The Automatic Carbon Efflux System (A.C.E.S)
A Multiport System for Measuring Carbon Dioxide Flux in Forest Soils and Woody Tissue
Developed by John Butnor, Chris Maier and Kurt Johnsen at the Forestry Sciences Laboratory, Research Triangle Park, North Carolina.
The Automated Carbon Efflux System (ACES) we developed is a multiport, dynamic gas sampling system that utilizes an open flow-through design to measure carbon dioxide fluxes from the forest floor or woody tissue with a variety of chamber styles. In the current design, sixteen soil chambers are measured sequentially (fixed or variable time step) using a single infra-red gas analyzer (IRGA). Air is supplied to each chamber in a push - pull fashion where air flow entering the chamber is greater than exiting maintaining a slight positive chamber pressure. Excess air is vented out the top of the chamber and ensures that the chamber pressure is held near ambient. Chamber pressure can be verified with a digital manometer. Flow rates are measured with mass flow meters. All chambers are continuously evacuated when not being sampled. The soil respiration chambers are constructed of pVC (25 cm diameter 10 cm height 4900 cm3) with a lexan lid. Each chamber has an air and soil thermocouple, pressure equilibration with the atmosphere and reflective insulation that prevents "greenhouse" heating in the chamber even in full sunlight. A soil moisture reflectometer is used to take soil moisture readings in each chamber and can be installed in a common location for continuous measurement. The ACES is fully automatic requiring only calibration checks twice per week. Under AC power the system can run continuously, using a DC power supply the ACES can go up to 48 hours without recharging.
ACES Chamber Design and Measurement Theory
The ACES uses an open system, dynamic chamber made of pVC pipe 10 cm tall with an inner diameter of 25 cm. A thin stainless steel collar is attached to the bottom of the cylinder to pierce into the soil (2 cm) and make a quality seal with the soil surface (Figure 1a). Air is supplied to the chamber in a push-pull fashion, where air flow entering the chamber is greater than exiting, creating a slight positive chamber pressure. Excess pressure is vented out of a 9 mm diameter port on the top of the of the chamber, ensuring that chamber pressure is near ambient and any leaks are pushing outward. System tests with a completely sealed null chamber demonstrated that a +10% pressure differential is completely relieved by the equilibration port. Air of known CO2 concentration enters the chamber through a ring-shaped diffuser that evenly distributes the flow and mixes with CO2 diffusing from the soil surface. The diffuser eliminates the need for a mixing fan in the chamber. CO2 enriched air is pulled into the sampler system through a another ring-shaped diffuser and its CO2 concentration is measured (Figure 1b).
Soil respiration is calculated by determining the molar flow rate of air passing through the chamber, the flux rate of CO2, and soil respiration is that flux expressed over the area of substrate measured:
- aFlow (mol/min)= (flow (lpm)/ 22.4a) * (273.15/ (273.15 + gas tempb)) + Bp(kpa)/ 101.3
- Flux (umol/s) = ( ?CO2 * aFlow) / 60
- Soil CO2 Flux (respiration) = Flux/ surface area (m2) = µmol/s-1m-2
- a 22.4 is the volume of 1 mole of air (dm3) at S.T.p.
- b air temperature in chamber (Co)
- adapted from (Coombs et al. 1986)
Finely adjusted sample pumps switch sequentially from chamber to chamber to make precise carbon evolution measures. Air is pumped at ½ to ¾ the sample flow rate to refresh air through the chambers when they are not being sampled. No air is pumped from the chamber, instead it flows out the pressure equilibration port. This reduces the build up of excess CO2 and the amount of time required to sample each chamber.
There are currently eleven ACES units in use, and four units in production. ACES has been used in a variety of forest and agricultural experiments across the United States to provide high quality soil and/or woody tissue respiration data.
Duke University, USDA Agricultural Research Service, Auburn University, Westvaco Corp., International paper Corp., Coweeta Hydrologic Laboratory (SRS), Jones Ecological Research Center, Virginia Tech, University of Georgia, University of Arizona, Columbia University (Biosphere 2), USDA Forest Service pacific Northwest Research Station, University of Washington, USDA Forest Service pacific Southwest Research Station, Michigan Tech
We would like to acknowledge Jim Vose (SRS, Coweeta Hydrologic Lab) and John Major (Canadian Forest Service) whose earlier gas sampling systems provided a framework for the ACES. Thanks also go to Mike Levine (Canadian Forest Service) whose soil chamber design influenced our design.
Combs, J., D.O. Hall, S.p. Long and J.M.O. Scurlock (eds.) 1986. Techniques in Bioproductivity and photosynthesis 2nd ed. pergamon press. Oxford, UK. 298 pp.
Fang, C. and J.B. Moncrieff. 1996. An improved dynamic chamber technique for measuring CO2 efflux from the surface of soil. Functional Ecology 10:297-305.