of all non‐methane hydrocarbons entering the atmosphere. Light and temperature regulate the instantaneous rate of isoprene emission but there is increasing evidence that they also affect the capacity for isoprene emission (i.e. the rate measured under standard conditions). We tested the rate of acclimation of the capacity for isoprene emission following step changes in growth conditions. Acclimation to new growth temperatures was very rapid, with most of the change occurring within a few hours and complete adjustment occurring within a day. Acclimation to new light levels was more complicated. Following a switch from low‐light growth conditions to standard assay conditions (30 °C and 1000 µmol photons m^?2 s^?1), there was a rapid (5–10 min) and a slightly slower (10–50 min) acclimation of the capacity for isoprene emission. After accounting for these short‐term changes, there was also a small, long‐term (4–6 d) acclimation of the isoprene emission capacity to the light level of growth conditions. We found no effect of growth conditions on the coefficients used to describe the instantaneous light and temperature response of isoprene emission. Therefore, current models of isoprene emission will only need to be altered to account for changes in the capacity for isoprene emission.     

Interactive effects of elevated CO2 and soil fertility on isoprene emissions from Quercus robur

The effects of global change on the emission rates of isoprene from plants are not clear. A factor that can influence the response of isoprene emission to elevated CO₂ concentrations is the availability of nutrients. Isoprene emission rate under standard conditions (leaf temperature: 30°C, photosynthetically active radiation (PAR): 1000 μmol photons m^?2 s^?1), photosynthesis, photosynthetic capacity, and leaf nitrogen (N) content were measured in Quercus robur grown in well‐ventilated greenhouses at ambient and elevated CO₂ (ambient plus 300 ppm) and two different soil fertilities. The results show that elevated CO₂ enhanced photosynthesis but leaf respiration rates were not affected by either the CO₂ or nutrient treatments. Isoprene emission rates and photosynthetic capacity were found to decrease with elevated CO₂, but an increase in nutrient availability had the converse effect. Leaf N content was significantly greater with increased nutrient availability, but unaffected by CO₂. Isoprene emission rates measured under these conditions were strongly correlated with photosynthetic capacity across the range of different treatments. This suggests that the effects of CO₂ and nutrient levels on allocation of carbon to isoprene production and emission under near‐saturating light largely depend on the effects on photosynthetic electron transport capacity.     

Monoterpene ‘thermometer’ of tropical forest‐atmosphere response to climate warming

Tropical forests absorb large amounts of atmospheric CO₂ through photosynthesis but elevated temperatures suppress this absorption and promote monoterpene emissions. Using ¹³CO₂ labeling, here we show that monoterpene emissions from tropical leaves derive from recent photosynthesis and demonstrate distinct temperature optima for five groups (Groups 1–5), potentially corresponding to different enzymatic temperature‐dependent reaction mechanisms within β‐ocimene synthases. As diurnal and seasonal leaf temperatures increased during the Amazonian 2015 El Niño event, leaf and landscape monoterpene emissions showed strong linear enrichments of β‐ocimenes (+4.4% °C^?1) at the expense of other monoterpene isomers. The observed inverse temperature response of α‐pinene (?0.8% °C^?1), typically assumed to be the dominant monoterpene with moderate reactivity, was not accurately simulated by current global emission models. Given that β‐ocimenes are highly reactive with respect to both atmospheric and biological oxidants, the results suggest that highly reactive β‐ocimenes may play important roles in the thermotolerance of photosynthesis by functioning as effective antioxidants within plants and as efficient atmospheric precursors of secondary organic aerosols. Thus, monoterpene composition may represent a new sensitive ‘thermometer’ of leaf oxidative stress and atmospheric reactivity, and therefore a new tool in future studies of warming impacts on tropical biosphere‐atmosphere carbon‐cycle feedbacks.     

Strong correlation between isoprene emission and gross photosynthetic capacity during leaf phenology of the tropical tree species Hymenaea courbaril with fundamental changes in volatile organic compounds emission composition during early leaf development

Changes of the volatile organic compounds (VOC) emission capacity and composition of different developmental stages of the tropical tree species Hymenaea courbaril were investigated under field conditions at a remote Amazonian rainforest site. The basal emission capacity of isoprene changed considerably over the course of leaf development, from young to mature and to senescent leaves, ultimately spanning a wide range of observed isoprene basal emission capacities from 0.7 to 111.5 µg C g^?1 h^?1 during the course of the year. By adjusting the standard emission factors for individual days, the diel courses of instantaneous isoprene emission rates could nevertheless adequately be modelled by a current isoprene algorithm. The results demonstrate the inadequacy of using one single standard emission factor to represent the VOC emission capacity of tropical vegetation for an entire seasonal cycle. A strong linear correlation between the isoprene emission capacity and the gross photosynthetic capacity (GP_max) covering all developmental stages and seasons was observed. The present results provide evidence that leaf photosynthetic properties may confer a valuable basis to model the seasonal variation of isoprenoid emission capacity; especially in tropical regions where the environmental conditions vary less than in temperate regions. In addition to induction and variability of isoprene emission during early leaf development, considerable amounts of monoterpenes were emitted in a light‐dependent manner exclusively in the period between bud break and leaf maturity. The fundamental change in emission composition during this stage as a consequence of resource availability (supply side control) or as a plant's response to the higher defence demand of young emerging leaves (demand‐side control) is discussed. The finding of a temporary emergence of monoterpene emission may be of general interest in understanding both the ecological functions of isoprenoid production and the regulatory processes involved.     

Isoprenoid emission in trees of Quercus pubescens and Quercus ilex with lifetime exposure to naturally high CO2 environment†

The long‐term effect of elevated atmospheric CO₂ on isoprenoid emissions from adult trees of two Mediterranean oak species (the monoterpene‐emitting Quercus ilex L. and the isoprene‐emitting Quercus pubescens Willd.) native to a high‐CO₂ environment was investigated. During two consecutive years, isoprenoid emission was monitored both at branch level, measuring the actual emissions under natural conditions, and at leaf level, measuring the basal emissions under the standard conditions of 30 °C and at light intensity of 1000 µmol m^?2 s^?1. Long‐term exposure to high atmospheric levels of CO₂ did not significantly affect the actual isoprenoid emissions. However, when leaves of plants grown in the control site were exposed for a short period to an elevated CO₂ level by rapidly switching the CO₂ concentration in the gas‐exchange cuvette, both isoprene and monoterpene basal emissions were clearly inhibited. These results generally confirm the inhibitory effect of elevated CO₂ on isoprenoid emission. The absence of a CO₂ effect on actual emissions might indicate higher leaf temperature at elevated CO₂, or an interaction with multiple stresses some of which (e.g. recurrent droughts) may compensate for the CO₂ effect in Mediterranean ecosystems. Under elevated CO₂, isoprene emission by Q. pubescens was also uncoupled from the previous day's air temperature. In addition, pronounced daily and seasonal variations of basal emission were observed under elevated CO₂ underlining that correction factors may be necessary to improve the realistic estimation of isoprene emissions with empirical algorithms in the future. A positive linear correlation of isoprenoid emission with the photosynthetic electron transport and in particular with its calculated fraction used for isoprenoid synthesis was found. The slope of this relationship was different for isoprene and monoterpenes, but did not change when plants were grown in either ambient or elevated CO₂. This suggests that physiological algorithms may usefully predict isoprenoid emission also under rising CO₂ levels.     

Soil–atmosphere exchange of greenhouse gases in a Eucalyptus marginata woodland, a clover-grass pasture, and Pinus radiata and Eucalyptus globulus plantations

Soils provide the largest terrestrial carbon store, the largest atmospheric CO₂ source, the largest terrestrial N₂O source and the largest terrestrial CH₄ sink, as mediated through root and soil microbial processes. A change in land use or management can alter these soil processes such that net greenhouse gas exchange may increase or decrease. We measured soil–atmosphere exchange of CO₂, N₂O and CH₄ in four adjacent land‐use systems (native eucalypt woodland, clover‐grass pasture, Pinus radiata and Eucalyptus globulus plantation) for short, but continuous, periods between October 2005 and June 2006 using an automated trace gas measurement system near Albany in southwest Western Australia. Mean N₂O emission in the pasture was 26.6 μg N m⁻² h⁻¹, significantly greater than in the natural and managed forests (< 2.0 μg N m⁻² h⁻¹). N₂O emission from pasture soil increased after rainfall events (up to 100 μg N m⁻² h⁻¹) and as soil water content increased into winter, whereas no soil water response was detected in the forest systems. Gross nitrification through ¹⁵N isotope dilution in all land‐use systems was small at water holding capacity < 30%, and under optimum soil water conditions gross nitrification ranged between < 0.1 and 1.0 mg N kg⁻¹ h⁻¹, being least in the native woodland/eucalypt plantation < pine plantation < pasture. Forest soils were a constant CH₄ sink, up to −20 μg C m⁻² h⁻¹ in the native woodland. Pasture soil was an occasional CH₄ source, but weak CH₄ sink overall (−3 μg C m⁻² h⁻¹). There were no strong correlations (R < 0.4) between CH₄ flux and soil moisture or temperature. Soil CO₂ emissions (35–55 mg C m⁻² h⁻¹) correlated with soil water content (R < 0.5) in all but the E. globulus plantation. Soil N₂O emissions from improved pastures can be considerable and comparable with intensively managed, irrigated and fertilised dairy pastures. In all land uses, soil N₂O emissions exceeded soil CH₄ uptake on a carbon dioxide equivalent basis. Overall, afforestation of improved pastures (i) decreases soil N₂O emissions and (ii) increases soil CH₄ uptake.     

Fine root dynamics and trace gas fluxes in two lowland tropical forest soils

Fine root dynamics have the potential to contribute significantly to ecosystem‐scale biogeochemical cycling, including the production and emission of greenhouse gases. This is particularly true in tropical forests which are often characterized as having large fine root biomass and rapid rates of root production and decomposition. We examined patterns in fine root dynamics on two soil types in a lowland moist Amazonian forest, and determined the effect of root decay on rates of C and N trace gas fluxes. Root production averaged 229 (±35) and 153 (±27) g m^?2 yr^?1 for years 1 and 2 of the study, respectively, and did not vary significantly with soil texture. Root decay was sensitive to soil texture with faster rates in the clay soil (k=?0.96 year^?1) than in the sandy loam soil (k=?0.61 year^?1), leading to greater standing stocks of dead roots in the sandy loam. Rates of nitrous oxide (N₂O) emissions were significantly greater in the clay soil (13±1 ng N cm^?2 h^?1) than in the sandy loam (1.4±0.2 ng N cm^?2 h^?1). Root mortality and decay following trenching doubled rates of N₂O emissions in the clay and tripled them in sandy loam over a 1‐year period. Trenching also increased nitric oxide fluxes, which were greater in the sandy loam than in the clay. We used trenching (clay only) and a mass balance approach to estimate the root contribution to soil respiration. In clay soil root respiration was 264–380 g C m^?2 yr^?1, accounting for 24% to 35% of the total soil CO₂ efflux. Estimates were similar using both approaches. In sandy loam, root respiration rates were slightly higher and more variable (521±206 g C m² yr^?1) and contributed 35% of the total soil respiration. Our results show that soil heterotrophs strongly dominate soil respiration in this forest, regardless of soil texture. Our results also suggest that fine root mortality and decomposition associated with disturbance and land‐use change can contribute significantly to increased rates of nitrogen trace gas emissions.     

Nitrogen addition reduces soil respiration in a mature tropical forest in southern China

Response of soil respiration (CO₂ emission) to simulated nitrogen (N) deposition in a mature tropical forest in southern China was studied from October 2005 to September 2006. The objective was to test the hypothesis that N addition would reduce soil respiration in N saturated tropical forests. Static chamber and gas chromatography techniques were used to quantify the soil respiration, following four‐levels of N treatments (Control, no N addition; Low‐N, 5 g N m^?2 yr^?1; Medium‐N, 10 g N m^?2 yr^?1; and High‐N, 15 g N m^?2 yr^?1 experimental inputs), which had been applied for 26 months before and continued throughout the respiration measurement period. Results showed that soil respiration exhibited a strong seasonal pattern, with the highest rates found in the warm and wet growing season (April–September) and the lowest rates in the dry dormant season (December–February). Soil respiration rates showed a significant positive exponential relationship with soil temperature, whereas soil moisture only affect soil respiration at dry conditions in the dormant season. Annual accumulative soil respiration was 601±30 g CO₂‐C m^?2 yr^?1 in the Controls. Annual mean soil respiration rate in the Control, Low‐N and Medium‐N treatments (69±3, 72±3 and 63±1 mg CO₂‐C m^?2 h^?1, respectively) did not differ significantly, whereas it was 14% lower in the High‐N treatment (58±3 mg CO₂‐C m^?2 h^?1) compared with the Control treatment, also the temperature sensitivity of respiration, Q₁₀ was reduced from 2.6 in the Control with 2.2 in the High‐N treatment. The decrease in soil respiration occurred in the warm and wet growing season and were correlated with a decrease in soil microbial activities and in fine root biomass in the N‐treated plots. Our results suggest that response of soil respiration to atmospheric N deposition in tropical forests is a decline, but it may vary depending on the rate of N deposition.     

Enhanced isoprene emission capacity and altered light responsiveness in aspen grown under elevated atmospheric CO2 concentration

Controversial evidence of CO₂‐responsiveness of isoprene emission has been reported in the literature with the response ranging from inhibition to enhancement, but the reasons for such differences are not understood. We studied isoprene emission characteristics of hybrid aspen (Populus tremula x P. tremuloides) grown under ambient (380 μmol mol^?1) and elevated (780 μmol mol^?1) [CO₂] to test the hypothesis that growth [CO₂] effects on isoprene emission are driven by modifications in substrate pool size, reflecting altered light use efficiency for isoprene synthesis. A novel in vivo method for estimation of the pool size of the immediate isoprene precursor, dimethylallyldiphosphate (DMADP) and the activity of isoprene synthase was used. Growth at elevated [CO₂] resulted in greater leaf thickness, more advanced development of mesophyll and moderately increased photosynthetic capacity due to morphological “upregulation”, but isoprene emission rate under growth light and temperature was not significantly different among ambient‐ and elevated‐[CO₂]‐grown plants independent of whether measured at 380 μmol mol^?1 or 780 μmol mol^?1 CO₂. However, DMADP pool size was significantly less in elevated‐[CO₂]‐grown plants, but this was compensated by increased isoprene synthase activity. Analysis of CO₂ and light response curves of isoprene emission demonstrated that the [CO₂] for maximum isoprene emission was shifted to lower [CO₂] in elevated‐[CO₂]‐grown plants. The light‐saturated isoprene emission rate (I_max,Q) was greater, but the quantum efficiency at given I_max,Q was less in elevated‐[CO₂]‐grown plants, especially at higher CO₂ measurement concentration, reflecting stronger DMADP limitation at lower light and higher [CO₂]. These results collectively demonstrate important shifts in light and CO₂‐responsiveness of isoprene emission in elevated‐[CO₂]‐acclimated plants that need consideration in modeling isoprene emissions in future climates.