SAWFACE facility details: the FACE plots
|
|
SAWFACE was under operation from 2011 to 2013. Since then it has not been operational but is back in the 2019 growing cycle and again wetland plants will receive CO2 enrichment. The FACE facility is installed within the Las Tablas de Daimiel National Park, in an area (≈900 m2) which has been covered by common reed during at least the last 25 years. The experimental area consisted of 6 octagonal CO2 enrichment rings (FACE plots) together with 6 control plots maintained at present day [CO2] FACE and control rings aredistributed randomly in the experimental area maintaining a distance greater than 6 meters between plots. Experimental plots were regularly arranged for logistical reasons (fencing, access gateways, wiring, pipes, etc.). The abundance of wild boars in the wetland forced us to minimize the size of the experimental area and install fencing to protect equipment and ring structures. Each FACE ring has 3 m in diameter (≈ 7 m2/plot) and consists of eight 1.2-m long polyethylene pipes (emission pipes) with a diameter of 16 mm (thickness of 0.5 mm) arranged to make an octagon. Each emission pipe is suspended horizontally at 20-30 cm above the macrophyte canopy, using eight stainless steel poles (4-m of height) located at the vertices of an octagon. These are assembled at the top with a detachable steel structure also forming an octagon.
|
The lower part of the poles is welded to flat stainless steel plates, which are attached to the wetland soil using perforated concrete blocks. This increased the octagon structure’s stability during flooding. The height of the emission tubes is regulated weekly according to vegetation height during its growth cycle by using stainless steel chains which are attached to the upper octagon structure).
The gas used for enrichment is liquefied ultrapure CO2 stored in a VT21 tank (≈40,000 kg), located outside the wetland (~400 m distance). Liquid CO2 is supplied to electrical heat exchangers which vaporized it. The resulting CO2 gas is channelled to a pressure regulator that reduced line pressure to 500-1,000 kPa. Finally CO2 gas is piped to FACE plots through 16-mm diameter polyamide tubing which was buried to minimize the risk of damage.
The gas used for enrichment is liquefied ultrapure CO2 stored in a VT21 tank (≈40,000 kg), located outside the wetland (~400 m distance). Liquid CO2 is supplied to electrical heat exchangers which vaporized it. The resulting CO2 gas is channelled to a pressure regulator that reduced line pressure to 500-1,000 kPa. Finally CO2 gas is piped to FACE plots through 16-mm diameter polyamide tubing which was buried to minimize the risk of damage.
The FACE rings functioningFACE rings are based on directionally controlled release of pure CO2 from jets (micro-holes) located in the emission pipes. By releasing CO2 through the jets the gas reaches sonic velocity so that, consistent with the theory of fluid mechanics, it creates a shock wave at the outlet and substantially improves mixing with air (Miglietta et al. 2001: New Phytologist 150:465–476). The reason for using micro-holes is to obtain a rapid mixing between CO2 and air, which achieves a significant simplification of construction and reduced capital cost of the FACE facility. Theoretically, considering a standard atmospheric pressure of 101.3 kPa, sonic velocity is achieved when the pressure inside the pipe is greater than 50 kPa, which occurred 86% of the time during the fertilization period. In order to produce the micro-holes each emission pipe was perforated with an ultra-short laser pulse (propellant; diameter <300 μm) every 2.4 cm, which gave a total of 50 jets per emission pipe and 400 jets per FACE ring. As fans are not used, it is unnecessary to build any additional control plots in order to evaluate ring structure effects. An automatic pressure regulator (1/4" electronic proportional pressure regulator MPT40-P3HPA12AS2VD1A, Parker) controls the amount of CO2 released in each FACE ring, and is operated by supplying variable voltage (0–10 V DC) that is translated into a pressure value (0-150 kPa). A digital-to-analog converter (DAC USB 3103, Measurement Computing Inc.) is used to convert the digital signal into an analog voltage value that operates the pressure regulators in each FACE plot.
|
Operational principleThe operational principle is the directional control of CO2 emission which is based on releasing the gas from horizontal pipes located in the upwind side of the FACE octagon plot following Okada et al. (2001: New Phytologist 150:251–260): when the wind speed was over 0.5 m s–1, CO2 was released from the most upwind emission adjacent pipes (2-4 pipes); when there was no wind (wind speed < m s–1), CO2 was emitted from every pipe (8 pipes in total). The use of two sets of pipes with holes located at different emitter spacing has not been considered in this design in order to simplify the ring structure and operation considering the small size of the FACE plots used. CO2 is managed using solenoid on/off valves (VE 151 HV, Parker), which commutate depending on the wind direction and velocity recorded by two 2D ultrasonic anemometers (WINDSONIC, Gill Instruments) located at the top of FACE-3 and FACE-5 plots. Solenoid valves are connected independently to the respective emission pipes of the FACE ring by means of 10-mm diameter, polyamide tube. These electrically activated valves were operated by relay controllers (1ADPDT R232 24-channel Relay Controller, National Control Devices Inc.). Wind data, as well as other additional meteorological measurements (2 IR-120 infra-red remote temperature sensors for leaf and soil temperatures and a NR-Lite net radiometer, both from Campbell Sci.) are stored in a data logger module (CR1000, Campbell Sci.).
|
