Cotton Strip
decomposition
Decomposition is a critical ecosystem process that mediates nutrient recycling and the carbon cycle in ecosystems. In freshwater systems, decomposition is often mediated by the microbial and invertebrate communities. Decomposition releases CO2 into the atmosphere. Faster decomposition of organic material can release more CO2 than what is being stored by the system, and could potentially contribute to global warming. Cotton strips are an easy, cheap, and highly standardised way of measuring decomposition rates, which can change rapidly when under treatments such as warming. Cotton strip experiments (CELLDEX) are being run worldwide now to provide reference rates of river and riparian decomposition around the globe.
Materials
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95% cellulose pure cotton strips (see Tiegs et al. 2013 for details on preparation)
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Coarse mesh leaf bags (10 mm aperture size)
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Brick (see illustration)
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Tensiometer
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Drying oven
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Dessicator
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Procedures
Setup
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Put cotton strips in mesh bags and close them firmly (e.g. with staples). Determine the number of cotton strips you want to experiment with depending on the number of replicates or sampling events you want to carry out.
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Attach mesh bags to bricks using zip ties so that they will sit above the sediment when in the water (see illustration).
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For deployment and collection use arm-length work gloves to prevent DNA contamination and protect against chemicals in the water. Bleach and rinse the gloves between each pond.
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Deployment
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Lay the bricks in the same location in each pond, ensuring that no mesh bags are trapped under the bricks (see illustration). Mesh bag collection periods will vary depending on the experiment carried out.
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Collection
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Slowly remove the bricks from the mesocosm floor to minimise disturbances to the system, using both hands.
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Remove the mesh bags from the bricks using scissors.
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Analysis
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The cotton strips first need to be carefully cleaned to remove any algae or other dirt from them. Gently wash them in a shallow tray of 80% ethanol, using a small paintbrush to remove debris. Hold the cotton strips by their edges, as that area will not be measured by the tensiometer.
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Dry the cotton strips on aluminum pans at 40 °C overnight, then store them in a dessicator for at least 2 hours. This removes all the water weight from the strips.
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Measure the tensile strength of the strips using the tensiometer. Refer to Tiegs et al. 2013 for their methodology. Tensile loss per day can be calculated as below:
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Tensile strength (reference strips) is the tensile strength of 10 strips not incubated in the field, washed in ethanol, and stored in a dessicator.
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Tensile Loss = 1 − ((Tensile Strength (treatment strips) / Tensile Strength (reference strips)) × 100) / Incubation Time​
Potential responses
Decomposition rates are known to respond positively to increases in temperature. These increases could be exponential, linear, or eventually plateau after reaching a certain "maximum" temperature. The graph below illustrates these potential response curves.
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ILlustrations
Place the bricks on the mesocosm floor, ensuring that no bags are trapped under the bricks. The zip ties used to tie the mesh bags to the bricks ensure they float above the sediment.
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The diagram to the left also includes leaf bags, which are an alternative method of measuring decomposition.