Atmospheric CO2 Research Group

Grassland, Soil and Water Research Laboratory
808 East Blackland Road
Temple, Texas 76502-9601

The Prairie CO₂ Gradient: Research with a View to the Past and Vision of the Future

Principle   
Scientists:        Hyrum B. Johnson, H. Wayne Polley, Charles R. Tischler,
                      Justin D. Derner, and Brian J. Wilsey
                      USDA/ARS, Grassland, Soil & Water Research Laboratory 


Cooperating:    William A. Dugas and Patricia C. Mielnick
                     Texas Agricultural Experiment Station, Blackland Research
                     Center


                     Robert B. Jackson and Laurel J. Anderson
                     Department of Botany, Duke University and Penn State University

The Prairie CO₂ Gradient (PCG) is a unique controlled environment facility for exposing a portion of intact rangeland to a continuous gradient in atmospheric carbon dioxide (CO₂) concentration. The PCG provides for a uniform daytime gradient in CO₂ concentration from near the Ice Age (c.18,000 years BP) level of 200 ppm (parts per million) to the current 360 ppm and from the current concentration to one predicted during the next century (550 ppm). The PCG is modeled after a prototype that has been operated for almost 10 years in a glasshouse at Temple,Texas, and is comprised of two tunnel-shaped chambers aligned parallel to each other along a north to south axis.

The two chambers in the PCG each are 1 m wide and tall, and are composed of ten 5-m long compartments containing undisturbed soil and vegetation. During daylight, photosynthesis of enclosed vegetation depletes the CO₂ concentration of air as it is moved unidirectionally through each chamber. The desired CO₂ gradient is maintained by automatically adjusting the rate of air flow through each chamber to changes in net CO₂ uptake by vegetation. Pure CO₂ is added during daylight to air introduced to one chamber (superambient chamber) to elevate the initial CO₂ concentration to 550 ppm. A continuous gradient in CO₂ concentration from 550 to 360 ppm is maintained. Ambient air is introduced during daylight to the other chamber (subambient chamber)to initiate a subambient CO₂ gradient from 360 ppm to 200 ppm. The direction of air flow in each chamber is reversed at night and the CO₂ of air input to the superambient chamber is elevated. Air flow rates are regulated to maintain night-time CO₂ concentrations at 150 ppm above daytime values along each chamber.

Each of the 20, 5-m lengths in the PCG is separated by ducts with chilled-water cooling coils that are used to automatically reset air temperature and humidity to values measured outside chambers. Soil in each of the 20 lengths with undisturbed vegetation is isolated to a depth of 90 cm from surrounding soil with a rubber-coated fabric 3 mm in thickness. Air dry bulb and dew point temperatures and atmospheric CO₂ concentration are measured regularly at extremes of each 5-m length. Water is added through a surface irrigation system to match precipitation.

The PCG was constructed on a rangeland dominated by C₃ annuals in spring and the decumbent C4 perennial grass Bothriochloa ischaemum (KR bluestem) and taller C₃ annual forb Amphiacharis dracunculoides (annual broomweed) during summer. Other warm-season C₃ forbs occur at relatively low densities. The long-term annual precipitation of 825 mm is well distributed over the year, but often is insufficient to support growth from July through September.

Carbon dioxide treatments were initiated with the PCG in April 1997 to test two central hypotheses.

1. Increasing atmospheric CO₂ from near glacial through present to possible future concentra- tions will enhance net primary production and thereby contribute to an increase in short- to medium-term (years) carbon (C) sequestration in soil. Carbon accumulation rates will be monitored directly and continuously as net CO₂ fluxes between the atmosphere and the vegetation and soils in each of the 20, 5-m lengths along the PCG, and will be integrated over daily to annual scales to quantify effects of CO₂ on ecosystem C balance. Measurements of aboveground and belowground biomass and soil C content will serve as a compliment to the CO₂ flux data.

2. Increasing atmospheric CO₂ concentration will reduce evapotranspiration and increase average levels of soil water, particularly during the usually dry summer. These changes will contribute to an increase in production and, possibly, to an increase in abundance of species that previously were limited by low water availability. Evapotranspiration from rangeland in 5-m lengths of each chamber will be calculated from regular measurements of air dry bulb and dew point temperatures at extremes of each length. We will regularly measure xylem pressure potential on selected species, volumetric soil water content, and species composition across the CO₂ gradient.