Science 7–10 · Year 9

Soil respiration: decomposers returning carbon to the air

Science understanding: Earth and space sciences (ACARA Year 9 carbon cycle; NSW has no Stage 5 content point on respiration or decomposers and teaches them in Stage 4: the focus area Change, whose Chemical change content has a practical investigation modelling respiration and photosynthesis; the focus area Cells and classification, which names respiration in mitochondria; and the focus area Living systems, whose Ecosystems content cycles matter through an ecosystem)

PracticalMedium risk

School laboratory, not for home

In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

The idea

Fungi and bacteria in moist soil respire the carbon in dead plant material back into carbon dioxide, so living soil gives carbon to the air while soil heated to kill its microbes gives far less.

Safety card

Medium riskA teacher supervises

Setting: In a school laboratory, with a teacher supervising, under the school's risk assessment. Not for home.

Hazards

soil can carry tetanus spores and other microorganisms, and bagged potting mix can carry Legionella bacteria
limewater (calcium hydroxide solution) irritates the eyes
hot, steaming soil and container from the microwave

Controls

use garden soil, not potting mix; wear gloves and a dust mask while handling soil, keep it damp to limit dust and wash hands afterwards
eye protection while pouring limewater
an adult heats and cools the soil
open the jars outdoors at the end and return the soil to the garden

Note

Write the risk assessment before the lesson under CSIS 1.7 (Risk Assessment, a pre-requisite for risk control) of the NSW Department of Education Chemical Safety in Schools package (Section 1, 2021 Technical Update), or through RiskAssess; keep the Safety Data Sheet for every chemical with the class register.

What you need

3 identical clear jars, 1 L, with airtight lids (or the cut 600 mL drink bottles of the SERC EarthLabs set-up)
300 g of moist garden soil with some fallen plant litter, split into three 100 g portions (garden soil, not bagged potting mix)
1 microwave oven to heat one moist 100 g portion on high for two minutes or until it steams (the SERC EarthLabs sterile control; adult), cooled to room temperature before use
3 small beakers each holding 20 mL of fresh clear limewater (saturated calcium hydroxide solution), or one carbon dioxide sensor (NDIR type, range to at least 5,000 ppm) sealed in each jar
1 thermometer, disposable gloves, a dust mask and a balance reading to 1 g

How to do it

Label the jars living soil, heated soil and empty control. Put 100 g of living soil in the first jar and 100 g of heated soil in the second, and add 10 mL of water to each so both are equally moist.
Stand a beaker of limewater on the soil surface in each jar (and in the empty jar), or seal a sensor inside each, then close the lids tightly.
Keep the three jars side by side at the same temperature out of direct sun, and record that temperature.
With limewater, look at each beaker every day for up to five days and score its cloudiness from 0 (clear) to 3 (milky); with sensors, log the carbon dioxide reading every minute for 60 minutes.
Convert a sensor rise to a mass: in 1.0 L of air at 25 degrees C and 101.3 kPa, 1.0 mg of carbon dioxide raises the reading by 556 ppm; the mass of carbon is 0.273 times the mass of carbon dioxide. Subtract the soil's volume from the jar volume first if the soil fills more than a tenth of the jar.
Run three jars of each kind (or pool three groups), compare the means, and as an extension repeat with living soil kept at 15 degrees C and at 30 degrees C.

What you should see

The limewater over living soil turns cloudy first: the SERC EarthLabs version reports tiny white flakes of calcium carbonate or cloudiness, with results in about 2 to 5 days. The heated-soil and empty jars stay clear or cloud only faintly from the air sealed in with them. With a sensor, the living-soil jar climbs steadily above its starting reading while the heated-soil jar tracks the empty control. Every 556 ppm rise in a 1.0 L headspace is 1.0 mg of carbon dioxide, which is 0.27 mg of carbon that decomposers have returned to the air (n = PV / RT = 0.0409 mol of air in 1.0 L at 25 degrees C; 1.0 mg / 44.01 g/mol = 22.7 micromol). The learner knows it worked when the living-soil jar exceeds both controls in every replicate.

What changes

What you change: whether the soil holds living decomposers (living or heated soil); temperature as an extension
What you measure: carbon dioxide released (limewater cloudiness score, or ppm rise per hour)
What you keep the same: mass and moisture of the soil
jar volume and seal
temperature
time

Common misconceptions

Each of these ideas is wrong, and the activity is a chance to test it.

Soil is just dead material; it is full of living organisms that respire.
Only animals release carbon dioxide; the fungi and bacteria that break down dead matter release it too.
Carbon in fallen plant litter stays locked in the ground; decomposers return much of it to the air as carbon dioxide.

Curriculum references

The NSW syllabus outcomes and Australian Curriculum v9 codes this activity supports. They are references, not a verified or complete curriculum alignment.

Science 7-10 Syllabus (2023), NSW Education Standards Authority (implemented from 2026; codes read at curriculum.nsw.edu.au on 22 and 23 September 2026)SC4-CHG-01 SC4-LIV-01 SC5-WS-03 SC5-WS-04
Australian Curriculum v9AC9S9U03AC9S9I02AC9S9I03

Sources

The pages the author read to write this activity.

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