Net Primary Productivity of Grass
Introduction:
During the growing seasons, tropical and temperate regions receive approximately 8,000 to 10,000 kcal/m2
each day. This energy includes UV light, infrared light, and white light. Of this energy, only a small amount of
the energy from white light (1-3% in the most productive zones) will be trapped by green plants to make
organic compounds (in the process of photosynthesis).
6CO2 +6H2O + light energy  C6H12O6 + 6O2
Photosynthesis results in the production of glucose, which can later be converted into other products in the plant
and provide for the growth of the plant. This results in an increase in biomass. Another term, which is used to
describe this process more quantitatively, is gross productivity (GPP), the amount of biomass produced by
photosynthesis per unit area over a specific time period. Gross productivity can be measured indirectly using
grass plants. Why indirectly?
The answer is due to the metabolic needs of the plant itself. That is, as the plant is producing glucose through
photosynthesis, at least one-half of this glucose is used to meet the plant’s own energy needs (cell respiration).
The energy that remains in plant tissues after respiration has occurred is called net primary productivity (NPP).
The NPPP represent the rate at which this organic matter is actually incorporated into plant tissue to produce
growth. Therefore, only this energy is available for consumers, and of that energy only a small portion (an
average of 10%) is available for use by them.
This relationship between GPP and NPP will be determined through calculations.
Net primary productivity =
(plant growth)
Gross primary productivity
(total photosynthesis)
–
Respiration
Materials:
Grass Seeds
Water
Flats
Ruler
Balance/Scale
Aluminum Foil
Soil
Paper Towels
Procedure:
You will calculate the primary productivity of one week of grass growth. You will be given one flat made of 3
plots. These plots have each been growing 15 seeds for about 2 weeks (the grass should be around 3 cm by
now).
1. Record the plot size = ________ cm2
For example a square plot size of 10cm would be 10 x 10 cm2 (100 cm2 )
2. Record the appearance of the grass.
3. Carefully remove the grass plants entirely from one plot. Keep the roots, they are part on the growth of
plants. Remove as much soil as possible. Carefully rinse off any remaining soil from the roots in the
lab sink, and gently shake off the excess water.
4. Weigh and record the plant mass. This will be your starting wet mass for covered and uncovered
plants.
5. Sandwich the plants between two pieces of paper towels, and place that in the incubator (for 72 hrs. = 3
days).
6. After the 3 days of drying record the weight of the grass plant. This will be recorded as starting dry
mass (g).
7. Completely cover one of the plots with aluminum foil and allow this one as well as the other uncovered
plot to grow for 1 more week.
8. Allow contents of flats to grow for 1 Week.
9. Remove the plants from the uncovered plot. Describe the appearance of the grass and then remove the
grass plants like you did in step 2.
10. Weigh and record the plant mass. This will be final wet weight for uncovered plants.
11. Follow drying instructions in step 5.
12. After the 3 days of drying record the weight of the grass plant. This will be recorded as final dry mass
(g).
13. Remove the foil from the covered plots. Describe the appearance of the grass, and then remove the
grass plants like you did in step 2.
14. Weigh and record the plant mass. This will be final wet weight for covered plants.
15. Follow drying instructions in step 5.
Data Collection
Visual Observations:
Uncovered Plant Observations Week 1:
Uncovered Plant Observations Week 2:
Covered Plant Observations Week 1:
Wet Weight Data Collected for Uncovered Grass
Starting Wet Mass (g) *
Final Wet Mass (g)
Difference in mass (g)
Wet Weight Data Collected for Covered Grass
Starting Wet Mass (g) *
Final Wet Mass (g)
Difference in mass (g)
Dry Weight Data Collected for Uncovered Grass
Starting Dry Mass (g) *
Final Dry Mass (g)
Difference in mass (g)
Dry Weight Data Collected for Covered Grass
Starting Dry Mass (g) *
Final Dry Mass (g)
Difference in mass (g)
* Should be the same mass (g)
Calculations
1.
Dry weight of Week 2 uncovered grass
- Dry weight of Week 1 grass
= Net Productivity
2.
Dry weight of Week 1 grass
- Dry weight of Week 2 covered grass
= Respiration
3.
Net Productivity
+ Respiration
= Gross productivity for one week
3. Repeat these calculations using wet weight.
4. You want to express your answer in appropriate units, so we will convert the area to 1 meter2. To do
this accurately please make sure you use the plot area you recorded in step 1 of the procedure.
Difference in dry mass (g)
Plot area (cm2)
Your Data Goes here:
_________(g)
_________ (cm2)
Difference in wet mass (g)
Plot area (cm2)
Your Data Goes here:
_________(g)
_________ (cm2)
x
(10,000 cm2)
(1m2)
=
net primary productivity in
__ g___
m2/week
x
(10,000 cm2)
(1m2)
=
net primary productivity in
__ g___
m2/week
x
(10,000 cm2)
(1m2)
=
net primary productivity in
__ g___
m2/week
x
(10,000 cm2)
(1m2)
=
net primary productivity in
__ g___
m2/week
Analysis Questions:
1. Why in part A was one set of plots harvested and the weight of the grass taken at the beginning of the
experiment?
2. What do the plots with the foil covering represent?
3. How would variations in respiration rates change your results? Under what conditions would you expect the
plants’ respiration rates to increase? Decrease?
4. Compare the difference in appearance between the foil-covered and the uncovered grass. If there is a
difference in appearance, explain the difference.
5. What units should be used to express productivity?
6. In your calculations, was there a significant difference between the dry weights and the wet weights? Which
of the two calculations would provide you with a more correct rate for gross productivity? Why?
7. If you were a field scientist and needed a quick answer, how could you minimize this difference?
8. Supposed you had been asked to measure the height of plants. Proposed an explanation to account for why
any growth may have taken place even though no sunlight was reaching the plant.
9. Standing biomass is the organic matter of the living organisms in an area. Due to the movement of animals,
this term is most often associated with just the plants of an area. The terms net productivity and standing
biomass are often mistakenly used interchangeably. Why would these terms not be interchangeable?
10. Graph the results of the respiration rate, NNP, and GPP experiment.
11. Choose a variable to change. Propose a hypothesis and a procedure using this new experiment.
Completed Analysis will included (word processed):
1. Calculations (show all work)
2. Completed Questions (Q&A form is acceptable)
3. Graphs
4. Proposed experiment