Biomass is an important concept to understand because it is the base of our food supplies, our lumber supplies (which affects building and paper supplies), and our ability to convert it into energy both as a direct fuel (burning biomass) and converting it into biofuels such as ethanol. The amount of biomass a plant can generate directly corresponds to its value as a biofuels crop. While many things can be converted into biofuels such as sugar and corn, non-food sources of biofuels such as switch grass allows for a greater production of fuel without cutting into our food supply chain. Thus, having a plant, such as switch grass, with high biomass productivity, allows for a greater potential of generating more biofuels. This simple study will allow students to see how the growth of a plant can help generate biomass for the potential of renewable fuels.
Biomass is the term that refers to the amount of biological material an organism or group of organisms contains. In other words, it is the “heaviness” of life. Plant biomass is an indicator of photosynthetic activity and growth. When a plant photosynthesizes it creates sugar, which it then uses to grow, reproduce, and store for later use. The amount of sugar that is initially produced is known as the Gross Productivity (or Gross Primary Productivity) of the plant and is directly related to the photosynthetic activity. However, this is generally very hard to measure since the plant constantly is using the sugar for life functions. (Note: Plants use cellular respiration just like animals to break that sugar down to use for life functions.) The main way a plant’s productivity is calculated is through its Net Productivity, or what is left over after the plant has used what is needed. (Just like a paycheck after taxes).
Typically, when scientists are measuring biomass in the field they can only measure the mass that is above the ground. Not only is it nearly impossible to obtain all the root mass, digging it up would cause great environmental stress. Therefore, the plants are cut down to ground level; the material is dried, and massed. However, on a small scale where the plants are self-contained, the total biomass, including the roots, can be determined.
Note: At the end of the experiment, the plants will be dried out so they will die. Students should know that ahead of time so they do not expect to take any plants home. If space and materials permit, an extra set up can be grown so the student can have one to take home with them.
As written, this lab simply shows how biomass is determined. To make it more interesting, two different types of seeds could be grown to determine, which generates more biomass and therefore a better source for biofuels. This would require another set of materials and double the space. Using a perennial seed, such as grass, and an annual seed, such as corn, could extend the discussion of this. Typically the corn will initially show a much greater growth rate and greater biomass production, but the long term of using perennials (less planting and fertilizer) can be brought to light.
The impact of fertilizers could also be studied. However, the fertilizer must be added at the beginning of the experiment before the water is added so that its mass is included in the overall weight. Adding fertilizer throughout the experiment will artificially add mass to those plants.
- Containers for planting: these could be small disposable cups, larger drink cups, egg cartons, small aluminum pans, or whatever the growing space allows. Note: the mass of the container and the dry soil should be less than the maximum mass limit of the electronic balance used to determine the biomass.
- Potting Soil: this soil must be completely dry at the beginning of the experiment. Fluctuating water levels in the soil make determining biomass difficult. Additionally, dry potting soil is extra messy and difficult to water initially.
- Seeds of some sort: the number of seeds planted will be determined by the size of the container. Easily manipulated seeds such as corn, peas, and sunflowers are good choices as they grow easily and quickly. More economic choices, such as grass seed and clover, are good, but obtaining a measurable biomass in a short amount of time may be difficult unless using a larger container such as a drink cup.
- Spray bottles or wash bottles for watering the plants.
- Light source for growing the plants: a brightly lit windowsill, grow lights, etc.
- Markers for labeling.
- Electronic balances.
- Distribute materials to each student group.
- Label the container with the group name.
- Fill the containers with soil and mass the soil and the container. Record the mass of the dry soil and the container in Table I. (It is important to know the mass of the dry soil for the end of the experiment).
- Add a small amount water to the soil (amount depends on the size) and gently stir the water and soil together until the water is absorbed. DO THIS OVER THE TRAY. Continue to add water to the soil until it is evenly moistened and sticks together but is not swampy. If any soil falls out of the container while it is being stirred, IT MUST BE PUT BACK IN THE CONTAINER. This is part of the original mass and is important for the final calculations. If using small containers such as small drinking cups or egg cartons, it may be easier to transfer the soil into a larger container to mix in the water and then transfer it back into the container.
- Mass the seeds to be added to the soil and container. Record the mass in Table I. Note: If using very small seeds such as grass seed or clover, simply have the students weigh out a specific amount such as 0.25 grams and then sprinkle them over the surface of the soil. Note: If using a seed such as sunflower seeds, which have a thick protective outer coating, the coating will be shed from the seedling as it grows. Since this was part of the original mass of the seed, the coating must be placed back in the container (just place it on top of the soil) and should not be discarded.
- Place the soil and seeds in the growing area. Gently water with a spray bottle or wash bottle.
- Allow the seeds to germinate and the plants to grow for several weeks. Water the plants as needed keeping the soil moist but not swampy.
- After the plants have reached a large enough size (at least 3 inches), stop watering them. After another two weeks the plants and soil should be completely dried out. To hasten this, a drying oven can be used on a very low temperature (especially if using plastic containers).
- Mass the cup, soil, and dried plants. Record this in Table I.
- Calculate the total biomass by subtracting the mass of the cup, dried soil, and seeds, from the new mass (#9).