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Know the factors that affect the usage of CO2 in a FACE system and adjust accordingly

Mahabubur Mollah1,3, Debra Partington2 and Glenn Fitzgerald1

1Department of Primary Industries – Horsham, 110 Natimuk Road, Horsham, Victoria 3401, Australia.
2
Department of Primary Industries – Hamilton, 915 Napier Road, Hamilton, Victoria 3300, Australia.
3
Corresponding author. Email: Mahabubur.Mollah@dpi.vic.gov.au

Abstract

A range of technologies/approaches have been used to assess the effect of elevated carbon dioxide (CO2) concentration on agricultural experimentations. Free-Air CO2 Enrichment (FACE) systems typically produce an environment more akin to field conditions than other available technologies. FACE is a field-based technique used by scientists to investigate how crop, forest and natural plant communities will respond to future levels of increased atmospheric CO2 concentration. The major operational cost of running a FACE system is the CO2 gas. However, the relative importance of the factors that affect usage of CO2 gas in FACE systems has not been well documented. To rectify this, data collected from the Australian Grains FACE (AGFACE) experiments during September – November 2010 were analysed to determine the effects of wind speed, relative humidity, air temperature, ring height and gap between fumigation tubes and top of crop canopy (gap) on kg of CO2 gas used per hour. Regression models showed that wind speed explained 74% of CO2 gas usage, followed by relative humidity (RH), temperature and ring height. The effects of ring height and gap were negligible. Overall the model accounted for 92.3% of the variation. The unaccounted gas usage was due to unaccounted factors and unexplained errors. Gas usage increased with increasing temperature and decreasing RH under each of six wind speed categories. The correlation between gas usage and temperature or RH was high (r2 = 0.86 – 0.90) for low wind (0 – 0.9 m/s) but declined gradually with increasing wind speed categories. The regression models will help the FACE community set and adjust the parameters of their FACE systems to run them more cost-effectively.

Key Words

AGFACE, FACE rings, CO2, CO2 usage, Australia

Introduction

Free Air CO2 Enrichment (FACE) technology provides a means by which the atmosphere around growing plants may be modified to realistically simulate future concentrations of atmospheric carbon dioxide (CO2). FACE systems usually inject pure CO2 from a series of horizontal pipes arranged as an octal (‘ring’) within which plants are grown. Experiments using FACE are required because most studies looking at the effect of elevated CO2 concentrations have been conducted in glasshouses or growth chambers, where there are often artefacts, such as increased temperature, reduced wind and rainfall interception that affect plant growth. In addition, there is, in theory, no limit to plant size, or growth problems caused by the constraint of enclosures in FACE systems, e.g. FACE experiments in forests. Historically and currently, there have been 38 FACE sites in Australia, China, Denmark, Finland, France, Germany, Hungary, India, Italy, Ireland, Japan, Netherlands, New Zealand, Sweden, Switzerland, UK and USA (FACE Database Management System 2012) in a range of environments and plant types. The single most expensive operating cost of these FACE systems is the use of CO2 gas. Researchers realised that CO2 gas usage in FACE systems was strongly related to wind speed (Hendrey et al. 1999). However, the relative importance of a range of factors that potentially affect usage of CO2 gas in FACE systems is little understood.

We hypothesised that the likely factors affecting CO2 usage in a FACE system are wind speed, relative humidity, air temperature, height of the FACE rings and gap between the horizontal fumigation tubes and top of the crop canopy. During September – November 2010 data were collected from a 16-m AGFACE ring at Horsham (3645′07″S latitude, 14206′52″E longitude, 127 m elevation) on all the likely factors listed above. The data on the use of CO2 (kg/h) were also recorded for the same period.

This paper establishes the relative importance of the factors that affect usage of CO2 gas in AGFACE systems and provides models which should help the FACE community to set and adjust the parameters of their FACE systems to allow them to run more cost-effectively.

Methods

In 2010, the readings collected automatically from an online flow meter (ABB TRIO – WIRL ST 40) in kg/h were averaged over 1 min intervals. Wind speeds recorded by the controllers of eight AGFACE rings were averaged over 1 min into a single figure to log along with the CO2 usage. The temperature (Temp) and relative humidity (RH) measured in a representative ring from the AGFACE site (Mollah et al. 2009) using a multiport CO2 analyser (Mollah and Fitzgerald 2010; Mollah et al. 2011) were used in the analysis. During the growing season the ‘ring height’ (height of the fumigation tubes above the soil surface) was increased several times to keep the fumigation (injection of CO2) tubes above the canopy. This created different ring volumes to be filled with CO2; therefore ring height was used as a factor in the analysis. The gap between the fumigation tubes and the top of the canopy (‘gap’) changed as the crop grew between each height adjustment of the ring. Canopy heights were measured every fortnight and the canopy growth for each day was interpolated and the gap between fumigation tube and canopy top calculated. Early in the season crop growth is slow, ‘ring height’ is static, and crop cover is not uniform. Therefore, data collected during spring 2010 (September – November) were analysed to determine the effects of wind speed, relative humidity, air temperature, ‘ring height’ and ‘gap’ on kg of CO2 gas used per hour.

GenStat Version 14 was used to fit a General Linear Model using 1-min average data to assess the relationship between CO2 usage (5.6 – 1183 kg/h), wind speed (0 – 5.9 m/s), temperature (-0.7 – 23.9 C), relative humidity (27 – 94 %), ring height (0.5 – 1.1 m) and gap (-26 – 109 mm).

Results

Wind speeds accounted for about three-quarters of CO2 gas usage in AGFACE

Analysis showed that 73.8% of the CO2 gas usage was explained by the wind speed alone and depended on the wind speed category (Table 1). The relative humidity, air temperature and their interaction; and the interaction between wind speed and RH accounted for about 18.5% of the gas consumption. Therefore, overall, the model accounted for 92.3% of the variation. The unaccounted gas usage was due to unaccounted factors and unexplained errors.

The ‘ring height’ had a statistically significant effect (P = 0.049) but accounted for only 0.27% of the gas usage. The ‘gap’ had no significant effect (P = 0.073) and accounted for only 0.23% of the gas usage. Due to their negligible contribution, the terms ‘gap’ and ‘ring height’ and interaction of the wind and air temperature were omitted from the model. The final model was fitted to assess the relationship between CO2 usage; wind speed, air temperature and relative humidity (equation 1).

CO2 usage (kg/h) = β1 + β2*RH(%) + 21.74*Temp(C) – 0.2082*(RH(%)* Temp(C) equation 1.

The values of β1 and β2 for all wind speed categories are provided to work out the CO2 usage for each wind speed category (Table 1).

Table 1. The values of β1 and β2 (equation 1) for all wind speed categories.

Wind speed category (m/s)

 

0.0 - 0.9

1.0 – 1.9

2.0 – 2.9

3.0 – 3.9

4.0 – 4.9

5 – 5.9

β1

712

683

603

499

531

577

β2

-6.7

-4.9

-1.7

2.08

2.71

2.99

For users’ convenience, the CO2 usage at various temperature and RH were calculated (Table 2 and Table 3) from the model (equation 1). The model, Table 2 and Table 3 will help engineers and scientists to estimate CO2 usage for a given wind speed category, temperature and relative humidity for their FACE systems. The model can assist in designing a cost-effective FACE system and set the parameters appropriately, e.g. top operational wind speed.

CO2 usage in AGFACE increased with increasing temperature and wind speed

Figure 1 and Table 2 show the impact of temperature on CO2 usage in AGFACE. The usage of CO2 gas in FACE systems is proportional to wind speed, corroborating with a previous study (Hendrey el al.1999). Gas usage reaches its peak when high winds combine with high temperatures (Fig.1). However, the correlation between gas usage and temperature was high (r2 = 0.86) for low wind (0 – 0.9 m/s) but declined gradually with increasing wind speed categories (e.g. r2 = 0.42 for wind speed category of 5 – 5.9 m/s) such that as wind speed increased, the effect of temperature decreased. There was no statistically significant interaction between wind speeds and air temperature.

Figure 1. Impact of temperature on CO2 usage.

Usually, high temperature is followed by high winds and they peak in the afternoon resulting in the highest usage of CO2 in the afternoon.

Our experiences suggest, reducing the upper limit of the wind speed (e.g. to 5 m/s) and shutting down the system on very hot days (say at > 35C) will save a significant amount of CO2. However, these may not be acceptable in some FACE systems, like AGFACE.

Past study (Jifon & Wolfe 2005) suggests that the interaction of elevated CO2 and high temperature (35C) reduces the CO2 stimulation of photosynthesis and productivity in bean and possibly other heat-sensitive species.

Table 2. Usage of CO2 (kg/h) at different temperatures and wind speed categories calculated using the model (equation 1) for mean RH (63.58%) during the experiment.

             

Wind speed category (m/s)

 

0.0 - 0.9

1.0 – 1.9

2.0 – 2.9

3.0 – 3.9

4.0 – 4.9

5.0 – 5.9

CO2 usage

Temp. (C)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

-5

265

314

424

517

640

680

0

330

379

490

582

705

745

5

396

445

555

648

771

811

10

461

510

621

713

836

876

15

527

576

686

779

902

942

20

592

641

752

844

967

1007

25

657

707

817

910

1033

1073

30

723

772

883

975

1098

1138

35

788

838

948

1041

1164

1204

40

854

903

1013

1106

1229

1269

45

919

969

1079

1172

1295

1335

CO2 usage in AGFACE decreased with increasing RH

CO2 gas usage declined with increasing R.H. (Table 3 and Fig. 2) and the correlation was high for low wind (e.g. r2 = 0.9 for 0 – 0.9 m/s and r2 = 0.8 for 1 – 1.9 m/s wind categories) but it declined gradually with increasing wind speed categories (e.g. r2 = 0.2 for wind speed category of 5 – 5.9 m/s). It is expected that the CO2 gas usage for a FACE system would be lower in cold and humid areas of Europe compared with hot and dry areas like Australia.

Table 3. The usage of CO2 at different RH and wind speed category calculated using the model (equation 1) for mean temperature (11.6C) during the experiment.

Wind speed category (m/s)

 

0.0 - 0.9

1.0 – 1.9

2.0 – 2.9

3.0 – 3.9

4.0 – 4.9

5.0 – 5.9

CO2 usage

R.H. (%)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

(kg/h)

0

848

903

885

928

874

852

10

784

838

842

894

871

860

20

719

772

799

860

868

868

30

655

707

757

826

865

876

40

590

641

714

792

863

884

50

526

576

671

758

860

892

60

462

511

628

724

857

901

70

397

445

585

690

854

909

80

333

380

542

655

851

917

90

268

314

500

621

849

925

100

204

249

457

587

846

933

Figure 2. Impact of RH on CO2 usage

Conclusions

Wind speed is most important single factor in AGFACE system accounting for about three-quarters of CO2 usage. This effect is expected to apply also to overseas FACE systems.

Around 18.5% of CO2 usage was explained by the combined effects of temperature, relative humidity and wind speed. FACE systems in hot and dry areas are expected to use relatively more CO2 than their counterparts in wet and cool areas.

If acceptable as the methodology, it is recommended to reduce the upper limit of the wind speed and shut down the system on hot days to save CO2.

References

FACE Database Management System (2012). http://public.ornl.gov/face/global_face.shtml, accessed 14 April 2012.

Jifon JL, Wolfe DW (2005). High temperature-induced sink limitation alters growth and photosynthetic acclimation to elevated CO2 in bean (Phaseolus Vulgaris L). J. Amer. Soc. Hort 130, 515 – 520.

Hendrey GR, Ellsworth DS, Lewin KF, Nagy J (1999). A free-air enrichment system for exposing tall forest vegetation to elevated atmospheric CO2. Global change biology 5, 293 – 309.

Mollah MR, Norton, RM, Huzzey J (2009). Australian grains free air carbon dioxide enrichment (AGFACE) facility: design and performance. Crop & Pasture Science 60, 697 – 707.

Mollah MR, Fitzgerald G (2010). The size of free air carbon dioxide enrichment (FACE) rings affects overall system performance: An Australian experience. Proceedings of 15th ASA Conference, 15 – 19 November 2010, Lincoln, New Zealand.

Mollah MR, Partington D, Fitzgerald G (2011). Understand distribution of carbon dioxide to interpret crop growth data: Australian grains free-air carbon dioxide enrichment experiment. Crop & Pasture Science, 62, 883–89.

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