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Geosciences LibreTexts

4.2: Soil Carbon and Respiration

  • Page ID
    14726
  • Learning Objectives

    • Define soil health.
    • Describe how soil health indicators are used to track changes in soil health over time.
    • Describe how soil health indicators are used to compare two different management practices.
    • Explain how soil organic matter influences aggregate stability.
    • Perform carbon cycle calculations for a Kansas farm.

    Soil respiration is the net release of CO2 from all living organisms in the soil, including bacteria, fungi, protists, earthworms, plant roots, etc. The rate of soil respiration has many different applications. It can be used assess soil health, with higher respiration rates indicating a more active, and thus healthy microbial community. It can also be used as part of a C balance to determine if soil under particular management practices is a net source or sink of C, which has direct application to carbon credits. Respiration rates can also be influenced by the quality of the substrate being consumed by the soil microbes. In this lab you will conduct tests to determine the relative soil health of two soils that have been managed differently for over a decade. The tests include a CO2 burst or a basal respiration test to determine the relative respiration rates, and a slake test to determine the stability of the soil aggregates. You will also work with your classmates to complete the Carbon Cycling problem set.

    Materials

    • Two or more soil samples that have been oven-dried at 35-40°C (for the CO2 burst test)
    • Two or more moist soil samples that have been recently collected (for the basal respiration test)
    • Soil roller
    • 30 cc (cm3) soil scoop and striker rod
    • Plastic beaker
    • Mesh screen
    • Pipette set to 9 mL
    • Water
    • Solvita® CO2 burst test paddle
    • Test jar with lid
    • Solvita® low-CO2 test paddle
    • Incubator (optional)
    • Solvita® Digital Color Reader and/or reference color charts
    • Two or more soil peds that are approximately the size of a golf ball
    • Coarse wire mesh or machine cloth
    • Two, 1 L sedimentation columns
    • Soil Carbon Cycling Problem Set

    Recommended Reading & Viewing

    Prelab Assignment

    Using the recommended reading and videos and the introduction to this lab, consider the questions listed below. These definitions/questions will provide a concise summary of the major concepts addressed in the lab. They also useful study notes for exams.

    1. Define soil health
    2. Define soil respiration
    3. Define carbon sequestration.
    4. Name three ways in which C is lost from the soil.
    5. Which of those three losses of C from the soil is the most significant?
    6. What practices might reduce or even reverse this loss in soil C, and thus maintain or improve soil health?
    7. Describe how soil organic matter impacts the stability of soil aggregates.

    Introduction

    Carbon Calculations

    Carbon dioxide is a product of respiration, one of several greenhouse gases emitted from soils. Greenhouse gases adsorb radiation emitted from the earth into space, reflecting the radiation to the earth, thus heating the earth in an effect commonly called the “greenhouse effect”. This process does help keep the planet’s climate at a suitable temperature, but if the concentration of greenhouse gases in the atmosphere is too high, this process is magnified, trapping too much heat and resulting in warmer global temperature averages and changes to local climates. This can result in significant changes that affect people around the world. Soils are important in moderating greenhouse gases and climate change.

    As microbes in the soil consume organic matter, they release carbon dioxide through respiration, which eventually makes its way to the soil surface and into the atmosphere. Undisturbed ecosystems, generally show a balance of C added to the soil and C lost from the soil to the atmosphere. In some cases, C accumulates in the soil when additions to C exceed C losses, resulting in soil features like a dark A horizon with lots of soil organic matter. This net increase in soil organic matter is called C sequestration. In disturbed soils, like tilled and cultivated soils, the physical disturbance of tillage temporarily increases aeration, resulting in higher rates of respiration and more C lost to the atmosphere. Over time, this causes a net decrease in soil C and net increases in atmospheric C.

    However, improved soil management practices like conservation tillage, no-till, or using cover crops can minimize these C losses or even increase soil C. Soil management for C was a significant topic in the 2015 United Nations Climate Change Conference in Paris. There, scientists recognized that agriculture can both be a source of C emissions and as a tool to fight climate change through C sequestration by helping limit the average global temperature increase to 1.5°C. Carbon credits can be used to subsidize farmers who change their management practices to increase the amount of C sequestered (stored) in the soil in perpetuity. Carbon credits function as agreements between a C emitter and someone who can manage land so that C is removed from the atmosphere through C sequestration. In activity one, you will perform the calculations that a hypothetical Kansas corn producer would do when considering an opportunity to sequester C through changes to tillage practices and then sell C credits.

    Measuring Soil Respiration

    Soil respiration is a measure of how much CO2 the soil is emitting and reflects the total CO2 emitted from all living organisms in the soil, including bacteria, fungi, earthworms, protists, roots, and others. Soil respiration measurements are used for calculating carbon balances, and can be used as a biological indicator of soil health. In Activities 2 and 3, your class will work together to quantify respiration rates of soil samples that have previously been collected and dried (Activity 2) or samples that have recently been collected and are still moist (Activity 3). Your class will also investigate the aggregate stability of soils using a slake test (Activity 4).

    Activity 1: Carbon Cycling Problem Set

    A soil problem set will be provided to you. Use your experience and knowledge from previous laboratory activities to complete these problems.

    Activity 2: Soil Respiration using Solvita® CO2 Tests

    Note: This protocol is an abbreviated version of the protocol that is provided with sampling kits from Solvita®. The full Solvita® will be provided to students to complete this activity.

    Basal versus CO2 Burst Tests

    The CO2 burst test is used in regions where prolonged periods of dry conditions are common. The CO2 burst test simulates the respiration rate after it is rewetted following a prolonged dry period, and uses soils that have previously been collected and dried. Samples used for the CO2 burst method must be rewetted. The basal test is used in more humid, temperate regions were soils rarely experience prolonged periods of dry conditions. The basal test uses soil samples that are minimally disturbed and have been recently collected. No additional moisture needs to be added to soils for the basal test. Your instructor will inform you which of these two tests you will be performing.

    Procedure

    For this experiment you will be using two similar soils preferable from the same area and of the same soil series. However, they should come from locations that have undergone contrasting soil management practices. One example is a soil that has been tilled annually compared to a soil that has been under no-till management and/or cover cropped. Another example is a tilled field compared to a soil from an adjacent, untilled fence row.

    1. Collect the soil using a clean trowel, shovel, or probe. Collect 12 or more sub-samples from a representative area to the depth, then composite the sub-samples into a single sample. If you are conducting the basal respiration test, skip to Step 4.
    2. Dry the soil samples in an oven at temperatures up to 45° oven until the soil reaches a steady state mass (several days to a week)
    3. The soil should pass through a 2 mm sieve. If the aggregates are too large, roll the samples using a soil roller until the aggregates are broken down enough to pass through the sieve. Do not use a soil grinder.
    4. Measure out a standard volume of soil using the 30 cc soil test scoop. Scoop enough soil so that soil is mounded on the top of the scoop. Then scrape off the excess sample using the striker bar, yielding exactly 30 cc of soil. Pour the sample into the plastic beaker and gently tap it to level out the sample. If you are conducting the basal respiration test, skip to Step 6.
    5. Place the mesh screen on top of the soil in the beaker, then gently add 9 mL of water to the soil using a pipette.
    6. Open the low-CO2 pouch and place the test paddle into the soil with the stake pointing down and the color indicator vieweable on top of the sample.
    7. Place the plastic beaker containing the soil sample and test paddle into the test jar, then tighly close the jar lid. Record the time the jar was closed on the jar lid. Keep the jar in a room at a stable 20°C for 24 hours, or place the jar in an incubator set to 20°C for 24 hours.
    8. Remove the test paddle after 24 hours and record the resulting color by either comparing it to a color chart, or by measuring the color using a digital color reader (DCR). The DCR will provide both the color measurement and the interpolated CO2-C ppm respired by the sample. If using the color chart, record the color number in Table 12.1, then use the “Range of Response Guidelines” from the Solvita® protocol to determine the CO2-C ppm respired. Record results using a DCR in Table 12.2.

    Table 12.1 Soil Respiration Results Using Color Chart

    Soil Color Measurement CO2-C ppm Respired Soil Biological Fertility Classes
    A
    B
    C

    Table 12.2 Soil Respiration Results Using Digital Color Reader

    Soil Color Measurement CO2-C ppm Respired Soil Biological Fertility Classes
    A
    B
    C

    Summarize the management practices for the soils as they where described by the instructor. Also briefly describe the soil (soil series, the horizon that was sampled, the depth of sampling, etc.).

    Which test did you perform, the basal respiration test or the CO2 burst test?

    How did respiration from the soils compare?

    Which soil is “healthier”? Explain the impact of soil management practices on the health of the soils.

    Activity 3: Slake Test

    Now you will conduct a slake test, which is a test that can be performed both in the field as well as in a laboratory. The slake test demonstrates how well a soil is able to withstand internal pressure that occurs during a sudden rewetting of the soil. Soils with more stable aggregates will be better able to withstand that pressure, and will exhibit less slaking than soils with less table aggregates. You will use two soils with contrasting soil management practices (see Activity 3). To perform the test, do the following:

    1. Fill two, 1 L sedimentation columns with water.
    2. Form a wire mesh into a “U shape” so that the two edges are supported by the rim of the column.
    3. Place soil peds that are approximately the size of a golf ball onto the wire mesh. The peds should be completely submerged.
    4. Observe the peds for at least 5 minutes.

    Which soil exhibited the most slaking?

    Describe how the history of soil management for the two soils compare, and explain how the soil management practices impacted this slake test.

    Assignment: Problem Set

    A problem set will be provided to you at the beginning of the laboratory session.

    Subsequent Lab Setup

    Some activities require more time than a single lab period. These activities must be set up ahead of time. One activity in the Soil Colloids explores the shrink-swell properties of soil. To prepare for this activity, do the following:

    • In two 500 ml beakers, create slurries of kaolinite and bentonite clays (this should be sufficient for the whole class). The slurries should be approximately the same consistency as a milk shake: slightly runny, but not as thick as a Frosty®.
    • You will be provided two petri dishes. Using a ruler, determine the interior volume of the dish. (Remember, volume of a cylinder = hπr2, where h is the height of the cylinder, and r is the radius.)
    • Using label tape, label the bottom of the two petri dishes with the following information:
      • Colloid type (Kaolinite or Bentonite)
      • Lab day/time
      • Table number
    • Using a spatula, fill both petri dishes with the appropriate clay slurry.
    • Set the petri dishes in the location designated by your instructor.

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