Explorations in Photosynthesis and Respiration

Originally posted on PASCO’s Blog. Blog is written by Bruce Taterka a teacher at West Morris Mendham High School in New Jersey

PASCO’s Wireless CO₂ Sensor provides a powerful opportunity for students to explore the effects of photosynthesis and respiration in their local environment. This blog will describe how students can use a “Pretzel Barrel CO₂ Chamber” to design experiments to measure rates of photosynthesis and respiration in a wide variety of settings and circumstances.

Background

Soil plays an important role in the carbon cycle and global climate because it acts as a carbon sink, sequestering CO₂ from the atmosphere in the form of organic matter. The upper layers of soil contain organic matter that is actively decomposed by the soil community: microbes, insects, worms and other animals.

While respiration by soil organisms decomposes organic matter and releases CO₂, the plant community is photosynthesizing and taking in CO₂ from the atmosphere. So in this part of the earth’s carbon cycle, CO₂ is moving in two directions – into the atmosphere from soil, and out of the atmosphere into plants. We refer to this transfer of carbon as “CO₂ flux.” With PASCO’s Wireless CO₂ Sensor, students can explore the carbon cycle, plant and soil biology, and climate change by measuring CO₂ flux.

The rate of CO₂ flux depends on a wide variety of factors. For soil, the rate of CO₂ production may be affected by:

• The content and type of organic matter in the soil
• Temperature
• Moisture
• Sunlight
• Soil chemistry

For the plant community, the rate of CO₂ production may be affected by:

• Type and density of plants
• Temperature
• Moisture
• Sunlight
• Soil chemistry

Carbon flux will vary with the weather, throughout the day, and the year as the season’s change. Measuring CO₂ flux provides an excellent jumping-off point for engaging in NGSS practices regarding the earth’s carbon cycle.

Students can define their research questions and problems, analyze the results, and generate designs to conduct a variety of controlled experiments using the chamber. Most experiments will identify the CO₂ concentration inside the chamber as the dependent variable and will test the effect of an independent variable of students’ choice.

On land, students can compare CO₂ flux on soil, lawn, forest, sunny vs. shady areas, and in a wide variety of other situations. In general, when the chamber is on bare soil or leaf litter, the CO₂ concentration inside the chamber will be expected to increase, usually within a short period such as 5 or 10 minutes. Figure 1. For longer investigations, the sensor can be placed in logging mode (storing data to internal memory) and left for ~24hrs before the batteries will be depleted.

Logging Mode Video

The chamber can be placed over plants that can fit inside it, in which case CO₂ concentration inside the chamber should decrease, although it may be counteracted by respiration. The chamber can even be used on water by wrapping a piece of foam insulation around it, allowing it to float.

In any type of environment, students can design their studies to investigate the effect of different variables on CO₂ flux.

How to make a Pretzel Barrel CO₂ Chamber

The chamber is made of a plastic pretzel container available in supermarkets.

• Draw a straight line around the container, then cut it in half using a razor knife to form two large plastic chambers open at the bottom and closed on top.
• You have two options here. You can cut a hole in the top for a CO2 sensor using an electric drill or a razor knife. The hole should allow the CO₂ sensor to fit snugly, keeping the container airtight; the hole can be adjusted with tape if necessary to create a tight seal around the sensor. Alternatively, the sensor can be placed inside the container since it’s wireless!
• The chamber is now ready to be used by placing it on top of soil and measuring CO₂ flux.

For aquatic environments, wrap a piece of foam pipe insulation around it.