The nitrogen cycle

Grade 9 Science Worksheet - Challenge Me

Circle the best answer.

2. 1.

   Ari reads a graph of two plots. By how many mg per kg do the two lines differ at week 6?

   • 10 mg per kg
   • 8 mg per kg
   • 34 mg per kg
   • 22 mg per kg
3. 2.

   Zee reads the graph. What was the bean plot's average gain per week from week 0 to week 8?

   Use the picture in question 1.

   • 2 mg per kg per week
   • 8 mg per kg per week
   • 1 mg per kg per week
   • 4 mg per kg per week
4. 3.

   Luz reads the graph. Which statement explains why the two lines moved apart?

   Use the picture in question 1.

   • Decomposers destroyed nitrogen in the grass plot, but not in the bean plot
   • Rhizobium in the bean roots added fixed nitrogen, while grass took nitrate up
   • Nitrogen taken up by grass was used up, and beans gained by chance
   • Bean leaves took N₂ gas from the air, but grass leaves could not
5. 4.

   In a model bean plot, Rhizobium bacteria are missing from the root nodules. Bo predicts what happens to the nitrogen the beans can use.

   • It stays the same, because bean leaves take N₂ from the air
   • It rises, because the bacteria were taking nitrogen from the beans
   • It stays the same, because soil nitrogen does not run out
   • It falls, because nothing in the nodules fixes N₂
6. 5.

   In a model soil sample, denitrifying bacteria become more active. Luz predicts what changes in the soil.

   • Nitrate stays equal, since decomposers destroy the extra nitrogen
   • More nitrate becomes N₂ gas, so soil nitrate falls
   • More N₂ becomes usable nitrogen, so soil nitrate rises
   • More ammonium becomes nitrite, so soil nitrite falls
7. 6.

   A pile of leaf remains and wastes lies in a model plot, and decomposers start work. Noah predicts what rises in the soil first.

   • Nothing, since decomposers destroy the nitrogen in remains
   • Nitrate, made straight from the remains
   • N₂ gas, released by the decomposers
   • Ammonium, made by ammonification

8. Read this first

   Use this

   Flow diagram (model, not to scale), soil and air. Boxes: N₂ gas in air; living things (organic nitrogen); wastes and remains; ammonium. Arrows: from N₂ gas in air to living things (organic nitrogen): process nitrogen fixation, by Rhizobium bacteria in legume root nodules. from living things (organic nitrogen) to wastes and remains: process wastes and remains pass to the soil. from wastes and remains to ammonium: process ammonification, by decomposer bacteria and fungi. Notebook, 3 lines. Line 1: Rhizobium bacteria in the pea root nodules fix N₂. Line 2: The pea plant also takes in N₂ gas through its leaves. Line 3: Remains of the pea plant are changed into ammonium by decomposers.

9. 7.

   Pia reads a notebook entry about a pea plot. Which line has a flaw?

   • Line 2: plants cannot use N₂ from the air until it is fixed
   • Line 3: decomposers destroy the nitrogen, so no ammonium forms
   • Line 1: Rhizobium harm the pea, so they should not fix N₂
   • No line has a flaw, since nitrogen a plant takes in is used up

10. Use this

    Flow diagram (model, not to scale), soil and air. Boxes: nitrate; N₂ gas in air. Arrows: from nitrate to N₂ gas in air: process denitrification, by denitrifying bacteria such as Pseudomonas and Clostridium. Notebook, 3 lines.

11. 8.

    Ren reads a notebook entry about denitrification. Which line has a flaw?

    • No line has a flaw, since denitrification and fixation are one step
    • Line 3: plants cannot use N₂, and fixation makes usable forms
    • Line 1: nitrifying bacteria, not denitrifying ones, change nitrate to N₂
    • Line 2: the nitrogen is used up, so nothing returns to the air

12. Use the table

    Card (model data). Soil nitrate at the start and at week 8.

    plot	crop	start (mg per kg)	week 8 (mg per kg)
    1	grass	15	11
    2	beans	15	23
    3	peas	12	18

    Notebook, 3 lines. Line 1: Grass roots took up nitrate, so the grass plot fell from 15 to 11 mg per kg. Line 2: So the grass plot lost nitrate. Line 3: The bean plot went from 15 to 23, so it gained 23 mg per kg.

13. 9.

    Tala reads a notebook entry about two plots. Which line has a flaw?

    • Line 3: the bean plot gained 23 − 15 = 8 mg per kg
    • Line 2: decomposers destroyed the nitrate the grass lost
    • Line 1: grass makes nitrate from the N₂ in the air
    • No line has a flaw, since the bean plot gained the full 23
14. 10.

    Kira's friend says bacteria in soil only harm plants, so soil would be better without them. Which reply is best?

    • Disagree: plants need no bacteria, since leaves take N₂ from the air
    • Disagree: soil bacteria turn nitrate into N₂ gas, which plants use
    • Disagree: soil bacteria fix N₂ and make nitrate that roots take up
    • Agree: bacteria compete with roots for the nitrogen in the soil