- The gross primary productivity (GPP) of an ecosystem is the rate at which energy is captured during photosynthesis. Thus, GPP is the total amount of photosynthetic energy captured in a given period of time. We know that plants must respire to provide energy for their life processes, which uses some of the energy of photosynthesis. The energy that remains in plant tissues after respiration has occurred is called net primary productivity (NPP). NPP is therefore the amount of biomass found in excess of that broken down by a plant's cell respiration. NPP represents the rate at which this organic matter is actually incorporated into plant tissues to produce growth. This relationship can be expressed as follows:
net primary productivity
(plant growth)= gross primary productivity
(total photosynthesis)- plant respiration - Only the energy represented by NPP is available for consumers, and of that energy only a small portion is actually used by them. We can express both GPP and NPP as energy per unit area per unit time, or in terms of dry mass per unit area per unit time. Examples of these are:
as Energy 
as Dry Mass kilocalories
meter2 / yeargrams
meter2 / year- In this lab you will measure net primary productivity for sample plots of different grass species, and compare their productivity. This lab requires approximately 3 weeks total time for plant growth, and 5 separate one-hour lab days.
MATERIALS:
Grass seed of several different species.
Light weight potting soil
Materials to make flats for sample plots
- This will depend on what material you decide to make the flats from. Plots of 10 x 10 cm, work well.
You can use commercial flats, just remember to measure them and use the appropriate measurements in your calculations.
You need 2 plots per species of grass.
Light source to provide uniform light to all samples.
Balance
Drying oven or other means of drying samples.
Drying paper or large beakers.PROCEDURE:
Pre-lab preparation
1. Prepare the sample plots. Plots of 10 x 10 cm are a good size for this lab experiment. You need 2 plots for each grass species being studied. Label the plot flats with #1 or #2, and the species of grass.
2. Count out the number of seeds you wish to plant, for each sample plot. You determine how many seeds you will use, depending on the plot size you have, but all plots must have the same number of seeds planted. Be sure to use plenty of seeds. Record your plot size ( as number of cm2 ), and number of seeds per plot in the DATA TABLE. Fill the plot flats with light potting soil gently pressed down. Spread the seeds evenly over the plot, and cover with a thin layer of potting soil. Water the plots and place them under the light source. Be sure each plot has as identical the amount of light as is possible.3. Water the plots with identical amounts of water, as needed, for the next two weeks. The grass should be 7 - 10 cm tall before the actual experiment begins.
Lab procedures
4. Take plot #1 for a species of grass, and carefully remove the entire grass plants from the soil. Remove as much soil as possible, but try to keep all the roots, they are part of the growth of the plant. Carefully rinse off any remaining soil from the roots in the lab sink, and gently shake off the excess water. Place the plants on drying paper or in a large beaker, and label with plot number and grass species.
- Repeat for plot #1 of the other species.
5. Place all the samples from step 4 in the drying oven for seventy-two hours (3 days).
Continue watering the #2 flats as before for the next week.
6. After the 3 day drying time, remove the samples from the drying oven and let them cool. When cool, find the mass of each sample. Record these masses in the appropriate place on the DATA TABLE as starting dry mass.
One week after step 4.
7. One week after you removed the #1 samples of each species of grass from the plots, repeat step 4 for the #2 sample plots. Properly label the samples and place them in the drying oven for seventy-two hours (3 days).
8. After the 3 day drying time, remove the samples from the drying oven and let them cool. When cool, find the mass of each sample. Record these masses in the appropriate place on the DATA TABLE as final dry mass.
DATA TABLE:
Plot size = ________ cm2Number of seeds planted per plot = _______
Mass data for samples. Grass Species name Starting dry mass
(g)Final dry mass
(g)Difference in mass
(g)CALCULATIONS:
9. Find the difference in mass for each species of grass by subtracting the starting dry mass from the final dry mass. Record your answers in the appropriate place on the DATA TABLE.
10. Calculate the net primary productivity for each species tested. You want to express your answer in appropriate units, so we will convert the area to 1 meter2. The example below shows the calculation using a 10 x 10 cm2 (100 cm2 ) plot size. Be sure to use your plot size in cm2.
Net Primary Productivity Grass species NPP in
g / M2 / week11. Of the species of grass tested, which species demonstrated the greatest net primary productivity?
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- Only the energy represented by NPP is available for consumers, and of that energy only a small portion is actually used by them. We can express both GPP and NPP as energy per unit area per unit time, or in terms of dry mass per unit area per unit time. Examples of these are:
MEASURING NET PRIMARY PRODUCTIVITY
OF GRASS
Laboratory Investigation
INTRODUCTION: