FRS310

Assignment #8

Decomposition Lab Activity

40 points

 

Introduction

Decomposition studies allow ecologists to measure decomposition rates in different environments.  We will use the litterbag technique to compare the decomposition rates of different species under different abiotic conditions.  We expect to see differences in decomposition rates between the treatments because moisture, temperature, and litter quality are all important in controlling the rate of decomposition.

 

What is decomposition?

In a natural ecosystem, decomposition is the combined result of physical breakage of leaves, leaching of dissolved components, microbial decomposition, and animal consumption.  For example, in a temperate (i.e., mid-latitude) stream setting, trees shed their leaves in the fall and these leaves enter the stream.  The breakdown process begins with the leaching of dissolved nutrients from the leaves and colonization of the leaves by microbes and fungi.  Fungi physically penetrate cellulose with hyphae and secrete exoenzymes to degrade organic matter while microbes metabolize simple monomers and polymers that leach from the leaves.  As microbes colonize and process leaves, they become “conditioned,” and stream insects begin to consume them.  Leaves conditioned with a film of microbes and fungi have been likened to “peanut butter on a cracker” by a prominent stream ecologist, an analogy which highlights the nutritional importance of the leaf colonists rather than the leaves themselves.  In terrestrial settings the process is the same, but due to environmental differences, leaf breakdown occurs on longer time scales than in aquatic systems.  Because breakdown occurs much more slowly in terrestrial ecosystems, the breakdown and decomposition products are more easily grouped into categories.  Dead plant material is referred to as litter when its original identity can still be distinguished.  As further decomposition degrades litter into an unrecognizable form, it becomes soil organic matter.  Fungi and microbes further degrade the easily metabolized components leaving behind humus, which is composed of chemically complex organic matter that resists decomposition.  Many factors affect leaf decomposition in terrestrial ecosystems, including temperature, moisture, nutrients, organism type and abundance, and the nature of the material itself.

 

Why is decomposition important?

In temperate ecosystems, leaves are part of a major pathway of energy flow and nutrient cycling in forest and stream ecosystems.  Nearly 100% of life on earth requires energy from carbon fixed by photosynthesis, so leaf breakdown represents a key step in the carbon cycle.  Photosynthesis uses energy from sunlight to fix gaseous carbon (CO₂) into carbohydrates (C₆H₁₂O₆).  This process stores the sunlight’s energy and sets up a redox (oxidation-reduction) gradient whereby organisms convert the fixed carbon back into CO₂, releasing the stored energy to sustain metabolic processes.  In terrestrial ecosystems, organic matter fixed by primary producers fuels the ecosystem by providing food for decomposers and consumers, which in turn provide food for predators.  In addition to the energy that is supplied to heterotrophs, nutrients are released from the organic material and become available to plants again.

 

Student Learning Objectives

• Students will be able to describe and implement a basic decomposition experiment.
• Students will be able to describe the major controls over decomposition rate.
• Students will be able to calculate the ‘decay coefficient’ to quantify decomposition rate.
• Students will be able to interpret differences in the rate of decomposition of different species and environmental treatments.

 

 

Part 1 – Experimental Setup

 

• Prepare your soil – ***don’t wet your soil until you’ve weighed it***
  1. Put enough soil in your pot to bring the level ~1-inch below the top. As you add soil, keep packing it down so that it doesn’t settle too much during the experiment.
  2. Once full of dry soil, weigh your pot and record the weight in table 2.
  3. Water your pot until it is at pot-holding capacity. You can check at the bottom of the pot to ensure the soil you can see there is wet.
  4. Let your pot set during class to drain
  5. Weigh your wet pot and record in table 2.
• Prepare your litter bags – you will need 3 bags per group.
  1. Take the pre-cut screen mesh and sew up two of the sides, leaving 1 side open to insert your leaves.
• Weigh your dried leaves and record the values in the table provided. Make sure to handle your leaves gently so that they don’t break into a lot of tiny pieces
• Place each leaf into a litter bag
  1. After weighing the leaves can place them in water increase their flexibility as you place them in litter bags
  2. Gently place the leaf into the bag
  3. Sew up the final side of the bag
• Weigh each bag+leaf combo and record in the table provided
• Place your litter bags into the soil
  1. Create a slot in the soil to place the litter bag
  2. Slide the litter bag into the soil
  3. Pack soil around the litter bag to ensure good contact between the leaf and soil
  4. Make sure to label the location of each bag.
• Once all litter bags have been placed into the soil, record the weight of your pot.
• Label your pot with your group name and your treatment.

 

Calculations

RWC = (Wet Pot Wt – Dry Pot Wt) / (Sat Pot Wt – Dry Pot Wt)

*note: Sat Pot Wt is the ‘Pot Wt’ from week 1

 

Treatments

• Amb T/Wet – Ambient temperature, saturated soil
• Amb T/Dry – Ambient temperature, dry soil
• Elev T/Wet – Elevated temperature, saturated soil
• Elev T/Dry – Elevated temperature, dry soil

 

Results

Table 1. Mass of leaves at different time intervals during the experiment.  The first ‘Date’ will be the date you put the leaves into the soil, the second date for each leaf will be the date you take them out of the soil, not the date you weigh the leaves.  So make sure to record the correct date. Make sure that your units are in mg.

 

		Species Name:
	Date	Leaf 1	Leaf 2	Leaf 3
Initial wt.
Harvest 1
Harvest 2
Harvest 3

 

 

 

 

 

 

 

 

Table 2. Weekly pot weight and the weekly water addition to ensure proper soil moisture

 

Date	Dry Pot Wt. (g)	Wet Pot Wt. (g)	RWC	Water Added (g)	Soil Temperature (C)
			NA		NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA
	NA

 

 

 

FRS310

Decomposition Lab – Part 2

 

Calculations

1. To complete this lab you will need to do 2 calculations for each of the collection periods: mean % dry mass (DM) remaining, leaf breakdown rate (k).
   1. %M_r: [1- (M­₀­-M_t)/M₀]*100

• %M_r= percent mass remaining
• M₀= initial DM, the mean DM from the handling loss leaf packs.
• M_t= final DM, the mean DM from each collection date.
• You will need the mean %DM remaining for each collection date!

1. 1. To calculate the breakdown rate, regress the natural log (ln) of %M_r (y-axis) on days of exposure (x-axis) using 100% remaining for Day 0. The negative slope of the regression line is equal to the decay coefficient (k).

 

 

		Dry Weight (g)
	Date	Leaf 1	Leaf 2	Leaf 3
Initial wt.
Harvest 1
Harvest 2
Harvest 3
MR

 

 

 

Report

• Although you have worked in groups, I would like each individual to turn in their own report. You can work together with the other members of your group, but I want the text in the report to be in your own words
• Introduction
  1. You should provide a brief description of:
     1. Why decomposition is important to understand
     2. What factors should control decomposition rates
  2. Hypotheses
     1. Will temperature or soil moisture be more important in controlling decomposition rates?
     2. Will the gymnosperm or angiosperm have faster decay rates?

• Materials and Methods
  1. You know the routine
• Results – what you need to present NOTE: We will work together to share our results with the other lab sections.
  1. 2 figures and 1 table
     1. Fig 1 – Plot the 4 treatments of the gymnosperms
        1. If there is more than one replicate for a treatment, you will need to first calculate the mean mass remaining for the gymnosperms at each time point
        2. Create a scatter plot with symbols with 4 lines. Each line should represent a different treatment
     2. Fig 2 – Plot the 4 treatments of the angiosperms
        1. If there is more than one replicate for a treatment, you will need to first calculate the mean mass remaining for the angiosperms at each time point
        2. Create a scatter plot with symbols with 4 lines. Each line should represent a different treatment

• Table – Report the k-coefficients for all treatments of both species
  1. Use the mean values of percent mass remaining at each time point to calculate the k-coefficient

1. Make sure you provide a written description of each figure you present

• Discussion
  1. Discuss the implications of the results you have presented – you can use the questions below to guide your discussion
     1. Which factor had the greatest impact on decomposition rates
        1. Temperature
        2. Soil moisture
        3. Litter quality
     2. Do you think the results discussed in ‘I’ above would change if you altered the conditions of this experiment (ie. different temperature, different soil moisture, different species)? How?

• How would you expect decomposition rates to vary across deciduous forests in the eastern US that occur in different climate conditions? Provide some spatial explanation for how decomposition would vary.

1. Do your results fit with expectations you had from topics discussed in this (and other) courses? Why or Why not?