A CO₂ Concentration Gradient Facility for Testing CO₂ Enrichment and Soil Effects on Grassland Ecosystem Function

The overall goal of this experiment is to determine how past and future increases in atmospheric carbon dioxide affect grassland ecosystem productivity, water balance, and carbon cycling. The study includes monitoring the effect of carbon dioxide on different soil types. Our experiment can help answer key ecological questions on the impacts of climate change on grasslands, including their potential to offset the effects of rising atmospheric.

CO2 Unique advantage of our approach lies in determining the impacts of CO2 enrichment since the beginning of the industrial revolution, and in determining how CO2 impacts may change in the future. Our approach for starting CO2 relies in using the plants we are studying to create their own CO2 gradients. This approach for manipulating CO2 and the impacts on grassland ecosystems was developed at our laboratory in the late 1980s.

The experiment documented here is the third generation of CO2 gradient experiments. To collect soil monoliths, use an open-ended steel box that is one meter square and one meter deep. The steel must be at least eight millimeters thick.

A base will later be welded on. Press the box down into the desired soil using a hydraulic press secured to the soil by three meter deep hele anchors. After pressing the box into the soil, excavate the surrounding soil with a backhoe.

Cut the monolith base free from the soil below and remove the monolith. Place a fiberglass wick against the soil. At the base of the box, the wick drains into a 10 liter reservoir attached to the base of the steel box shown here already welded in place.

The reservoir collects water that may drain through the monolith during the experiment, and also provides a means to sample the water for chemical analyses. Plant the species to be studied in the monoliths at densities appropriate to your studies species. If necessary, kill preexisting vegetation on the monoliths with a non residual herbicide like glyphosate.

For tall grass, prairie species, 56 plants over the square meter is suitable. Eight seedlings of seven species arrange the species in a Latin square pattern using a unique randomization for each monolith grass species used here includes side oes, grandma, little blue stem, Indian grass, and white Tritons. Forbes species used were pitcher, Sage Canada, golden rod, and Illinois bundle flower.

A legume used drip or aerial irrigation to keep the plants well water during establishment. The goal is to minimize water stress while the plants are establishing water in amounts and frequencies appropriate to the study species. After the plant establishment phase, maintain the plants under ambient rainfall until the chamber construction is complete.

The monolith chambers are fitted into trenches that are about seven meters wide, 1.5 meters deep and 60 meters long. Each trench fits two chambers and each chamber fits 10 interconnected sections in each trench. Fit 10 containers made of heavy steel with one meter between each container.

These form the base of each section and each will hold four monoliths. Join the adjacent sections with sheet metal ducts to provide a pathway for airflow. Install a cooling coil inside each duct.

The coil is supplied with 10 degrees Celsius water from a 161 kilowatt refrigeration unit. The flow of chilled water to each coil is regulated by a control valve that responds to chamber air temperature. Place four 4, 540 kilogram capacity scales in each five meter section container onto each scale, place a monolith with established prairie plants.

Each five meter section should contain monoliths from two soil types in random order within the section randomize the soil Type pairings in each section include sandy loam in the pairings of every other section. To complete the chamber cover each section with 0.15 millimeter greenhouse film, which is commonly used in climate manipulation experiments To access the plants as needed. Install zippered openings with draft flaps on the covers.

The cover can be removed when needed for sampling or maintenance. Keep the vegetation covered throughout the growing season as long as the photosynthetic capacity of the vegetation is enough to maintain the CO2 gradient. Sample the air at the entry and exit of each chamber every 20 minutes.

Route the air through filtered airlines to infrared gas analyzers, which immediately measure the CO2 concentration. Similarly, measure the water vapor every 20 minutes. Also, for each section, use shielded fine wire thermal couples to measure the ambient temperature every 20 minutes at the air entry, the section midpoint and at the air exit.

Using the sample temperature data, regulate the cooling coils to maintain a consistent midsection ambient air temperature that is consistent between sections. Lastly, measure the photosynthetic photon flux density incident on the chamber. Using a quantum sensor, ambient air is pulled into the super ambient chamber by the fans at the chamber entrance.

Use a mass flow controller to inject pure CO2 into the chamber and maintain the concentration at 500 microliters per liter of air. Also regulate the speed of the fans to achieve a CO2 level of 390 microliters per liter of air exiting the chamber. Use the CO2 in photosynthetic photon flux density measurements.

To maintain this parameter Control of lower speed is the most critical aspect of maintaining the prescribed CO2 gradient. For the sub ambient chamber, introduce ambient air and regulate the fan speed to achieve an exiting CO2 level of 250 microliters per liter during the night hours. Reverse the air flow through both chambers and set the gas injection and fans to meet the following parameters in the super ambient chamber.

Enrich the incoming air to 530 microliters of CO2 per liter and regulate the flow so that the air exits at 640. Microliters of CO2 per liter in the sub ambient chamber, adjust the air, so CO2 levels are 390 microliters per liter at the entry and 530 microliters per liter at the exit for precipitation. Apply the mean growing season.

Rainfall to each monolith. Use a drip irrigation system from a local water source to approximate the seasonal rainfall pattern. Measure water applications using a digital flow meter.

It's critically important that the plants in the monolith are watered well enough to avoid severe drought stress. In this way, they're photosynthetic rates remain high enough to create and maintain the CO2 gradient. This can be helped if some monoliths are planted with a sink plant with a high photosynthetic rate.

Over seven years of operation, chambers of prairie grass monoliths were maintained at a linear atmospheric CO2 concentration or ca with only small discontinuities. Apart from one section, temperature and vapor pressure deficit also remained constant measured in the top 20 centimeters of soil. Volumetric soil water content varied linearly along the CA gradient on two of the three soils in the study, only in the silty clay soil was there.

No change in this parameter. Plant productivity was measured using the above ground net primary productivity metric. This varied linearly with CA on all of the soils.

The lowest response to CA occurred with the clay soil and the greatest response was seen with Sandy loam. The Mesic C four Tallgrass Sarga Newan was the most abundant plant in the experiment. It was most strongly affected by CA on the sandy loam and only marginally affected by CA on the clay soil.

The Zurich C four Midgrass BTU Lua Kerti Pendula was the next most abundant species overall, and on silty clay soil in sub ambient ca. It was the most abundant. Its productivity was most affected by ca on silty clay and least affected by ca on Clay.

After watching this video, you should have a good understanding of how to establish and maintain experimental vegetation for regulating a CO2 concentration gradient and understand the critical parameters that need to be measured in order to control that CO2 gradient. This facility requires about two years to construct, but provides the capacity for decades long studies plant responses to past and future CO2 levels. Such long-term studies are critical to help understand the effects of rising atmospheric CO2 on grassland carbon cycling.

This approach can be applied to any plant species that would fit within the chamber.