Monoterpene emission from soils in orange plantations of the valencian citrus belt, Spain

Phys. Chem. Earth (B), Vol. 24, NO. 6, pp. 695-698, 1999 0 1999 Elsevier Science Ltd All rights reserved 1464- 1909/99/$ - see front matter

Pergamon

PII: S1464-1909(99)00067-2

Monoterpene Emission from Soils in Orange Plantations of the Valencian Citrus Belt, Spain R. Steinbrecher’, K. Hauff’, J. Riissler’, M. Diirr2, G. Seufert’ ‘Fraunhofer-Institute for Atmospheric Environmental Research (IFU) D-82467 Garmisch-Partenkirchen, Germany 2CEC-Joint Research Centre, Environment Institute, I-2 1020 Ispra, (VA), Italy Received 24 April 1999; accepted 22 October 1999

Abstract. During the mesoscalic experiment performed in June 1997 within the framework of the BEMA-project on Biogenic Emissions in the Mediterranean Area, two test sites with orange and mandarin fields were equipped with soil enclosures to analyse type and amount of terpenoid emissions from soils. Relevant emissions were found at both sites to mainly consist of limonene, but a couple of other mono- and sesquiterpenes could also be observed. For both sites and Citrus species, the terpenoid emissions were in the range of 1500-3000 pmol mm2sec.’ from soils under the trees covered with litter, and of IO-100 pmol me2 sec.’ from bare soils between rows. The origin of emissions from leaf and fruit litter was experimentally confirmed by removal of the uppermost soil layers. 0 1999 Elsevier Science Ltd. All rights reserved

2. Methods 2.1 Location The cuvette-measurements of terpene emissions from soils of a Citrus sinensis and Citrus clementii plantation were performed on two different plots ca. 3km (site B) and 20km (site D) from seashore near Burriana/Spain (0’ 12’ W, 39” 57’ N) between June 4th and June 16th, 1997. Orange trees (Citrus sinensis) and mandarine trees (Citrus clementii) were planted in rows in these plantations. The leaf area index (LAI) of the orange and mandarin trees at site B ranged between 4 to 7 and 3 and 5 at site D, respectively (R. Lenz, personal communication). The soil under the trees was covered with fresh green and older black fallen fruits and leaves. Due to herbicide treatment by the farmers, there was no ground cover vegetation at all and the soil between trees was bare.

1 Introduction Volatile organic compounds (VOC) are emitted by many plant species. Due to their high reactivity with active oxygen species, VOC significantly influence tropospheric air chemistry (e.g. Trainer et al., 1987). The European joint project BEMA (Biogenic Emission in the Mediterranean Area) aims at quantifying biogenic VOC emission rates. An important goal is to identify in detail the controlling factors of emission from vegetation in order to understand biosphere - atmosphere interactions and to quantify their role in tropospheric ozone formation in relation to anthropogenic emissions (e.g., Seufert et al. 1997). Besides natural ecosystems, managed lands like orange plantations may influence air chemistry locally and on regional scale. In the past a lot of attention was given to characterising VOC emissions from plants (see, for example, the compilation by Benjamin et al. 1996). However, VOC emission Tom soils, especially of compounds influencing air chemistry, are poorly characterised. Extensive studies were performed only on monoterpene emission from Scats pine and Norwe

2.2 Environmental parameters The measurement of VOC emission from soils was performed in June 1997. The weather in the Valencian citrus belt was usually bright with a few clouds in the afternoons. Leaf temperatures of 35 - 38°C were observed during this campaign while soil surface temperatures reached values up to 54 “C. Photosynthetic active radiation (PAR) reached values up to ca. 1900 pE. Through daily measurements the water content of the upper soil layer was pursuited by tak-

-. Correspondence to: G. Se&t.

gian spruce forests and Stone pine forests (Janson 1993, Seufert et al. 1995, Steinbrecher et al. 1997a). The origin of VOC emission from these soils is mainly the needle litter on the ground. Janson (1993) also discussed an emission from the roots of Scats pine trees. In orange plantations, soil emissions from VOC may originate !?om the same sources. A characterisation and quantification of these sources is lacking up to now. Therefore, the objective of this study was to quantify the emission of reactive VOC from the soil of orange plantations and to analyse the controlling factors.

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ing soil samples around noon after the method described in Hartge and Korn (I 992). From 07-06- 1997 to l5-06- 1997 it was very stable with a mean value of 5.3 % (std. error = 6.4 % for n = 45 values). Only on 5th and 6th of June values reached up to 10.9 % and 7.5 %, respectively, because of a thundershower in the evening of the 4th of June. 2.3 Cuvette Measurements: Identical soil cuvettes were used in the measurements at the two plots. They were built mostly of Teflon (transparent foil; thickness: 50 pm) and mounted on an aluminium frame of40 cm diameter anchored 5 cm into the ground. A slowly running, large bladed impeller was installed in each cuvette for mixing the air volume. Filtered ambient air, used to flush the cuvettes, had a residence time of about 2 min. Inlet and outlet fluxes were measured by massflow controllers to guarantee zero pressure difference (< 1Pa) inside the enclosures compared to ambient. Sensors for recording the photosynthetic active radiation were mounted on the cuvettes. Temperature measurements at site D were performed with four NiCr-Ni-thermocouples within each cuvette and two IR-sensors (OS65-V-R4-2 - NewportOmega) to record soil surface temperature. At site B, the temperature in the enclosures was monitored with shielded and ventilated Pt 1000 probes for air temperature and with Pt 1000 insertion probes to measure average soil temperature within the uppermost soil layer O-3cm. All sensors used, flow controllers and flow meters were calibrated before the field campaign. All given parameters are related to soil area and dry weight, respectively. 2.4 Terpenoid sampling and anaIysis At site D, the VOC-concentrations in the cuvette inlet and outlet air were analysed after concentration for 20 min (flow rate: 120 ml mid’) on mixed bed tubes (Carbosieve S-III/ Carboxen 1003 and Carbotrap) with an online GCsystem (Airmotec HCIOIO BTX Monitor). Measurement frequency was 1 h. The trapped compounds were thermodesorbed at 350 ‘C and cryofocused at -30 “C on a 18 cm * 0.53 mm id capillary precolumn filled with Carbopack B. The injection of the compounds onto the capillary column (DB 1701, 12 m * 0.18 id, 0.4 pm df; Restek) was achieved by heating up the precolumn to 350 “C within I sec. A linear temperature program starting at 35 “C and ending at 150 “C (rate: 8 “C min“) guaranteed the separation of the NMHC. All compounds were detected by flame ionisation (FID). The VOC-spectrum emitted by the soil was dominated by monoterpenes: limonene >> sabinene > p-cymene/l,l cineol > myrcene > A’-carene > a-pinene > f%pinene. The compounds were identified and quantified according the procedures given by Steinbrecher et al. (1997b). At site B, monoterpenes were trapped in glass tubes filled with 125 mg Tenax TA (Aldrich, 20-35 mesh) and placed in a cooled sampling device as described by Staudt et al. (1995). 3 to 6 I of air were sampled at the inlet and outlet

ports of the enclosure at a massflow controlled rate of 100 to 200 ml min.’ alter a prepurging time of 5 to 10 minutes on a bypass line. Sampling time was 30 minutes every hour. Air samples were analysed by a gas chromatograph (GC CP9001, Chrompack) equipped with a desorption unit (TCT/PTI CP4001, Chrompack), a fused silica capillary column (25 m x 0.32 mm, df: 1.2 pm CP-Sil 8 CB, Chrompack) and a FID. The carrier gas was helium (85 kPa) and the desorption and separation programs presented the following sequence: 3 min precooling at -lOO”C, IO min desorption at 2OO”C, 1 min injection at 200°C. GCoven: 4 min at 65°C 2.5 “C mitt“ to 80°C 2.0 “C min-’ to 100°C and 20°C mitt“ to 240°C. Gaseous and liquid calibration standards prepared from commercial authentic mono&penes of high purity (Fluka Aldrich, 95-99% purity) allowed the peak identification and quantification for 28 different mono- and sesquiterpenes. In both analytical systems, peak identification was verified by GC-MS-analysis (P. Ciccioli, CNR-Rome and B. Larsen, JRC-lspra). 2.5 Emission rates Emissions rates were calculated on the basis of the airflow through the cuvette and the concentration difference of the compounds between inlet and outlet. For emission factor calculation the emission rates were standardised to 30°C using the following equation (after Guenther et al. 1991) in a non-linear regression analysis of the data sets: Ea=

Em*ea*“-rs)

E, = emission factor in pmol mm2total soil surtace s-’ at 30 “C Ea = emission rate [pmol rnw2s-‘1 a= 0.090 “c-1 T= temperature of the soil [“Cl T,= 30°C 2.6 Statistics For investigating the influence of the various environmental parameters on the isoprenoid emission rate of the soil, the statistical software package of SPSS for Windows 6.1.2 (1995) was used. The relevant statistical procedures are given in the text.

3. Results and discussion The diurnal course of terpenoid emissions was recorded in four enclosures installed on litter covered soils and on bare soils of an orange and mandarin field at site B. The emission rates shown in Figure 1 are in the range of 500 and 3000 pmol me2 se’ horn soils covered with litter of orange and mandarin, and of 15 to 100 pmol mm2s-’ 6om bare soils between the trees. As shown already in BEMA-report (1997) by Steinbrecher et al., limonene represents more than 80% of litter emissions, in addition to some sabinene, myrcene, a-pinene and a-terpinolene. Rest 13 stays for the

R. Steinbrecher et al.: Monoterpene Emission from Soils in Orange m

limonene

m

~myrcene m a-terpinolene -x-enclosure air temperature

sabinene

m a-pinene Irest 13 . soil temperature 0-3cm

pmol m2 set-I !3500

degree Celsius

Navel late -with litter

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at O-3cm depth, as emissions obviously did originate from the layer of organic material on the soil at the soilatmosphere interface (see Figure 2). The temperature of the uppermost 3cm soil layer was always higher than air temperature and reached values up to 43 degrees in the full sun exposed cuvette on bare soil between the small Mandarin trees. At this place, the surface temperature measured discuontinuously with a portable IR thermometer reached values up to 54 degrees. In contrast, the enclosures in the Navel late field with mature and large trees were in full and half shadow, soil temperatures compared to air temperature were slightly lower during the day and higher during the night with differences never exceeding 5 degrees. Soil emissions of isoprenoids appeared to be under temperature control, but it remains difficult to decide what kind of temperature is most important. The best relation of emissions was to soil temperature 0-3cm and surface temperature, respectively. A non-linear regression analysis (Spss-Statistic-Software Vers 5.0.1) revealed e.g. for limonene coefftcients of determinations between 0.65 (n=9; I I-06-97; Site D) and 0.87 (n=22; 06-06-97; Site D).

00 05 07 09 II 12 13 14 I5 16 18 20 22 14Kl 3000 ,

Mandarin -with titter .

2500 I

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Figure 1: Diurnal cycle of terpenoid emissions and of air and soil temperature in four soil enclosures installed in the Navel late and Mandarin plots near Burriana. sum of all other terpenoids emitted in trace amounts like A’-carene, p-cymene, R-phellandrene, as well as the sesquiterpenes l3-caryophyllene and a-humulene. In Figure I, the diurnal variation of emission rates is given together with air temperature in the enclosure and with soil temperature

Figure 2: Emission factors of monoterpenes for investigated soil types related to ground surface. The bars denote the emission factors for each soil type (number of soil plots investigated: soil with organic layer 4 plots (mean soil covering: 30 %), bare soil 2 plots and bare soil after removal of 3 cm upper soil layer 2 plots).

The error bars note the standard deviation of the mean sum of the monoterpene emission factors; soil with organic layer n = 64, bare soil n = 23 and bare soil after removal of 3 cm upper soil layer n = 23. In addition to the diurnal course of emission rates, a specific experiment was performed at site D to test further the origin of soil emissions. Normalising monoterpene emis-

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R. Steinbrecher et al.: Monoterpene Emission from Soils in Orange Plantations

sion rates to 30 “C surface temperature leads to an estimate that the total monoterpene emission from an orange plantation amounts to 1200 pmol mq2 soil surface s-r (Figure 2). This high emission is due to the organic layer on the surface. Afier removing tiuits, blossoms and leaves the emission was reduced by 90%. Further 3cm removal of soil surtace caused a decline in the emission to almost zero. In consequence all measured emission is due to monoterpene emission of the organic layer of the soil, without any contribution of the roots. Further, the high limonene content in the emission pattern can be attributed to emission from the fiuit litter. As demonstrated in a separate experiment, fruit litter emits ca 3 times more limonene compared to leaf litter (Table 1).

emitted from the soils is assumed to be depending on the highly variable management and harvesting praxis of individual fields.

Acknowledgements We would like to acknowledge raters from CEAM/V&“cia during

the campaign.

detached leaves 414 (27)

1405 (57)

sabinene

247 (13)

191 (16)

p-cymene/l,Scineol

139 (17)

144 (19)

is greatly acknowledged.

- Report 1997: Report on the BEMA (Valencia-Spain),

measuring exercises at Burriana

April-May-September

EN, Brussels and Luxenbourg. Benjamin,

1996.

EUR

Report

17336

I997

M. T., Sudol, M., Bloch, L. and Wirier, A. M. Low-emitting

forests:

a

taxonomic

monoterpene

methodology

emission

for

assigning

rates. Atmospheric

u&m

isoprene

Environment

30,

and 1437-

1452.1996.

myrcene

47 (5)

89 (9)

A3-carene

30 (7)

53 (5)

a-pinene

29 (7)

28 (2)

S-pinene

18 (2)

18 (2)

C monoterpenes

for their support

support t?om the CEC-ENV4_CT95-

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Measurements of monoterpene emission from the soil of the orange plantation showed that the emission was mainly due to the organic layer of the soil and tiuit litter. The composition of the organic layer with fruit litter, leaf litter and old blossoms did influences the emission pattern as well as the source strength of the monoterpenes from the soil of a citrus orchard with temperature as main controlling tactor. Therefore, in assessing the VOC emission from the soils in orange plantations, beside soil temperature the organic matter, especially the fruit litter, covering the soil is a key parameter to be determined. Considering differences of up to 25 OC between one and the other plot within a distance of a few meters, especially in open-rowed type of modern plantations, this heterogeneity in soil surface temperatures can not be resolved in regional surface temperature maps up to now. In addition, parametrisation and upscaling of VOC emissions from citrus fields may tinther be complicated by the t&t that the total amount of VOC

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Table 1: Emission factors (in ng g-’ dry weight hei at 30 “C surface temperature) of monoterpenes emitted from the organic soil layer in an orange plantation (Site D, Burriana, Spain). In bracketts the asymptotic standard error (n=9; SPSS for Windows Ver. 6.1.2, 1995).

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