Regional Isoprenoid Emission from Eucalyptus grandis Forests in Northeastern Argentina.
Eucalyptus species are strong source of isoprenoid emission. The objective of the present study was to estimate isoprenoid emissions from Eucalyptus grandis forest crops in the Entre Rios province in the Mesopotamia region of Argentina. The emission rates of isoprene and monoterpenes were measured on individual leaves under; controlled environmental conditions. At 30C and with a PPFD of 1000 uM m-2 s-1 the emission rate of isoprene emission was 12.5 1.9 nM m-2 s-1 and monoterpenes 15.1 3.1 nM m-2 s-1. Emission was significantly affected by leaf position and decreased significantly from East to West. The most abundant compound emitted was limonene, accounting for between 50 and 68% of the total monoterpene emission. The time course of the isoprene emission course showed its strong dependence on light.
On sunny days 42% of the isoprene emission comes from top of the canopy and only 2% from bottom of the canopy. But on overcast days, the relative contribution of diffuse radiation may be larger and can exceed the contribution of direct radiation. A model procedure in a Geographic Information System was implemented to estimate isoprene emissions at a regional scale. A forest inventory, data from a meteorological station and leaf area indices derived from satellite data served as inputs for the model. For the Entre Rios province (78781 km2), the isoprene emission totals up to 39.5 t d-1 on a clear summer day. The methodology applied to estimate isoprenoid emissions on a regional scale contributes to the understanding of carbon exchange between biosphere and atmosphere.
Keywords: Biogenic emission; Isoprenoid; Gas exchange measurements; Eucalyptus grandis; Argentina
Terrestrial ecosystems play an important role in carbon cycling, and interact in multiple ways with atmospheric processes. The emission of volatile organic compounds (mainly isoprene and monoterpenes) from terrestrial vegetation represents a global input of 1 Pg of carbon into the atmosphere (Guenther et al., 2012). Isoprenoids are highly reactive and as a consequence significantly affect the chemical and physical properties of the atmosphere (Chameides et al., 1988; Centritto et al., 2011); through the formation of tropospheric O3 and secondary organic aerosols, in addition to influencing the lifetime of powerful greenhouse gases including methane (Trainer et al., 1987; Fuentes et al., 2000; Claeys et al., 2004).
Isoprene (C5) and monoterpenes (C10) are formed in the chloroplast through the methyl erythritol phosphate pathway using newly fixed carbon during photosynthesis (Brilli et al., 2007; Grote et al., 2014).
Experimental evidence shows that isoprenoid emission depends upon many factors that are likely to be affected by global change (Penuelas and Staudt, 2010; Li and Sharkey, 2013; Grote et al., 2014; Sharkey and Monson, 2014): (1) environmental conditions that raise emission, such as temperature (Sharkey and Loreto, 1993; Fares et al., 2011; Brilli et al., 2013) and photosynthetically active radiation (Sharkey and Loreto, 1993; Loreto and Centritto, 2008); (2) factors that decrease emission rates, such as improper spectral composition of the light (Pallozzi et al., 2013a, b), abiotic and biotic stresses (Brilli et al., 2007; Loreto and Schnitzler, 2010; Niinemets, 2010;
Brilli et al., 2013; Harrison et al., 2013); (3) atmosphere concentration of CO2 and O3 (Lerdau, 2007; Loreto et al., 2007); (4) plant form, development and functional type (Brilli et al., 2013; Harrison et al., 2013; Loreto and Fineschi, 2015); and (5) land-use changes (Geron et al., 2006; Ciccioli et al., 2014; Sharkey and Monson, 2014). Quantitative predictions of global isoprenoid emissions to climate change are extremely complex due to the multifaceted interactions amongst the multiple determinants that control emission rates.
Nonetheless, global change is expected to dramatically increase the level of isoprenoid emission, mostly through the influence of rising temperatures (Penuelas and Staudt, 2010; Sharkey and Monson, 2014).
At present, models estimate that the global emission of isoprene, the most abundant VOC, accounts for approximately 0.5 Pg C yr-1 (Arneth et al., 2008; Ashworth et al., 2013), whereas the annual estimate of monoterpene emissions ranges between 0.03 and 0.15 Pg (Ashworth et al., 2013). However, scaling up isoprenoid emissions from the leaf to larger scale remains a major challenge, as the amount and components of isoprenoid emission vary geographically depending upon the vegetation type, the dominant species and canopy structure (Ashworth et al., 2013; Guenther, 2013; Grote et al., 2014). Furthermore, little information regarding isoprenoid emission in the southernmost regions of South America is currently available.
The objective of the present study was therefore to estimate isoprenoid emissions from Eucalyptus grandis forests in the Entre Rios province. Here we present a case study aimed at implementing a methodology for upscaling leaf level data to estimate potential emissions at a canopy scale. We have focused on isoprene, as unlike monoterpenes, isoprene emissions can be modelled with relatively high reliability (Arneth et al., 2008). Meteorological data and spatial distributed leaf area indices derived from data of the Moderate Resolution Imaging Spectroradiometer (MODIS) were used to estimate forest emissions during the study period by applying methodologies described by Muller et al. (2008).
Materials and Methods
The Entre Rios province of Argentina occurs between 30.2 and 33.8 Southern Latitude and 57.8 and 60.5 Western Longitude, with an area of 78781 km2. The climate is subtropical; with an average maximum temperature over the past 30 years of 32.3C (January) and 17.9C (July). The mean annual precipitation is 1345 mm, with the maximum in April (151 mm) and minimum in July (53 mm). Eucalyptus grandis is the most important commercial species and most planted tree in the Entre Rios province: E. grandis plantations cover nearly 103000 ha (SAyDS, 2007; MAGyP - DPF - Area SIG e Inventario Forestal 2008/2009). The average time when plantation destination is wood sawmills is 10-15 years. One-year old saplings were planted in spring of 2002. Measurements were performed on eight-year old plants, approximately 24-25 m in height. The forest stand had a density of 1111 trees per ha.
Gas Exchange Measurements
Photosynthesis (An), stomatal conductance (gs), intercellular [CO2] (Ci) and isoprenoid emissions were measured in situ between 10:00 and 16:00 h. A round portion (6 cm2) of fully expanded leaf of E. grandis was clamped in the cuvette of the portable IRGA system (LI-6400, Lincoln, Nebraska, USA). To measure the basal rate of isoprenoid emissions, all gas exchange measurements were made in ambient [CO2] (380 M M-1), at PPFD (photosynthetic photon flux density) of 1000 uM m-2 s-1, relative humidity of 50-55% and leaf temperature of 30C. The measurements were made in March 2010 on leaves selected from the centre, east and west parts of ten E. grandis trees.
When An became steady, the chamber outflow was disconnected from IRGA and diverted into a silcosteel cartridge packed with 200 mg of Tenax (Markes International Limited, UK). A volume of 2 L of air was pumped through the trap at a rate of 200 mL min-1. The cartridges were then analyzed through a thermal desorber UNITY (Markes International Limited, UK) by using a gas chromatograph (GC-Agilent 6850, Agilent Technologies, Wilmington, DE, USA) equipped with a splitless injector and a HP-5MS capillary column (30 m in length, 250 m i.d. and 0.25 m film thickness) and coupled with a mass selective detector (MS-Agilent 5975C, Agilent Technologies, Wilmington, DE, USA). Helium was used as a carrier gas. The concentration of each volatile compound was calculated by comparison with the peak area of a gaseous standard.
The GC-MS system was calibrated weekly using cylinders with a standard of each detected compound (Rivoira, Milan, Italy) and the concentration of each volatile compound was calculated by comparison with the peak area of the gaseous standard. Different compounds were identified via the NIST library provided with the GC/MS Chem Station software (Agilent). Gas chromatography peak retention time was substantiated by analysis of parent ions and main fragments on the spectra. Following isoprenoid sampling, measurements of dark respiration (Rd) were also made at ambient CO2 concentration on the same leaves by switching off the light in the cuvette and measuring the CO2 emission rate.
Meteorological input data for the isoprene emission model consisted of global radiation, temperature and relative humidity. The data were measured at one of the meteorological stations of the Instituto Nacional de Tecnologia Agropecuaria (INTA), located in Concordia in Eastern Entre Rios at a height of 48 m asl (31 23' S, 58 02' W). The model calculations were carried out using the meteorological values for a day in late summer (March 24, 2010) with clear sky, a maximum mean hourly global radiation level of 888 W m-2, temperatures between 13C (in the early morning) and 30C (in the late afternoon) and values of relative humidity between 26% (in the afternoon) and 97% (in the morning). The leaf area indices were taken from a NASA MODIS product (http://modis.gsfc.nasa.gov), which provides composite images every 8 days, with a spatial resolution of 0.5' (roughly 1 km). We used LAI data from 22 to 29 March 2010 as this covered the measurement period.
The model calculations were carried out for 70 LAI classes, ranging from 0.1 to 7, the highest LAI of the MODIS product. Twenty eight percent of LAI values were between 1.0 and 2.0, 22% between 2.0 and 3.0 and 29% between 6.0 and 7.0. Isoprenoid emissions were calculated for all LAI classes (in steps of 0.1) on the grid of leaf area indices. The emissions were weighed in each grid cell with the portion of area covered by Eucalyptus forests as given by the forest inventory. The maps were created with a Geographical Information System, ArcGIS (ESRI, Redlands, USA), which allows software development with Visual Basic to realize the model computations.
Dependence of Isoprene Emissions on Temperature and PPFD
The dependence of isoprene emission EISO (nM m-2 s-1) per leaf area on temperature and radiation was calculated as (European Emission Agency, 2007):
Where, EISO,S is isoprene emission at standard conditions (leaf temperature TS = 30C and photosynthetic photon flux density active radiation IS = 1000 uM quanta m-2 s-1), CT and CPPFD are correction factors which take into account deviations from standard conditions. For the dependence of the emissions on leaf temperature and PPFD we used semi-empirical equations recommended by Guenther et al. (1993). The correction factor CT describes the dependence of the isoprene emission on leaf temperature T:
Where, R (= 8.314 J-1 M-1 K-1) is the universal gas constant, cT1 (= 95 000 J M-1), cT2 (= 230 000 J M-1) and TM (= 314 K) are empirical coefficients and cT3 (= 0.96) assures that CT is equal to one at standard temperature (Guenther, 1997). The correction factor increases with temperature, but decreases after a maximum at 39C. The dependence on PPFD is given by:
Where, I is PPFD (uM m-2 s-1 per leaf area) and a (= 0.0027) and cL1 (= 1.066) are empirical coefficients. The function is zero at night and reaches its maximum asymptotically with increasing radiation. The two functions are suitable to describe the isoprene emission variation for a number of different plants including (1994), de Pury and Farquhar (1997), Campbell and Norman (1998) and Friend (2001) to compute the radiation absorbed by sunlit and shaded leaves in the canopy. The radiation absorbed by shaded leaves takes into account diffuse radiation as well as scattered radiation. The radiation absorbed by sunlit leaves takes into account the additional direct irradiation from the sun.
Leaf temperature was calculated with the leaf energy budget of Campbell and Norman (1998) and Monteith and Unsworth (2008). The energy budget requires the energy absorbed by the leaf, which comprises PPFD, near-infrared radiation (NIR) and long-wave radiation from sun, sky and soil. The contributions of long-wave radiation from the canopy, sky and soil are calculated with the Stefan- Boltzmann equation (Goudriaan and van Laar, 1994). The absorption of short-wave and long-wave radiation depends on leaf height in the canopy as described by Wang and Leuning's model (1998). Sky emissivity was calculated according to Brunt's equation (Brunt, 1932), using FAO parameterization (FAO, 1990). The energy budget also takes into account energy lost through thermal radiation, by heat flow due to the difference between the leaf and ambient air, and by transpiration.
To calculate transpiration the measured stomatal conductance (Table 1) was used, and its variation in response to PPFD was estimated using a rectangular hyperbola (Jarvis, 1976; Baldocchi et al., 1987).
The energy budget equation is solved by iterative application of the Newton-Raphson method (Dai et al., 2004; Steinbrecher et al., 2009). As the absorbed shortwave radiation is different for sunlit and shaded leaves, different leaf temperatures were calculated for these leaves.
Canopy Model for Radiation
Because isoprene emission is strongly influenced by radiation, a model considering light extinction in the canopy was applied to scale up isoprene emissions from leaves to the whole canopy. To apply the radiation canopy model, the radiation above the canopy has to be subdivided into its components, e.g. direct and diffuse radiation.
These components were estimated on the basis of global radiation and solar elevation angle according to the methodology described by de Pury and Farquhar (1997).
One half of the energy flux is assumed to be in the range of the photosynthetically active radiation (400 - 700 nm) (Monteith and Unsworth, 2008). The direct and the diffuse photosynthetically active radiation is then calculated with a methodology described by de Pury and Farquhar (1997). To convert the energy flux to quantum flux a conversion factor of 4.57 u M quanta J-1 can be applied, for diffuse radiation the conversion factor is 4.24 u M quanta J-1 (McCree, 1972).
The resultant direct and diffuse PPFD values serve as inputs for the canopy model. The radiation canopy model applied equations and parameters given by Goudriaan and van Laar Eucalyptus (Guenther et al., 1993).
Canopy Model for Biogenic Emissions
The output of the canopy model for radiation are the sunlit and shaded absorbed PPFD values, Isun and Isha, at the depth of the canopy corresponding to the cumulated leaf area index L downwards from the top of the canopy.
Canopy isoprene emission, EISO, is then calculated with equation (1), which is integrated over the cumulated leaf area index L downwards from the top of the canopy:
The first and the second summand on the right side of this equation represent isoprene emission by the sunlit and shaded portions of the canopy, respectively. The variables fsun and fsha are the fractions of sunlit and shaded leaves at the canopy depth corresponding to the cumulated leaf area index L, which adopts values between the integral limits L = 0 and L = LAI, whereas LAI is the leaf area index of the canopy. The integrals were computed numerically using the Newton-Cotes method (Tornig, 1979). For the application of this method seven values of the integrand were calculated for different values of the cumulated leaf area index L. The values of the integrand are then multiplied with coefficients given by the Newton-Cotes method and summed up. The method was checked with test functions and exhibited a very high accuracy with integration errors of less than 1%.
The model procedure (Fig. 1) is carried out for LAI values up to LAI = 7. For each LAI value emission values for sunlit and shaded leaves at seven different canopy heights are used to calculate the whole canopy emission by integration. The model procedure is carried for a whole day in time steps of one hour. The daily emission of isoprene is spatially distributed corresponding to the MODIS LAI and the occurrence of Eucalyptus in the forest inventory.
Gas Exchange Measurements
There were significant differences in gas-exchange parameters among leaf location (Table 1). Photosynthesis was significantly higher in leaves to the east and centre of the tree than of west. Stomatal conductance was not statistically different between the central and west leaves, but significantly increased in the east leaves. The different dynamics of An and gs resulted in a significantly higher An in east than central leaves, while no differences were detected between east and west leaves. There were no significant differences in Rd among the different leaf location.
Table 1: Net photosynthesis (An), dark respiration (Rd), stomatal conductance (gs) and intercellular [CO2] (Ci) of E. grandis
An###mol CO2 m-2 s-1###9.50 0.86 b###9.60 0.79 b###7.24 0.57 a
Rd###mol CO2 m-2 s-1 -2.69 0.27###-2.37 0.32###-2.54 0.30
gs###mol H2O m-2 s-1###0.079 0.010 a###0.101 0.009 b###0.076 0.004 a
Ci###mol CO2 mol-1###170 9 a###206 9 b###191 7 ab
Table 2: Emission of isoprene and monoterpenes (nmol m-2 s-1) from E. grandis measured at leaf temperature 30 C and PPFD of 1000 umol m-2 s-1
Isoprene###11.1 b###18.8 c###7.4 a
-pinene###0.994 b###1.254 c###0.638 a
Camphene###0.112 a###0.174 b###0.122 a
Sabinene###0.294 a###0.233 a###0.597 b
-pinene###0.227 b###0.348 c###0.074 a
-myrcene###0.884 b###1.749 c###0.270 a
-phellandrene###0.068 b###0.155 c###0.016 a
3-carene###0.078 ab###0.110 b###0.032 a
a-terpinene###0.053 b###0.050 b###0.009 a
p-cymene###0.857 b###1.031 c###0.400 a
1-8 cineole###0.962 b###2.038 c###0.695 a
Limonene###9.788 b###16.850 c###2.948 a
-terpinene###0.098 b###0.195 c###0.024 a
-terpinolene###0.078 a###0.196 b###0.034 a
Linalool###0.128b###0.046 a###0.065 a
###Total###14.7 b###24.5 c###6.0 a
Isoprene and monoterpene emission was also significantly affected by leaf position (Table 2). Emissions of both compounds decreased significantly from east to west. In general, a similar pattern was seen also in the predominant monoterpene compounds of the emitted blend of VOCs. The most abundant compound emitted was limonene, accounting for between 50 and 68% of the total monoterpene emission in the different leaf types, followed by 1-8 cineole, b-myrcene and a-pinene.
The photosynthetically active radiation above the canopy derived from the global radiation reached values of about 361 and 74 W m-2 for direct and diffuse radiation (Fig. 2), respectively. The estimated leaf temperatures typically differed by a few degrees from the air temperature. The computations suggest that at midday, in a canopy with a high LAI, sunlit leaves were about 2 K warmer than air, while shaded leaf temperature hardly differed from air temperature. As expected, PPFD absorbed by the leaves within a canopy decreased with the canopy depth, as a result of the attenuation of diffuse light (data not shown).
At LAI = 7 the isoprene emission on the selected day (March 24, 2010) reached a value of 51.8 mg m-2 d-1. The diurnal variation of the isoprene emission at LAI = 7 was a maximum of 7.9 mg m-2 h-1 (Fig. 3). In such a canopy, 42% of the isoprene emission comes from the top of the canopy (L = 0 - 1), and only 2% from the bottom. The time course of the isoprene emission course showed its strong dependence on light (Fig. 4). The spatial distribution of the emission (Fig. 5) was determined by the occurrence of Eucalyptus forests, which are mainly found in the Eastern part of the Entre Rios province. The maps show averaged values of 100 LAI grid cells with a resolution of 5' x 5'. For the whole study area the isoprene emission totalled 39.5 t d-1.
Eucalyptus species are strong isoprene and monoterpene emitters, with large amounts of monoterpenes stored in specialised leaf cavities (Ashworth et al., 2013; Brilli et al., 2013). In such species, monoterpenes are largely emitted by evaporation in response to mechanical stress and warming (Ashworth et al., 2013). However, it has been demonstrated that emission rate of monoterpenes is not completely decoupled from their biosynthesis (Staudt et al., 1997; Komenda and Koppmann, 2002), as between 30-90% of total emission derives from light-dependent newly synthesized monoterpenes. The values of isoprene and monoterpene emissions recorded in this study reported in the literature for a number of Eucalyptus species (He et al., 2000; Brilli et al., 2013). Furthermore, isoprene measurements carried out above canopies of other species and ecosystems are also available, such as scrubland, grassland and bogs in regions with temperate and tropical climate (Pacifico et al., 2009).
The maximum fluxes of isoprene above forest canopies ranges between 2.2 mg m-2 h-1 for a tropical evergreen broadleaf forest in Amazonia (Rinne et al., 2002; Muller et al., 2008) and 29 mg m-2 h-1 for a temperate deciduous broadleaf forest in Michigan (Pressley et al., 2005). Our estimate of the maximum isoprene flux (7.9 mg m-2 h-1) for the Eucalyptus forest is thus within this measurement range.
Due to the asymptotic behaviour of the light dependency (Equation 3), maximum isoprene emission is easily achieved by leaves exposed to strong direct radiation. In contrast, shaded leaves are exposed to lower diffuse radiation, thus an increase of diffuse radiation leads to a more efficiently increase of isoprene emissions than found in sunlit leaves. Shaded leaves therefore constitute an important part of the canopy, and contribute significantly to biogenic emissions. In the present study, direct and diffuse PAR fluxes have been computed based on the global radiation for a clear sky. On such a day, diffuse radiation is much smaller than direct radiation, and exhibits little variation over the course of the day. However, during overcast skies, the contribution of diffused radiation may be larger and can exceed the contribution of the direct radiation.
Therefore, to estimate biogenic emissions over longer periods it would be recommended to apply a method, which takes into account cloudiness, or to measure both components of radiation towards their use as inputs for the radiation canopy model. The light-dependence of isoprene emissions is relatively similar for a variety of plants (see e.g. Pacifico, 2009), so these estimations of isoprene emissions can be considered to be quite reliable.
Isoprene is predominantly emitted from the sunlit fraction of the canopy, but also the shaded fraction exhibited to diffuse light. Light strongly determines isoprene emissions and the equation that describes this dependency is well known (Equation 3). In contrast, monoterpene emissions depend strongly on temperature, and a exponential function has been proposed to describe the temperature dependency (Tingey et al., 1980). Approaches exist to describe the biosynthesis and emission of terpenoids in a mechanistic way. Back et al. (2005) described the synthesis of monoterpenes in a model including photosynthesis and photorespiration, which also requires a high number of model parameters not known in the present study. Other models (Niinemets and Reichstein, 2002; Noe et al., 2006) take into account monoterpene emission kinetics, related to the pools where monoterpenes are stored.
These models require a greater number of input parameters, but in future studies may possibly be considered to replace the current empirical equation for monoterpenes.
The separation of the two components of PPFD (i.e., direct and diffuse PPFD) allows describing the light- dependent processes in the canopy. Due to the asymptotic behaviour of the light dependency (Equation 3), the maximum level of isoprene emission is easily reached for leaves exposed to strong direct radiation. In contrast, shaded leaves are exposed to lower diffuse radiation and an increase in diffuse radiation leads to a more efficient increase in isoprene emission than in sunlit leaves. Shaded leaves therefore constitute an important part of the canopy, and contribute significantly to biogenic emissions (Fig. 3).
In overcast skies the contribution of diffuse radiation to emission of isoprenoids may be even larger and exceed the contribution of direct radiation.
The difference between leaf and air temperature is strongly affected by the radiation absorbed the leaves. Leaves at the top of a canopy receive more shortwave radiation than leaves inside the canopy because radiation decreases with canopy depth. However, leaves at the top of a canopy receive relatively little thermal radiation from the sky in comparison with leaves fully surrounded by canopy, because the sky has a lower emissivity (approximately 0.80 in our case study) than leaves, which have an emissivity of nearly one. The wind speed profile within the canopy - with decreasing wind speeds from top to down (Monteith and Unsworth, 2007) - leads to a decreasing tendency for the energy exchange by transpiration and convection with ambient air (Campbell and Norman, 1998). When the model is performed with a leaf temperature equal to air temperature, the daily isoprene emission becomes 3% smaller and the maximum isoprene emission is reduced by 0.4 mg m-2 s-1.
This indicates that in the present study of a single day, the difference between air and leaf temperature does not have a major influence on the emission estimates. However, on a global scale, isoprene emission is reported to be 18% higher when leaf temperature is used instead of air temperature (Muller et al., 2008); as a consequence, to conduct a more comprehensive approach an estimation of leaf temperatures is preferable.
The present study utilises the results of field gas exchange measurements of a Eucalyptus species with the aim of estimating spatially resolved and time-dependent isoprene emissions in the Entre Rios province of Argentina. These measurements, combined with meteorological measurements, forest inventory and satellite-based data (leaf area indices), suggest emissions of up to approximately 39.5 t d-1 isoprene in the course of a clear summer day. Future studies could take into account a higher resolution of meteorological input data, in addition to leaf age nutrient conditions, soil moisture and stress situations of the plant. Furthermore, other model approaches could be tested taking into consideration the internal CO2 concentration of leaves. For this, however, more gas exchange and field measurements are needed. These measurements could include seasonal dependencies of the emissions and should cover the whole spectrum of tree species in the Mesopotamian region of Argentina.
K.R. gratefully acknowledges support from the Centrum fur internationale Migration und Entwicklung (CIM), Germany. Financial support was provided by INTA (proj. AERN 294421 and PNNAT 1128024) and by Ministero dell'Istruzione dell'Universita e della Ricerca of Italy: PRIN 2010-2011 "PRO-ROOT" and Progetto Premiale 2012 "Aqua". The authors wish to thank field work assistance to Eugenia Beget and Alejandra Kemerer and to Dr. Matthew Haworth who assisted in the proof-reading of the manuscript.
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|Publication:||International Journal of Agriculture and Biology|
|Date:||Jun 30, 2016|
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