Assign the reading the night before the scheduled activity. Teachers may wish to have students complete a “close reading” while using strategies such as annotation.
Write the following statement on the board, or project, as a “do now” or “warm up” activity.
What are the factors that affect decomposition?
Spend the first 10 minutes of the class period discussing the question posted. Hopefully, students will discuss some of the following from the reading:
Type of Detritivore
There are numerous types of detritivores, they are specialized to break down different materials
What type of detritivores do you think break down the leaves your group collected and why?
Type of Material
Deciduous vs. Evergreen Evergreen take longer to break down
Propose a reason (or two) why evergreens take longer to break down.
Moist environments have faster rates of decay than dry environments
Both moisture and temperature are simple experimental design questions that are expected of students for NYS standardized exams. Either of these factors could be assigned as a follow up design lab.
Warmer environments have faster rates of decay than cool environments
Based on this discussion, have students create & share a hypothesis about which leaf species will decompose the most before they begin the experiment.
Place students into their lab groups and provide them with both the leaves and soil that they collected. If you did not choose to run the first experiment, you will need to provide these materials. Demonstrate to students how to create a 4" x 4" packet. They will need to measure out approximately a 5"x10" piece of screen and then fold it into a packet. (You may wish to use this as a challenge to each group to see who can figure it out first. If you choose to do this, have the students explain it to you FIRST before allowing them to cut it out, otherwise you may waste quite a bit of screen.)
Students should follow the procedure in the handout to create their leaf litter packets. Remember to show students how to properly tare the scales and to make sure to use a piece of paper as a “weighing dish” so that small pieces of leaves are counted when making packets. Once students have created their packets and buried them about halfway into the soil, have them check the moisture of the soil by observation. If the soil needs water, have them either spray or pour a SMALL amount of water throughout the surface of the soil. Soil should be damp but not wet. We have found that for a quart size container, 100 mL of water is sufficient for 3 weeks if fresh soil is used. Make sure to poke holes in the tops of the containers to allow air circulation. Students will need to place their containers somewhere safe in the room where they can occasionally check the moisture level and add water as needed (once per week). Decomposition should be fully underway by the third week and results should be obtainable.
After approximately three weeks, have students GENTLY unearth their soil packets and brush as much soil off as possible. If there is a lot of soil stuck inside of the litter bags, rinse all the bags under running water and hang up to dry overnight. Using a piece of paper as a weighing dish, students should weigh each packet and record their results and place their group data on the board to share with the class using the data table below.
Mass of Decomposed Leaves
Allow students time to create bar graphs comparing the decomposition rates. (Teacher may wish to extend students by working with a graphical computer program, such as Excel).
The teacher may wish at this point to make comparisons depending on the type of tree species that have been studied. For example: evergreens vs. deciduous or different species of oak, etc. Each species has its own specific nutrients, which may be preferable to certain organisms (just as we may have a preference to lettuce types) and some may be more difficult to digest. These ideas are expanded on more fully in the herbivory lesson plan.
Here is a reference list of common trees and their nutrient levels:
Northern red oak
Eastern white pine
Northern white cedar
Additional nutrient data for other species can be found online at the Foliar Chemistry Database of the Northeastern Ecosystem Research Cooperative (www.geospatialcareers.net).
As a follow up homework assignment, the teacher may wish to use one of the factors discussed in the "Do Now" and have students set up a hypothetical experiment to test a factor. If time permits, the teacher may wish the lab groups to fully conduct the experiments, collect and analyze the data.
Use students responses to the laboratory questions, graph and optional extension homework assignment to evaluate how well students understand the stated objectives. Pay particular attention to question #2 - which tree species had the fastest vs the slowest decomposition rate. Ask students if they can hypothesize WHY these differences may have occurred. Students may focus on nutrient differences, but may also talk about the types of decomposers, which will be the focus of Lesson 3.