Measuring Soil Respiration

Transcription

1 Measuring Soil Respiration Background Decomposition in forest soils is a key ecosystem function that in part determines the productivity and health of the trees growing there. The process of decomposition releases large quantities of essential nutrients that are tied up in dead organic matter. These nutrients are released into the soil solution thereby making them available to plant roots. If decomposition of a forest is impaired by drought, acid rain or some other disturbance, the vegetation may experience nutrient imbalances. In this exercise we will investigate the rate at which carbon is released from soil in the form of carbon dioxide (CO2). The conversion of organic carbon to CO2 by decomposers (mainly bacteria and fungi) is called respiration, much the same as the cellular respiration that occurs within your own cells. The surface layer of soil in a forest is commonly called the forest floor and is made up mostly of dead organic material. The decomposition rate of the forest floor determines the nutrient supply to trees. Ecologists measure the amount of organic matter (or carbon) contained in the forest floor by measuring the thickness (cm) or mass per unit area (kg C ha -1 ). The quantity of carbon is determined by a dynamic balance between C inputs and C outputs. The main output of carbon from the forest floor is through decomposer respiration. Several environmental factors control the rate of decomposition. Since respiration is a biological process carried out by bacteria and fungi, it is very sensitive to temperature. In most soils, the respiration rate peaks at about 25 C and declines as temperature varies. Soil moisture also affects respiration; very dry or very wet conditions reduce respiration rates. One of the most common methods for measuring soil respiration is the sodalime method; this method is able to remove water vapor from the air in a reversible reaction. The water produced by this reaction remains temporarily adsorbed (ads) to the soda lime. The soda lime method involves covering a plot of ground with a chamber and then placing a pre-weighed, open dish of soda lime within it. As the soil organisms release CO2 to the chamber it is quickly absorbed by the soda lime (along with water vapor). After 24 hours, the chamber is removed and the soda lime is dried at 105 C to evaporate the water and is then weighed. The increase in mass of the soda lime is attributable to CO2. The procedures in this experiment take several days to complete because of drying times and incubation times.

2 Below is a brief overview of the time frame for this experiment, more detailed instructions are below: Put soda lime in jars in the oven to dry 4 days before experiment Day 1, Weigh soda lime (approx 10 minutes) and set up incubation chambers in the field 2-3 days later, Retrieve soda lime and put in oven to dry Day 3, go over calculations, write up exercise. Pre-Lab Tasks The soda lime must be dried and weighed before the lab session. Drying takes 24 to 48 hours. Gather your field equipment and supplies before the lab session. Equipment Drying oven (set at 105 C) Analytical balance (0.001 g range) Soil thermometers Digital thermometers Materials Soda lime, granular Glass jars (10 to 18; 15- to 50-mL) Spatulas Desiccator Plastic Ziploc sandwich bags (8 to 12) Aluminum weighing boats Chambers (10 to 18 containers, 3-L cylinders) Labeling tape Kitchen knife or trowels (2) Clipboards Sharpies Safety Considerations Soda lime is slightly caustic. Read the safety instructions on the MSDS sheets that come with the soda lime. Procedures A. Prepare the Soda Lime 1- Two or three days before experiment Label each glass jar with a piece of tape and permanent marker. Add approximately 8 grams of soda lime to each jar. Place the jars with soda lime in an oven at 105 C for at least 24 hours to evaporate the water from the granules. 2- Day of experiment a- Remove jars from the oven and place in a desiccator to cool for 2 to 5 minutes. Remove jars from desiccator one at a time, weigh to the nearest

3 tenth-milligram (0.0001g) and cover immediately. Record this as the preincubation dry mass that includes the soda lime and jar. b- Select your sampling locations. You should use at least 5 replicate chambers at each location. c- Place a chamber upside down on a relatively flat area of the forest floor. The rim of the chamber must make an air tight seal with the soil surface, so carefully remove twigs, small rocks that are in the way without disturbing the leaves and soil surface under the chamber. Leave the chamber loose on the surface for now. d- Obtain a jar containing soda lime. Remove the cap and place the jar under the chamber resting on the soil. Make sure it is not likely to tip over. e- Slowly and carefully push down on and rotate the chamber to force the edges about 0.5 to 1 cm into the forest floor. Place a weight on the chamber (like a medium-sized rock or branch) to maintain pressure and keep it from blowing away. f- Record the number of the soda lime jar and the number and location of the chamber. Repeat these steps for each of the chambers at each site. g- At one of the sites, place an opened jar of soda lime in an upright chamber and seal the chamber with a lid. This will serve as a blank to document the amount of CO 2 absorbed from the air in the chamber and during the opening and closing of the jars. B- Incubation: Let all chambers incubate for 24 to 48 hours as shown in the photo. C- Processing: 1- Return to the field to retrieve the chambers. Remove the chamber and cover the soda lime jar immediately. Measure and record the date, time, air temperature, soil temperature, and soil water content.

4 Air Temperature: Measure air temperature at 0.5 m. Plan how you will account for the daily fluctuation of air temperature and as the weather changes. Soil Temperature: Measure soil temperature just below the surface at about 5 cm. Soil Moisture (see soil moisture protocol) Soil water content varies significantly on a scale of meters, so take at least four soil samples from each sampling location each about 1 meter apart and combine these. 2- Back at the lab, place all the jars uncovered in the oven at 105 C. Dry for at least 24 hours to evaporate water from the soda lime. 3- Remove the dry soda lime from the oven and place in a desiccator to cool for 5 minutes. Remove jars one at a time from the desiccator, weigh to the nearest tenth-milligram. Record this as the post-incubation dry mass (which includes the mass of the jar) D. Calculations 1- Drying the soda lime after the incubation drives off the water that was absorbed and also the water that was produced by CO2 absorption. In order to compensate for this underestimation we multiply by a correction factor (1.69). 2- Initial Dry Mass of Soda Lime (g) = Mi 3- Final Dry Mass of Soda Lime (g) = Mf 4- Mass Change of Blank (g) = Mb = Mf(blank) - Mi(blank) 5- Mass Change of Sample (mg) = dm = (Mf - Mi - Mb) * CO2 Absorbed by soda lime (mg CO2) = dm * Soil Respiration (mg CO2 / m2 / d) = CO2 Absorbed / Area / Days of Incubation E- Further Analysis: 1- Calculate the average soil respiration and standard deviations for each sample location (or perform a statistical test). Put these values in a table. Then write two to three paragraphs describing and interpreting the results of your experiment. 2- Examine the temperatures and soil water content data you collected. Does any of this data help explain the differences in soil respiration between sampling locations? Explain. 3- The temperature and water content data you collected represent point-intime measurements. Do you think the temperatures and water content

5 values are representative of the microclimate during the entire incubation period? What would be a more accurate way to quantify the microclimate during the incubation period? 4- Are there other factors that could explain the differing rates of soil respiration between your sampling locations (such as amount of organic matter, vegetation type, slope, aspect, etc.)? Explain how they would affect respiration rates. Soil Respiration Data: Initial Site # Jar # Dry Mass (g) Final Dry Mass (g) CO2 Absorbed (mg) Chamber Area (m²) # Days Incubation (d) Soil Respiration (mg/co2/m²/d)