Name_____________________________Date_____________________________________Block________
Grass Productivity Lab
Introduction:
Net primary productivity (NPP) is defined as the amount of carbon from the atmosphere that gets
added to green plants per unit of time. It is a rate, the quantity of new vegetable matter added per day, per
week or per year. Net primary productivity is calculated as the total gain of biomass from photosynthesis
minus the losses due to plant respiration. It is this net gain that is available to other organisms as food. The
higher the NPP is in a region, the higher the overall biomass and diversity.
You will measure NPP by comparing the changes in dry mass of plant growth over the course of several
days (at least one week). Data will be collected from four samples of 2 unknown species of grass. You will
follow the plant removal procedure which measures the net primary productivity as an increase in dry weight
over several days. You will also calculate GPP by adding respiration (R) loss to the measured NPP.
Research Questions (CHOOSE ONE): Which grass seed will have a greater net primary productivity? Which
grass seed will have a greater gross primary productivity? How does respiration rate affect gross primary
productivity? How does respiration rate affect net primary productivity?
Example Hypothesis: Grass seed A will have a higher productivity than grass seed B, because seed A has a
greater moisture retaining ability and will therefore be able to conduct photosynthesis more efficiently than
grass seed B.
Variables:
Independent:
Dependent:
Controls:
Procedure:
Pre-lab Preparation
1. Fill the plots with potting soil to near the top.
2. Plant 100 seeds in each plot so that the seeds are evenly spaced.
3. Cover the seeds with a thin layer of the potting soil.
4. Water the plots with 50mL of water to ensure that the soil is saturated but has no standing water.
5. Place the plots to expose them to sunlight or under artificial lights designed for plants.
6. Continue to water the seed with 50mL of water each day.
7. It will take two to three weeks for the plants to be large enough to start the experiment.
1.
2.
3.
4.
5.
6.
7.
Procedure – Plant Removal Procedure
Trim ALL plants to a standard height of 4cm.
Harvest all plants from ONE plot by removing them gently from the soil (be sure to remove the root system as
well).
Place the plants on an open container made from aluminum foil and put in the drying oven for 48 hours at a
temperature between 900 C and 950 C.
After the plants are dry, weigh and record the dry mass of 100 plants. This is the starting dry mass of
UNCOVERED plants.
Cover one of the remaining plots with aluminum foil. Poke a few holes in the foil. Leave the other plot
uncovered.
Allow the remaining grass to grow in the 2 plots for one more week.
Harvest all grass plants from the uncovered plot by removing them from the soil. Repeat the drying and
weighing procedure. Record this as the final dry mass of UNCOVERED plants.
8. Harvest all grass plants from the covered plot by removing them from the soil. Repeat the drying and weighing
procedure. Record this as the dry mass of COVERED plants.
9. Find the difference in dry mass. This is the number of grams of carbon per 100 plants. Scale this value up by
using the number of plants growing in 1m2 (see step 2 on the sample data).
10. Then convert units of cm2 to m2 and also the number of days between samples into years.
11. Calculate the net primary productivity in units of grams of carbon per square meter per year.
12. Record samples 2, 3, and 4 by sharing data with other groups in the class.
Data Collection and Processing
Table 1: Starting and ending dry masses and change in dry mass of uncovered plants, and dry mass of covered plants
over one week period for 100 plants in a 100cm2 plot of grass seed A
Sample
Number of
total plants
growing in plot
Starting Dry
Mass of 100
uncovered
plants (g)
Final Dry Mass
of 100
uncovered
plants (g)
Δ Difference
in Dry Mass (g)
Final – Starting
Dry Mass of
100 covered
plants (g)
1
2
3
4
Avg.
Table 2: Starting and ending dry masses and change in dry mass of uncovered plants, and dry mass of covered plants
over one week period for 100 plants in a 100cm2 plot of grass seed B
Sample
1
2
3
4
Avg.
Number of
total plants
growing in plot
Starting Dry
Mass of 100
uncovered
plants (g)
Final Dry Mass
of 100
uncovered
plants (g)
Δ Difference
in Dry Mass (g)
Final – Starting
Dry Mass of
100 covered
plants (g)
Sample Data
1. final dry mass – starting dry mass = grams net primary productivity (NPP)
Starting Dry Mass = 33.23 grams
Final Dry Mass= 34.67 grams
Net primary productivity (NPP) 34.67 – 33.23 = 1.44 grams/100cm2/week*
Find the average NPP for each grass type. You will use the average number to calculate the total NPP/m2/year for grass A
and grass B.
*time between samples
2.
Calculate average NPP per 1m2 (100 cm = 1 m then 10,000 cm2 = 1 m2)
average NPP
X grams
=
2
100 cm
10,000 cm2
1.44 grams
x grams
=
= 144 grams/m2 /week
2
100 cm
10,000 cm2
3. Calculate the NPP per year
NPP grams/m2
x NPP in 1 year
=
7 days*
365 days
144 grams/m2
x NPP in 1 year
=
= 7508.57 grams/m2 /year
7 days*
365 days
* The number of days between samples. In this example, 7 days passed between starting and final mass
4. Calculate the average GPP
GPP = NPP + R
R = average starting dry mass – average dry mass of covered grass
5. Calculate the GPP per year
GPP grams/m2
x GPP in 1 year
=
7 days*
365 days
* The number of days between samples. In this example, 7 days passed between starting and final mass of covered grass
Data Processing - Calculations:
1. Calculate the net primary productivity in units of grams of carbon per square meter per year (grams/m2/year) for
grass A. Show all of your work! Use the Sample Data on the previous page to guide you in this process.
2. Calculate the net primary productivity in units of grams of carbon per square meter per year (grams/m2/year) for
grass B. Show all of your work! Use the Sample Data on the previous page to guide you in this process.
3. Suppose the grass plants for A and B continued to grow at the same rate that you calculated for a year. What
would be the net productivity for a field that is 1 km2? (1000m2 = 1 km2)
4. Calculate the GPP in units of grams of carbon per square meter per year for grass seed A. SHOW YOUR WORK!
5. Calculate the GPP in units of grams of carbon per square meter per year for grass seed B. SHOW YOUR WORK!
Discussion, Evaluation and Conclusion
Discussion
1. What is net primary productivity?
2. What is the difference between net primary productivity and gross primary productivity?
3. Why is the net primary productivity more meaningful to us than gross primary productivity?
4. Is it better for ecosystems to have a high NPP or a low NPP? Why? How does the NPP impact other organisms in
the food web?
5. What do the plants with foil cover represent?
6. Explain how variations in respiration rates would affect your GPP results. Under what conditions would you
expect the plants’ respiration rates to increase? Decrease?
7. If there was growth in the covered plots, account for why that growth might have taken place even though no
sunlight was reaching the plant.
8. Why is it important to study the productivity of moisture retaining grass seed and regular grass seed? What
implications does this have on society (water resources, landscapers, people in dry climates…)?
9.
Grass seed could have been replanted on the plot from which the grass was removed and the amount of new
plant growth compared and recorded as productivity. What would be the reliability of such data?
10. How does your answer in #9 relate to a farmer’s use of crop rotation on his farm?
Evaluation
1. What procedures in this experiment could have caused variability or error in the collection of your data?
2. Specifically how could the above procedures be changed to allow this experiment to run more smoothly?
Conclusion
1. Which type of grass seed was more productive?
2. What specific data allowed you to make this conclusion? (Actually cite the processed data here!)
3. What is the reason that one grass seed is has (less/same/ more) productivity than the other?