Received 27 February 1996; revision received 25 July 1996; revision accepted 16 October 1996
Introduction
The importance of Free Air CO2 Enrichment (FACE) concentrations and on the global change factors (Schulze
experiments for the study of the impact of rising atmo- & Mooney 1993; Kimball et al. 1995).
spheric CO2 on terrestrial ecosystems has been often The possibility of a metered release of carbon dioxide
outlined (Drake et al. 1985; Hendrey 1992). FACE systems in the field was first investigated in the late ’60s anddo have several advantages over conventional experi- early ’70s (Kretchman 1962; cited by Allen et al. 1992;mentation; they cause no detectable disturbance of the Harper et al. 1973a; Harper et al. 1973b) and paralleled aplant growth environment other than elevating the CO2 number of studies made to fumigate field plots with airconcentration and a large amount of plant material may pollutants (see McLeod 1993 for review). Technologicallybe fumigated, thus minimizing edge effects, and allowing updated FACE systems have been recently developedmultidisciplinary studies (Hendrey et al. 1993). Accord- to expose small to large field plots to elevated CO2ingly, major effort is currently made by the international concentrations under otherwise natural conditions. Inscientific community to use FACE systems to assess, particular, the Brookhaven National Laboratory (BNL)experimentally, the potential impact of elevated atmo- FACE systems have been successfully tested for CO2spheric CO2 concentrations on natural and cultivated enrichment of field plots ranging from 18 to 30 m inecosystems as well as to assess the impact that natural diameter, for grasses and pasture plants (Blum 1993;and cultivated ecosystems have on atmospheric CO2 Jongen et al. 1995), cotton (Lewin et al. 1992; Lewin et al.
1994), wheat (Kimball et al. 1995) and loblolly pine
(Hendrey et al. 1993). A Mini-FACE system that wasCorrespondence: Franco Miglietta, fax 1 39/55-308910, e-mail
[email protected] developed in Italy has allowed CO2 enrichment over an
Fig. 1 Sketch diagram showing the assemblage of the different
Mid-FACE components (see text for explanation).
Fig. 3 CO2 concentration gradient measured during a windy
day in the Mid-FACE prototype along wind direction (see
text for details). (a) shows that CO2 concentrations are higher
on the upwind side of the ring (right) and lower on the
downwind side of the ring (left). Vertical bars denote standard
deviation of the mean. Frequency distribution of one minute
average wind speed during prototype performance test are
shown in (b).
inadequate from a biological perspective. Short-termFig. 2 Performance of the Mid-FACE prototype. The fraction
spatial gradients are an unavoidable consequence of non-of time in which the one minute average CO2 concentrations
uniformity of CO2 emissions and mixing within FACEread at the centre of the array was within the 10% of targetrings operated under variable windy conditions and these(solid line) and 20% (dashed line) of target, is plotted vs.
wind speed. Data were averaged over a period of 29 days. have been always detected when spatial performances of
FACE system have been evaluated (Hendrey 1992; Lewin
et al. 1994; Walklate et al. 1996). However, over periods
of days or more such gradients average out resulting inarea of 1 m2 (Miglietta et al. 1996) and a linear FACE
system has been used for vineyard crops (Bindi et al. more uniform patterns of CO2 concentration over the
experimental area.1995). Preliminary tests of another FACE system have
been also reported in the literature (Reece et al. 1995). The most important disadvantages of FACE systems
are the elevated initial capital costs and the high operatingOne of the major concerns with FACE systems is the
spatial distribution of atmospheric CO2 concentrations costs due the large quantity of CO2 used (Schulze &
Mooney 1993). A new FACE system (Mid-FACE) basedwithin the rings. The presence of long-term spatial gradi-
ents in CO2 concentration over the FACE area that would on the same basic technological approach used by the
BNL-FACE but on a smaller scale was developed in Italy.result in plants standing on a side of the ring being
exposed to higher (or lower) mean CO2 concentrations The rationale for this attempt was to provide a less costly
and reliable facility with adequate performance to allowthan those standing on the opposite side, would be
DE S I GN AND P E R FORMANCE O F A FAC E S Y S T EM 419
Fig. 4 Horizontal CO2 concentrations measured using Draeger diffusion tubes at three different heights and times of the day
(see legend on the figures) in the Mid-FACE prototype. Isolines indicate points having the same mean CO2 concentration (µmolmol–1); the solid circle denotes the Mid-FACE area defined as ‘sweet spot’.
for experiments with elevated CO2 under field conditions. Materials and methodsThe principal design objective was finding an acceptable
compromise between the quality of temporal and spatial Mid-FACE designCO2 control and a significant reduction of both capital
cost and total CO2 use. This objective was pursued by The Mid-FACE system consists of circular array of emis-
sion pipes, carbon dioxide supply components, CO2reducing the overall size of the FACE ring to 8 m in
diameter, although it was clear that such a reduction concentration monitoring components and a PC-based
control program (Fig. 1). The array has 24 vertical ventwould have affected system performance due to the
unavoidable presence of an edge effect near gas outlets pipes each containing multiple gas emitter ports. Each
pipe is connected to a common 8 m-diameter toroidal(Walklate et al. 1996) and would have reduced signific-
antly the area usable for biological studies. distribution plenum through controlled on/off valves
based on commercially available double-effect actuatorsThis paper describes this Mid-FACE system and pro-
vides relevant information to evaluate its performance in (Mod. DEJRM 16x125, Vesta, Rovigo, I). A high volume
blower (Mod.SM250/6, APEM, Firenze, Italy) injects airterms of spatial and temporal CO2 control. Principal
results of the Potato-FACE experiment in the summer into the plenum. Due to resistances in the plenum and
in the vertical vent pipes the nominal null load flow rate1995 using this facility are given in a separate paper
(Miglietta et al. 1997). of the blower (15 m3min–1) is almost halved. Pure carbon
DE S I GN AND P E R FORMANCE O F A FAC E S Y S T EM 423
Agropyrum caninum. The vegetation was uniformly cut at readings taken every second were averaged over one
minute and stored in computer files. A gas analyser feda height of 50 cm and the systemwas almost continuously
operated from 22 January to 20 February 1995 at a target with air sampled at the centre of the array was used to
control the system.concentration of 560 µmol mol–1. Records of one minuteaverage CO2 concentration measured in the centre of the Diffusion tube measurements were made on 9
February. These tubes accurately estimate mean CO2circular array were taken over the period of continuous
operation. Both horizontal and vertical distribution of concentration over a period of time ranging from 1 to
8 h. Readings are made by recording progressive colourCO2 concentrations were measured intermittently during
prototype development. Spatial distribution of CO2 con- changes of a specific reactant along a graded scale.
Such colour change is caused by diffusion of CO2centrations was measured using both infra-red gas ana-
lysers (WMA-2, PPS, Hitchin, UK) and diffusion tubes along the tube and since diffusion increases with
increasing CO2 concentrations, the length of the coloured(Mod.500/a-D, Dragerwerk AG, Lubeck, Germany). In
the first case, five IRGAs were cross-calibrated using a area is proportional to the cumulated levels of atmo-
spheric CO2 concentrations. The tubes were at firstcalibration mixture (SOL, Firenze, Italy) and fed with air
sampled at canopy height on a spatial transect traced out tested against an infra-red gas analyser and this
permitted to calibrate successive readings. A total ofalong the prevailing wind direction over a period of a
day. Air was sampled at the centre of the ring and at 2 24 diffusion tubes (500–20,000 µmol mol–1
h–1) were then placed at three heights above the canopyand 3.5 m from that point on both directions. Sampling
tubes of the same length (5 m) and of 4 mm diameter (0, 15 and 30 cm) and were read every hour after
being opened at 09.00 hours, using a caliper.were used to sample air at a rate of 15 L min–1. IRGA
Fig. 8 Horizontal CO2 concentrations
measured over a day with variable
wind speed in the 560 µmol mol–1 ring.Surfaces indicate areas having the same
mean (M) and standard deviation (S)
of CO2 concentrations (µmol mol–1) forthe whole day (D), for the period with
growth, but it then remained unchanged at a height
of 58 cm from late June to harvest.
Mid-FACE performances were evaluated throughout
the experimental season both by continuously recording
in computer files one minute average CO2 concentrations
measured in the centre of each array and by performing
measurements of spatial distribution of CO2 concentra-
tion within the FACE plot. Spatial distribution measure-
ments were made from 20 August to 9 September in
the different Mid-FACE rings. During this period air
was sampled from 16 different positions within each
CO2 enriched array. Spatial arrangement of sampling
points was defined on the basis of prototype perform-
ance data by defining two concentric circles enclosing
56% (outer circle) and 14% (inner circle) of the total
FACE plot area. The area defined by the outer circle
was supposed to correspond to the ‘sweet spot’ sensu
Lewin et al. (1992) in which CO2 concentration were
minimally affected by wind speed and direction. Air
sampling was made using a membrane pump and an
automatic multiport sampling device (ALD, Massa-
Carrara). Air was sampled for 10 consecutive minutes
from each port and fed into a gas analyser. Measure-
ments were made for the three CO2 enriched rings by
moving the multiport sampler between them. Before
each series of measurements the gas analysers were
calibrated using a gas calibration cylinder. The multiport
device and the gas analyser were both connected to
the central control computer and both one minute CO2concentrations measured in the centre of the array and
one minute averages measured for the different positionsFig. 9 Frequency distribution of measurements made at the
centre and at the outer and inner circles traced out within within the circle, were stored in computer files.the Mid-FACE ring (see text for explanation). Symbol legend:
Centre j, inner u and outer circles sResults and discussion
(b) Mid-FACE operational set-up. Five Mid-FACE rings(a) Mid-FACE prototype
were constructed from March to May 1995. These were
then used from 10 June to 4 September for the potato The analysis of Mid-FACE prototype performances was
very valuable for subsequent development and designexperiment which is described in a separate paper
(Miglietta et al. 1996b). Three out of five rings were of the operational set-up. Prototype data measured
over 29 days showed that the variability of CO2used for free air CO2 enrichment in an experimental
design involving four different atmospheric CO2 concen- concentration in the centre of the ring area largely
depended on wind speed (Fig. 2). The stronger thetrations (ambient, 460, 560 and 660 µmol mol–1). CO2fumigation was initially made for 24 h a day, but since wind, the lower was the amplitude of CO2 concentration
fluctuations around the mean or, in other terms, thenatural CO2 concentrations during night-time were very
often above 500 µmol mol–1, night fumigation was fraction of time in which one minute average of CO2concentrations deviated by less than 10 or 20% from aceased from early August onwards. For easier compar-
ison all the performance data reported in this paper pre-set target concentration. Under windy conditions,
noticeable difference was observed between CO2 concen-refer to measurements made during daylight hours,
only. The potato crop grew up to 55 cm at the time trations measured on upwind and downwind sides of
the ring. This difference was as high as 90 µmol mol–of flowering and remained at about this height until
senescence started in late August. The height of 1 (Fig. 3) and was caused by unavoidable non-
uniformity of CO2 concentrations within the ring.emission ports and of the sampling ports were increased
twice from emergence to late June to follow crop Although large, this gradient was comparable to that
DE S I GN AND P E R FORMANCE O F A FAC E S Y S T EM 425
Fig. 10 The four surface graphs show
the spatial distribution of the fresh
tuber weight measured at the time of
potato crop harvest both in the control
and FACE rings. Grey tones denote
fresh tuber yield per plant as shown
by the vertical scale. This representation
of data gives a clear visual impression
that plant production responded to
increasing CO2 levels with increased
tuber production and that there were
no spatial gradients in tuber weight
per plant in the Mid-FACE rings.
reported for other systems under similar conditions of the potato crop until harvest. A few interruptions for
equipment maintenance and minor problems occurred(Hileman et al. 1992; Hendrey 1992). Regression analysis
of the gradient revealed that a central area existed, during the experiment and these amounted to a total
of 49 h. Concentration data for a few days were notwithin the ring, in which even under high wind speed
the absolute difference between the CO2 concentration available due to inadvertent loss of data during copy
operations of computer files. The daily mean andmeasured at any point and in the centre of the ring
was less than 10% of target (Fig. 3). This area had a standard deviation of daily means of CO2 concentrations
measured for 74 consecutive days in the centres of thediameter of 6 m and defined the central ‘sweet spot’.
Draeger diffusion tubes gathered additional informa- three rings are reported in Table 1. Frequency distribu-
tion of wind directions are shown in Fig. 5. Meantion on the horizontal and vertical distribution of CO2concentrations within the prototype ring. A clear daily wind speed calculated over daylight hours was
1.1860.47 m s–1 and the frequency distribution of onehorizontal CO2 gradient was detectable over the first
measurement hour at the canopy level and both at 15 minute average wind speed recorded during the
experiment is given in Fig. 5.and 30 cm above it (Fig. 4). As expected CO2concentrations were lower on the downwind side of As shown for the Mid-FACE prototype, the variability
of CO2 concentration read at the centre of the arraythe ring, but values read in the centre of the array did
not differ between sampling heights (Fig. 4). The were dependent on wind speed, but were not affected
by wind direction (Fig. 6). Over the entire experiment,horizontal gradient was less pronounced after 4 and
7 h and this was likely due to the fact that wind the fraction of time in which one minute average CO2concentrations in the centre of rings deviated by lessdirections varied from NW to NE during the measure-
ment period (Fig. 4). than 10% from the targets were 66, 74 and 83% for
the 660, 560 and 460 µmol mol–1 Mid-FACE rings,Overall, Mid-FACE prototype performances were
inferior, in terms of both spatial and temporal CO2 respectively. Performance of the 560 µmol mol–1 ringwas inferior to those reported for the 550 µmol mol–1control, to those achieved by the BNL-FACE (Lewin
et al. 1994), but, nevertheless, the design was considered rings of BNL-FACE in Arizona (Lewin et al. 1994) and
the 600 µmol mol–1 BNL system installed in Switzerlandto meet construction criteria and objectives.
(Jongen at al., 1995). This contradicts previous experience
made with larger systems that showed that variability(b) Mid-FACE operational set-up
of CO2 concentrations decreases with decreasing array
diameter (J. Nagy, personal communication). However,The operational Mid-FACE set-up constructed in the
spring 1995 was continuously operated from emergence it must be considered that the location of the Mid-
FACE experiment was subjected to a high variability 20% of the target for more than 65 and 97% of the
time, respectively.in wind directions and high turbulence caused by the
topography of the site, the presence of a forested hill, Spatial uniformity of CO2 concentrations over the
experimental areas was adequate despite the unavoid-a vineyard and a number of isolated trees located in
the vicinity of the experimental field. able presence of short-term concentration gradients
under windy conditions. Hopefully, the distribution ofSpatial performance of the operational set-up was
comparable to that of the prototype. As expected, an wind directions over the period of the experiment did
compensate, in the long term, these short-term gradientshorizontal concentration gradient was detected during
a day of almost unidirectional wind of variable speed so that CO2 effects on tuber growth were uniformly
distributed within FACE areas (Fig. 10).(Fig. 7a), whereas no clear horizontal gradients were
apparent during a day in which wind had variable It is important to outline that the Mid-FACE allowed
to perform a FACE experiment with a limited budget.direction (Fig. 7b). A special case is reported in Fig. 8
where spatial distribution of CO2 concentrations is This was possible as a low cost system was developed
and used, on average, less than 1800 kg CO2 per dayshown for a day with variable winds by comparing
performance data under windy (. 2 m s–1) and non- for the three rings during daylight hours. Cost reduction
in the Mid-FACE, as compared to other larger FACEwindy periods (, 0.8 m s–1). The graphs clearly show
the existence of a spatial gradient during windy systems, did not result from the simple scaling-down
of the size. Low cost material used for assembling theconditions which disappears when the wind is calm.
When calculated over a period of a week, the rings, the simplified design of on/off valves, the use
of low cost flow controllers, IRGAs and meteorologicalfrequency distribution of one minute average CO2concentrations measured at the level of the outer circle instrumentation allowed total capital cost for the facility
(3 FACE rings plus controls) of less than US$ 40, 000.of the sweet spot did not differ from that measured
at the level of the inner circle, thus suggesting that Together with the previously developed Mini-FACE
system (Miglietta et al. 1996), the Mid-FACE designwind direction varied either uniformly or randomly
during the period of measurements. This was true for provided evidence that small FACE systems are indeed
applicable in field experiments. Further system tuningthe 660, 560 and 460 µmol mol–1 rings (Fig. 9).
Nevertheless, the fact that CO2 concentrations measured and technological advances as those suggested by
Walklate et al. (1996) have the potential to improve theboth in the outer and in the inner circle deviated more
from the mean than those measured in the centre of performances of these small systems further and to
increase their efficiency in CO2 use.the array, indicated that significant CO2 concentration
gradients did occur during wind episodes (Fig. 9).
Overall, spatial distribution data that were presentedAcknowledgementssuggested that all the plants within the sweet spot of
the Mid-FACE rings were exposed to the same mean Authors wish to thank Fabio and Mauro Menegoli of GEOGASlong-term CO2 level. Conclusive evidence for this is spa for financial and technical support to this research.
Suggestions, recommendations and conversations made withpresented in Fig. 10 where tuber yields (fresh weight)John Nagy, George Hendrey, Keith Lewin and Andy McLeodmeasured at harvest on individual plants grown withinhave been very valuable in the design of the fumigation
the Mid-FACE sweet spots are presented. Data showdevice. Lucilla Cerio, Vincenzo Lo Duca, Francesco Vaccari,
very clearly that the mean potato yield was increased Aldo Salvadori, Giacomo Tagliaferri, Gennaro Dibonito, Filippoin response to increasing aerial CO2 concentration and Busiello are also acknowledged for their technical assistance
during prototype development and the experiment. This workthat such an increase was uniformly distributed overcontributes to the GCTE Core Project (Focus 3, Potato Network)the entire CO2 enriched areas. No spatial gradients inwhich is a part of the International Geosphere-Biosphere
tuber yield were detectable in any of the rings.Programme (IGBP). It also partly contributes to CROPCH-ANGE, a research project funded by the EU-ENVIRONMENTProgramme (Contract n. EV5VCT920169)
Conclusions
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