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United States Department of Agriculture

Agricultural Research Service

Technical Overview
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Plot layout (ring diagram): each of the 30 plots is enclosed within a 3.7 m diameter plastic flange that is buried to 60cm depth (hexagon shape). All plots have a 40+ cm buffer from the flange edge to the “core” portion of the plots, which has an area of 7 m2. This is divided into a 4.1 m2 northern mixed grass prairie (NMP) portion and a 2.9 m2 WEED portion by a 25cm deep steel flange. The NMP side has a 1.5 m2 harvest grid, a 20cm wide soil sampling area around the perimeter, and soil water content (to 80cm), air and soil temperature sensors (5, 10, 45 cm). A minirhizotron tube is installed at a 25o angle to a depth of 52cm; and trace gas sampling rings are located around the harvest grid. The field has 4 datalogger/power stations from which power is provided and sensor data is recorded. All power cables and sensor wires are housed in 3.8 cm ID PVC conduit, for protection. 
 
  
Mini-FACE system: Pure CO2 is supplied from a 50 ton liquid CO2 tank on the site. Each of the 10 Elevated CO2 plots is surrounded by a plastic CO2 injection pipe with 3.4 m diameter. The pipe has a line of laser-drilled, 300 um holes with 15.9mm spacing, through which pure CO2 is injected around the perimeter (non-directional) of the plots at a pressure of about 10 bars, resulting in rapid mixing with ambient air.  
  
The elevated CO2 is dispersed in the plot by the wind. Each plot has an IRGA (PP Systems WMA-4) which transmits the CO2 concentration (sampled near the center of the plot) to a Control Unit located in the field trailer. Injection tubing is connected to a proportional valve array and valves are adjusted based on CO2 concentration and wind speed (measured with a sonic anemometer at the trailer). Each plot is adjusted every 4 seconds. To monitor “CO2 contamination”, CO2 concentration is constantly monitored in 6 Ambient CO2 plots with a multi-port scanner system and LI-7000 IRGA (LI-COR). 

The PHACE heating treatments utilize a PID (proportional-integral-derivative) feedback system with infrared radiometers to sense canopy temperatures.  A datalogger is used to collect the radiometer data and run the PID control.  The infrared radiometers measure the canopy temperature differential between the heated plots and reference plots.  This differential is then used by the PID system to control the output of the heaters.  A warming set point of a 1.5 C° increase during daytime and 3 C° increase during nighttime is used.  Each of the heated plots has six 1000 watt infrared heaters to warm the canopy.  The reference plots have dummy heaters to insure shading from the heating infrastructure is similar across all plots.

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Last Modified: 10/17/2008
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